Emerging Perspectives on Chemical Weed Management Tactics in Container Ornamental Production in the United States

Abstract

Weed management remains a critical challenge in the U.S. container ornamental production industry, where weeds not only compete with crops for limited resources but also harbor pests and pathogens, thereby diminishing plant quality and marketability. The paper explores the economic impact of weed infestations, herbicide resistance development, and the limited availability of selective herbicides for ornamental crops in the United States. This review synthesizes current chemical weed control tactics, focusing not only on both preemergence and postemergence herbicides commonly used in ornamental nurseries, but also organic alternatives and integrated weed management (IWM) approaches as complementary strategies by evaluating their effectiveness, crop safety, and usage. There is a critical need for research in the areas of alternative chemical options such as insecticides, miticides (e.g., Zerotol and Tetra Curb Max), and organic products for liverwort control in greenhouses. Although essential oils and plant-based extracts show some potential, their effectiveness and practical use remain largely unexplored.

1. Introduction

The ornamental horticulture industry has experienced significant growth both in the United States and globally. This industry encompasses nursery production (container and field); landscape horticulture, which focuses on enhancing outdoor spaces with plants; Christmas tree production; and floriculture, which involves the cultivation, distribution, and marketing of ornamental plants for gardens and floristry. Ornamental plants, cultivated primarily for their decorative appeal, include annuals, perennials, and foliage plants [1].

As a billion-dollar industry, ornamental horticulture plays a crucial role in both economic and environmental landscapes. In 2023, the number of producers increased to 10,216, up from 8949 in 2022, with the 28 key program states seeing a 17% rise in grower numbers. Total sales across all floriculture crops reached $6.69 billion, holding steady from 2022, while production area expanded to 851 million square feet. Among crop categories, annual bedding and garden plants led with $2.48 billion, followed by herbaceous perennials ($1.04 billion) and potted flowering plants ($1.00 billion). Florida and California remained the top-selling states, together accounting for 33% of total U.S. sales. The industry also undertook 6527 operations with hired labor, averaging 16.4 workers at peak season per operation [2]. Beyond its economic significance, the ornamental plant industry also enhances aesthetic value, psychological well-being, and productivity [3]. Given the increasing consumer interest in ornamental plants, researchers have focused on understanding plant preferences and how market trends respond to economic shifts such as changes in income and pricing.

One of the major challenges in nursery container production is weed management, as weeds compete with ornamental plants for essential resources such as nutrients, water, light, and space within a restricted area. This competition negatively impacts plant quality, reducing their market value and, in severe cases, leading to plant mortality. Additionally, weeds can serve as hosts for insects, pests, and pathogens, further compromising the health and profitability of nursery crops. Effective weed control is critical, yet post-emergence weed control options in container nurseries are limited, with only a select group of graminicides available for certain ornamental crops. As a result, weed control relies primarily on pre-emergence herbicides supplemented by hand weeding, which is both labor-intensive and costly—often exceeding $9884.20 per hectare [4,5].

The non-chemical weed control strategies for ornamental crop production include scouting, sanitation practices, hand weeding, mulching, irrigation management, substrate stratification, mulch disks (or geo disks), and lid bags and fertilizer placement, which have been discussed in the previous paper [6]. However, non-chemical weed control practices alone cannot control weeds effectively and additionally there are high cost and labor requirements associated with manual weed control. Therefore, chemical weed control is an essential component of effective weed management programs in nursery container production system. The purpose of this review is to provide an overview of chemical weed control strategies in ornamental production specific to the United States and to identify knowledge gaps where current practices could potentially be improved or further research is required.

2. Effects of Weeds on Nursery Ornamental Crop Production

Weeds are traditionally seen as direct competitors to crops for essential resources such as light, water, and soil nutrients. In confined spaces like nursery containers, this competition becomes even more pronounced, leading to significant reductions in ornamental plant growth and overall quality.

2.1. Direct Competition Between Weeds and Ornamentals

Previous research conducted by [7] has demonstrated that weed competition significantly reduces the growth and biomass of container-grown ornamentals. For instance, Ilex crenata Thunb. (Japanese holly) and Ligustrum spp. exhibited shoot dry weight reductions of up to 53% when competing with weeds such as Bidens alba (L.) DC., Euphorbia maculata L., and Eclipta prostrata (L.) L.at high weed densities (100% weed coverage). The impact varied by container size, with larger containers (56.8 L) showing the most severe biomass reduction, particularly under full weed competition [7]. Furthermore, weed competition not only affected shoot biomass but also influenced nutrient uptake. Foliar nitrogen and potassium concentrations declined in plants subjected to high weed densities, further indicating the competitive effects of weeds on ornamental crop nutrition as foliar nitrogen concentrations were significantly reduced in Ilex crenata grown in 3.8 L, 11.4 L, and 56.8 L containers under 50% and 100% weed coverage, while potassium levels declined across all container sizes, indicating that weeds outcompeted ornamentals for essential nutrients. The presence of Oxaliscorniculata and Carex pensylvanica in naturally inoculated containers further contributed to competition effects during the later stages of the experiment. Additionally, growth index reductions were most pronounced in ligustrum grown in 11.4 L and 24.7 L containers, where plants without weed competition were 47% to 61% larger compared to those subjected to high weed densities. These findings highlight the substantial impact of weed interference on both biomass accumulation and nutrient availability in container-grown ornamental crops [7]. However, regardless of the severity of competition, weed-infested plants not only lose vigor but also have reduced marketability than their weed-free counterparts. The presence of weeds reduces the aesthetic appeal of the plants, making them less attractive to customers [8].

In the case of ‘Fashion’ azaleas, the presence of just one Eclipta weed per pot reduced shoot growth by 43%, highlighting the direct impact of weed competition on ornamental crops [8]. Weeds compete with ornamental plants for critical resources such as water, nutrients, and space, often stunting the growth of the intended plants. Even in smaller containers (3.78 L), a single weed can disrupt plant development [4]. Coffeeweed (Sesbania exaltata) significantly suppressed the growth of pyracantha (Pyracantha coccinea), Andorra juniper (Juniperus horizontalis ‘Plumosa’), and Japanese holly (Ilex crenata), whereas cocklebur (Xanthium strumarium) suppressed pyracantha and Andorra juniper but had no notable effect on Japanese holly. In contrast, bittercress (Cardamine hirsuta) exhibited no significant competitive impact on any of the ornamental species [9]. These findings highlight the species-specific nature of weed interference and the need for targeted weed management strategies in container-grown nursery crops.

Potentilla fruticosa ‘Gold Drop’ exhibited extreme sensitivity to weed competition, with grassy weeds reducing its growth even at densities of less than one plant per container. Their research underscores the economic importance of early weed control in container-grown ornamentals, as even minimal weed interference can significantly hinder growth and overall plant quality [10]. The presence of a single redroot pigweed (Amaranthus retroflexus) or large crabgrass (Digitaria sanguinalis) plant per 2.4 L nursery container reduced Japanese holly growth by 47% and 60%, respectively. This study provides clear evidence of the detrimental effects of common weeds in container production and emphasizes the competitive advantage weeds have over ornamentals in limited-space environments [11].

The extent of competition varies depending on the weed species and the type of ornamental plant. In woody plants, for instance, competition from certain weeds severely limits plant growth during container production, while others may have a lesser effect. Weed competition was shown to affect plant biomass and delay production, even when plant growth indicators were similar between treatments. This suggests that the negative effects of weeds are not limited to direct growth inhibition but also extend to production timelines. As weed density increases, competition effects intensify, with more severe impacts on plant development. If weed infestations become too dense, the damage may reach a point where no further negative effects are observable, as the plants are already heavily compromised [7].

The competitive effects of three major weed species in container-grown ornamentals Coffeeweed (Sesbania herbacea, previously Sesbania exaltata), Cocklebur (Xanthium strumarium) and Bittercress (Cardamine spp.) in the southern United States and reinforced the necessity of effective weed management to prevent severe reductions in plant growth and quality [8]. Despite these early studies, limited recent research has addressed the suppression effects of persistent weed species currently infesting container nurseries. These findings emphasize the ongoing challenge of weed interference in ornamental nursery production and the importance of continued research to identify sustainable control strategies.

2.2. Weeds as Hosts for Pests and Pathogens

Weeds not only diminish the aesthetic quality of ornamental plants in nursery production but also serve as reservoirs for various pests, including insects such as whiteflies and thrips, as well as mites, slugs, and snails. Additionally, certain weed species, such as woodsorrel (Oxalis spp.) and bittercress (Cardamine spp.), act as hosts for plant viruses like Tobacco rattle virus (TRV) and these viruses can be transmitted to susceptible ornamental crops by thrips, exacerbating disease management challenges [12]. Studies have documented the role of weeds such as Solanum sarrachoides in harboring Potato leaf roll virus (PLRV), which has been associated with high infection rates in susceptible crops [13]. Similarly, Datura stramonium has been identified as a major host for PLRV in Morocco [14]. In the context of ornamental crops, weeds such as Amaranthus retroflexus and Chenopodium album have been shown to carry Tobacco rattle virus, which negatively impacts plants like sugar beets and potatoes [15]. Additionally, weeds like Pueraria montana (kudzu) can host Phakopsora pachyrhizi, the causal agent of soybean rust, which can spread to other leguminous plants [16]. Consequently, weed removal from greenhouse containers, benches, and floors is essential for maintaining both plant quality and effective pest management strategies.

2.3. Economic Impact of Weeds in Ornamental Nurseries

The economic consequences of weed infestations in nursery container production are substantial. Weeds not only compete for resources but also lead to increased labor and chemical control costs, reducing profit margins for growers. Weed control represents one of the most significant expenses in ornamental crop production. Herbicides, while effective, make up a substantial portion of these costs, particularly in large-scale nursery operations. Furthermore, restrictions on the interstate movement of infested ornamental plants can limit market access, adding financial strain to growers like- presence of Rotylenchulus reniformis, a nematode found in association with ornamental nursery weeds (Florida black jack (Bidens pilosa); Common Purslane (Portulaca oleracea) and santa maria (Parthenium hysterophorus) in Florida, has led to regulatory restrictions that further impact the economic viability of nursery operations [17]. Weed infestations in ornamental plant nurseries present significant economic challenges, affecting both production costs and the overall profitability of the industry.

In container-grown nursery production, weed control is a critical component of variable costs. A study on Buxus microphylla var. japonica ‘Green Beauty’ revealed that weed management expenses, encompassing hand weeding and herbicide applications, constituted approximately 38% of the total production cost per shrub in field settings and 7% in number 1 containers [18]. The financial implications of weed infestations extend to labor costs associated with manual removal. Reports indicate that nurseries may spend between $1235.53 and $9884.20 per hectare on hand weeding, depending on the severity of weed pressure and the effectiveness of preemergent herbicide programs [5]. These costs do not account for potential crop losses or reduced growth resulting from weed competition. In Michigan, the ornamental industry significantly contributes to the state’s economy. The total financial impact of nursery and landscape production, including related industries, is estimated at $1.26 billion [19]. Effective weed management is essential to maintain and enhance this economic contribution, as uncontrolled weeds can lead to increased production costs and reduced marketability of ornamental plants.

Over the past 15 years, container production has emerged as a rapidly growing segment within the U.S. nursery industry. However, weed infestations pose significant challenges in this sector, as they compete with ornamental crops for limited resources such as water, nutrients, and air within the confined space of containers. This competition can lead to reduced plant growth and vigor, ultimately affecting the aesthetic quality and market value of the ornamental plants [4]. The economic damage resulting from weed infestations is substantial. Estimates suggest that the cost of manual weed removal, including labor and associated expenses, can be as high as $9900 per hectare [5]. These figures highlight the critical need for efficient and cost-effective weed management strategies in the ornamental industry. In nursery ornamental crop production, weeds pose a multifaceted challenge by competing for essential resources, acting as reservoirs for pests and pathogens, and causing significant economic losses. Effective weed management strategies are crucial to maintaining plant quality and profitability.

3. Chemical Methods

Manual weed removal is often labor-intensive, costly, and can inadvertently harm ornamental plants due to intertwined root systems. While non-chemical methods, such as sanitation, mulching, and hand weeding, play a role in integrated weed management programs, chemical control remains an essential component due to its efficiency and cost-effectiveness. Maintaining a weed-free nursery environment requires stringent sanitation practices to prevent weed seed introduction and dispersal. Weeds and their seeds can enter greenhouses through infested plant materials, growing media, tools, equipment, and even on workers’ clothing. Additionally, weed seeds may be dispersed by wind (e.g., dandelion, horseweed, groundsel), irrigation water (e.g., chickweed), or natural propulsion mechanisms, as seen in woodsorrel and bittercress, which can propel seeds up to 12 feet in nurseries. Once established, weeds can produce large quantities of seeds that contribute to persistent weed problems, increasing competition with crops and serving as habitats for pests [20].

3.1. Preemergence Herbicides for Ornamental Production

Preemergence herbicides are weed control agents applied to the soil before weed seeds begin to sprout. After application water is applied at top inch of soil to activate them. Their primary function the primary function of pre-emergence herbicides is to kill the newly emerged seedlings at the very early stages, such as before the 1- to 2-leaf stage. These herbicides reduce weed pressure early on, helping maintain plant health, aesthetic and economic value. While effective, the use of preemergence herbicides must be carefully managed to avoid crop injury. Some ornamental plants are sensitive to these chemicals, necessitating careful selection and application to prevent damage. Some common active ingredients, which are known for their ability to control a wide range of weed species by disrupting cell division or photosynthesis in emerging seedlings, discussed in Table 1.

Isoxaben + trifluralin, also known as Snapshot TG as trade name, has demonstrated high safety as a preemergence herbicide for numerous herbaceous ornamental crops while effectively controlling a wide range of grass and broadleaf weeds [21,22]. Isoxaben and trifluralin combinations are used for weed control in container-grown herbaceous flowering perennials [23]. For optimal results, herbicides should be applied in late summer, early fall, or early spring before weeds emerge or immediately following cultivation. To activate the herbicide, at least half an inch of water must be applied within three days after application. In container-grown plants, it is essential to hand- water before application and irrigate the area immediately after treatment [24].

Isoxaben—classified as a benzamide herbicide, functions by disrupting cellulose biosynthesis in emerging seedlings, thereby inhibiting their development [25]. This herbicide is commercially available for field nursery crop production in a 75% active dry flowable (DF) formulation. Typically applied at rates ranging from 0.56 to 1.12 kg/hectare, isoxaben provides effective preemergence control against a broad spectrum of broadleaf weeds. Additionally, it is approved for use on more than 400 different plant species. While isoxaben demonstrated excellent control over numerous broadleaf weed species, it was less effective against grasses including quackgrass, Johnsongrass. However, when combined with oryzalin, isoxaben offered a broader spectrum of weed control with strong safety for most container and field-grown woody nursery crops [26]. The higher crop tolerance of isoxaben makes it particularly beneficial in cases where crop sensitivity to herbicides is a concern [27].

Trifluralin—is a grass-active herbicide commonly utilized in container nurseries. However, its availability in the nursery industry is currently limited to granular formulation, making it impractical for large-scale field nurseries. Trifluralin, along with oryzalin, pendimethalin, and prodiamine, belongs to the dinitroaniline (DNA) herbicide group, which functions by inhibiting root formation. Among these, oryzalin is highly effective in controlling grasses and certain small-seeded broadleaf weeds. To enhance its weed control spectrum, oryzalin is often combined with other preemergence herbicides such as oxyfluorfen, isoxaben, or simazine, leading to broad-spectrum preemergence weed suppression [28,29,30].

Oryzalin—is typically applied at a rate of 4.94–9.88 kg/hectare a.i. and is highly effective in controlling grasses and certain small-seeded broadleaf weeds [28]. The herbicide Surflan, which contains oryzalin, is approved for use on over 350 woody landscape species. When applied to woody field-grown nursery stock, it is generally noninjurious [28,31,32]. Oryzalin is often combined with other preemergence herbicides, such as oxyfluorfen, isoxaben, or simazine, to achieve broad-spectrum weed control for preemergence applications [4,28,29,30].

Pendimthalin—herbicide is a cost-effective option for preventing weed growth in the ornamental nursery industry. It provides broad-spectrum control of over 50 species of grassy and broadleaf weeds while maintaining a low application cost per hectare. Additionally, it is labeled for use on more than 335 ornamental plant species. It should be applied only to established plantings. For seedbeds, transplant beds, or liners, application should be postponed until the plants are well-rooted. It is important to ensure that soil or planting media has settled firmly after transplanting, without cracks that could expose roots directly to the herbicide. Optimal weed control and plant safety occur when the product is applied evenly across the soil or media surface. Only plant species listed on the product label should be introduced into areas treated with Pendulum products the previous season to prevent potential injury. When using this product with container-grown ornamentals, delay application to bare root liners for 2 to 4 weeks after transplanting. it can be effectively used as part of landscape and grounds maintenance programs to provide long-lasting preemergence control of many annual grasses and select broadleaf weeds. Treated areas, such as mulch beds, roadsides, parking areas, and similar spaces, should be cleared of existing weeds prior to application [33].

Prodiamine—herbicide is generally safe for many woody ornamental plants; however, its sprayable formulation can be harmful to numerous herbaceous species. The granular version (trade name Regalkade G) is a safer alternative for use on ornamentals. Barricade 4FL should be applied in the fall and/or spring, either before weed seeds germinate or after existing weeds have been removed. For longer-lasting control, higher application rates are recommended, though the total annual application must not exceed 2.58 kg/ha. The product can be applied as a broadcast, over-the-top, or directed spray, but treatments on newly transplanted ornamentals should be delayed until the soil has fully settled around the plants. Caution is advised when applying prodiamine to rapidly growing tissue or buds, particularly in the spring, as this may temporarily damage new growth. To reduce the risk of injury, over-the-top applications should be postponed until new vegetation has hardened off, unless prior experience confirms plant tolerance. After application—especially on deciduous species—overhead irrigation should be applied immediately to move the herbicide from plant surfaces to the soil. Pre-watering before application may also enhance this process [34].

Dimethenamid-P—effectively controls various annual grasses, sedges, and broadleaf weeds in containers and field-grown woody nursery stock, landscape plantings, and groundcovers. For the best results, it needs to be applied to weed-free soil or substrates and irrigate the area shortly after application. It should not be used in greenhouses or enclosed structures. Additionally, avoid spraying over broadleaf ornamental plants in the spring when young, tender growth is present. Instead, directed applications can help minimize the risk of damage to delicate foliage. This herbicide is not suitable for use on begonias or ornamental grasses. Reports indicate potential injury can happen to certain herbaceous ornamental species, including Dianthus spp., Rudbeckia hirta, Stachys byzantine, Salvia spp., and others [35].

Indazilflam—an effective preemergence herbicide targets annual grasses such as crabgrass, goosegrass, foxtails, barnyardgrass, and annual bluegrass, along with broadleaf weeds like bittercress, doveweed, oxalis, and spurge, as well as annual sedges emerging from seed. The SC formulation may also provide some level of postemergence suppression, particularly demonstrating effective control over small seedling broadleaf weeds like oxalis. When applied to newly planted ornamentals, it is recommended to water the plants beforehand to help settle the soil. To activate the herbicide, approximately half an inch of rainfall or irrigation should follow the application as soon as possible. This product is not suitable for use in propagation beds, herbaceous perennials, or liner production. Reports indicate that it may cause injury to certain plant species, including hydrangea and euonymus [36].

Table 1. Commonly used pre-emergence herbicides for effective weed control in ornamental production (adapted from [37,38,39,40].

Active Ingredients	Trade Name	WSSA * Group and Mechanism of Action	Usage				Notes
			Greenhouse		Containers	Field
			With Crop	In Soil
Trifluralin 2.0%, and Isoxaben 0.5%.	Snapshot TG	3 (microtubule assembly inhibitors) +29 (Cellulose biosynthesis inhibition)	No	No	Yes	Yes	The Combination of trifluralin and isoxaben improves weed control for both grasses and residual weed control. Limited applicability in (open) greenhouses due to volatility issue.
Isoxaben	Gallery	29	No	No	Yes	Yes	Safest herbicide controls most annual broadleaf weeds and grasses like eclipta, hairy bittercress and spotted spurge.
Oryzalin	Surflan	3	No	no	No	Yes	A pre-emergence surface-applied herbicide used for control of annual grasses and broad-leaved weeds in fruit trees and around ornamentals. Exhibit mammalian toxicity and a potential carcinogen [41].
Trifluralin	Treflan	3	No	surface application on containers [42]	Yes	Yes	Incorporated into the soil, providing long-lasting suppression of various annual grasses and broadleaf weeds, including large crabgrass, foxtail, pigweed, and carelessweed [40].
Pendimethalin	Pendulum	3	No	Yes [43]	Yes	Yes	Can be used with oxadiazon and oxyfluorfen (OH2) provides improved pre-emergence control of annual grasses and some broadleaf weeds [44]. Widely used in nursery crops; effective in reducing annual grass populations [45]. Combining pendimethalin with dimethenamid-P (FreeHand) enhances broad-spectrum weed control [46].
Prodiamine	Barricade	3	Depends on crop tolerance [47]	No	No (landscape ornamental only)	No	Commonly used for preventing weed seed germination; effective for annual grass weeds [44]. Combining oxyfluorfen with prodiamine (RegalStar) improves broadleaf weed suppression [27].
Dimethenamid-P	Tower	15 (very-long-chain fatty acid (VLCFA) synthesis inhibition)	No	No	Yes (woody plants)	Yes	Effective in controlling broadleaf and grass weeds in container production [46].
Indazilfam	Marengo	29	No	Yes (on gravel/under benches)	Yes	Yes	Provides long residual weed control, particularly for broadleaf species.

* WSSA abbreviation of Weed Science Society of America.

Table 1. Commonly used pre-emergence herbicides for effective weed control in ornamental production (adapted from [37,38,39,40].

Active Ingredients	Trade Name	WSSA * Group and Mechanism of Action	Usage				Notes
			Greenhouse		Containers	Field
			With Crop	In Soil
Trifluralin 2.0%, and Isoxaben 0.5%.	Snapshot TG	3 (microtubule assembly inhibitors) +29 (Cellulose biosynthesis inhibition)	No	No	Yes	Yes	The Combination of trifluralin and isoxaben improves weed control for both grasses and residual weed control. Limited applicability in (open) greenhouses due to volatility issue.
Isoxaben	Gallery	29	No	No	Yes	Yes	Safest herbicide controls most annual broadleaf weeds and grasses like eclipta, hairy bittercress and spotted spurge.
Oryzalin	Surflan	3	No	no	No	Yes	A pre-emergence surface-applied herbicide used for control of annual grasses and broad-leaved weeds in fruit trees and around ornamentals. Exhibit mammalian toxicity and a potential carcinogen [41].
Trifluralin	Treflan	3	No	surface application on containers [42]	Yes	Yes	Incorporated into the soil, providing long-lasting suppression of various annual grasses and broadleaf weeds, including large crabgrass, foxtail, pigweed, and carelessweed [40].
Pendimethalin	Pendulum	3	No	Yes [43]	Yes	Yes	Can be used with oxadiazon and oxyfluorfen (OH2) provides improved pre-emergence control of annual grasses and some broadleaf weeds [44]. Widely used in nursery crops; effective in reducing annual grass populations [45]. Combining pendimethalin with dimethenamid-P (FreeHand) enhances broad-spectrum weed control [46].
Prodiamine	Barricade	3	Depends on crop tolerance [47]	No	No (landscape ornamental only)	No	Commonly used for preventing weed seed germination; effective for annual grass weeds [44]. Combining oxyfluorfen with prodiamine (RegalStar) improves broadleaf weed suppression [27].
Dimethenamid-P	Tower	15 (very-long-chain fatty acid (VLCFA) synthesis inhibition)	No	No	Yes (woody plants)	Yes	Effective in controlling broadleaf and grass weeds in container production [46].
Indazilfam	Marengo	29	No	Yes (on gravel/under benches)	Yes	Yes	Provides long residual weed control, particularly for broadleaf species.

* WSSA abbreviation of Weed Science Society of America.

3.2. Postemergence Herbicides for Ornamental Production

Postemergence herbicides are used to control weeds that have already sprouted and are visible above the ground. These herbicides may be selective—targeting specific weed species while preserving ornamental crops—or nonselective, offering broad-spectrum weed control. However, in ornamental settings, the availability of effective selective postemergence herbicides is limited. Nonselective types are generally divided into two categories: contact and systemic herbicides. Contact herbicides, such as diquat and pelargonic acid, destroy only the green tissues directly exposed to the spray. They are effective for controlling small annual weeds but may only “burn back” larger annual or perennial weeds without eliminating the root system. Systemic herbicides, including glyphosate and glufosinate, are absorbed by foliage and translocated throughout the plant, reaching growing points in roots and shoots (more information is given in Table 2). Glyphosate is highly effective against both annual and perennial weeds due to its extensive movement within the plant. Glufosinate, while also systemic, has more limited translocation, primarily affecting treated leaves without significant movement to distant growing points [48].

Fluazifop-butyl—This herbicide provides selective postemergence control of both annual and perennial grasses. It can be applied over the top of various ornamentals, including many woody and herbaceous species in container and field nurseries, landscapes, and greenhouses. However, to avoid potential damage to sensitive plants, directed sprays are necessary. To enhance weed control, cultivation for two to three weeks after application may be beneficial. For optimal effectiveness, thorough spray coverage is essential, ensuring the surface is coated but avoiding excessive runoff [49]. Extensive phytotoxicity testing of fluazifop-butyl on woody plants has shown minimal adverse effects, except for some red-flowering azalea cultivars [27].

Glufosinate is a non-selective, postemergence herbicide that is often perceived as having primarily “contact action.” However, while some degree of translocation occurs, its movement is limited to within treated leaves rather than being transported over longer distances to new growth points. Compared to glyphosate, glufosinate exhibits more rapid symptom development, typically within 48 h, but its effectiveness against perennial weeds, such as bindweed, goldenrod, and mugwort, is usually limited to temporary suppression. Due to its reduced translocation, glufosinate may have certain advantages over glyphosate for landscape trimming and edging applications. In general, glufosinate is less effective than glyphosate in controlling perennial weeds and grasses, though some exceptions exist. It has demonstrated superior efficacy against horsenettle, white clover, and cutleaf evening primrose and is also effective against many glyphosate-resistant weed species [48].

Clethodim, a postemergent graminicide, is applied once grass weeds have emerged and are in their early growth stages for optimal effectiveness. It is generally considered to be one of the better graminicides for perennial grass control, including bermudagrass and fescues. It is best to spray clethodim under favorable weather conditions, ensuring no rainfall is expected for at least 24 h after application. Since clethodim is applied using a sprayer, it is important to monitor wind conditions and follow best application practices to minimize drift. When using clethodim in a tank mix, the addition of crop oil is necessary. Clethodim is also commonly combined with glyphosate for improved weed control. Additionally, because poor water quality can hinder the uptake and movement of clethodim within plants, using a spray-grade dry ammonium sulfate water conditioner is recommended to enhance its effectiveness [50].

Paraquat is a non-selective, contact herbicide which rapidly destroys the foliage it contacts but does not translocate to underground tissues, allowing regrowth from roots. Due to its high mammalian toxicity (oral LD50 = 150 ppm), paraquat is classified as a restricted-use herbicide, necessitating careful handling during mixing and application. In contrast, glufosinate, an amino acid derivative, also acts as a contact herbicide but has significantly lower mammalian toxicity (oral LD50 = 2000 ppm), making it a safer alternative to paraquat in similar application scenarios [27].

Diquat (Reward), a postemergence contact herbicide, is effective at controlling small annual weeds. While it can injure larger annual and perennial weeds, it does not completely eradicate them. Research indicates that applying Reward at a spray volume of 2 gallons per 1000 square feet is more effective than lower-volume applications. Some key benefits of Reward include its ability to quickly eliminate small seedling weeds at a relatively low cost. Additionally, minor spray drift typically results in only superficial damage to landscape plants without systemic movement that could harm desirable vegetation. Unlike some herbicides, Reward remains effective in both cool and warm temperatures. However, its limitations include poor control of perennial and well-established weeds, as well as a higher mammalian toxicity level compared to alternatives such as Scythe, Finale, and Roundup-Pro [48].

Glyphosate is one of the most widely used translocated herbicides in nursery production, offering broad-spectrum weed control. However, incorrect application rates frequently lead to nursery crop injuries. For optimal efficacy, glyphosate should be applied at a 1% solution for annual weeds less than 6 inches tall, a 1.5% solution for taller annual weeds, and a 3% to 5% solution for perennial weeds. Misapplication of glyphosate is a common cause of herbicide-related damage in nurseries, underscoring the importance of proper dosing and application techniques [27].

Flumioxazin- offers preemergence and early postemergence control for a variety of annual broadleaf weeds and provides preemergence control for several annual grasses in established containers and field-grown woody ornamentals. It is typically applied as a directed spray to avoid damaging desirable foliage, but it can be broadcast to certain conifers. Often combined with a non-selective herbicide, like glyphosate, for both preemergence and postemergence weed control. When combined with flumioxazin, these mixtures can enhance the control of glyphosate-resistant weeds, especially when applied to young seedlings. It effectively controls many common annual broadleaf weeds found in containers and field-grown nursery crops, such as bittercress, spurge, woodsorrel, groundsel, and chickweed, among others. Control of eclipta has been good in some locations, although it has varied in others. While crabgrass is generally controlled, its effectiveness diminishes with lower doses or over time [51].

Table 2. Commonly used synthetic post-emergence herbicides for effective weed control in ornamental production [38,40,52,53,54].

Active Ingredient	Trade Name	WSSA * Group and Mechanism of Action	Usage				Notes
			Greenhouse		Container	Field
			With Crop	In Soil
Fluazifop-butyl	Fusilade ll	1 (inhibit acetyl-CoA carboxylase (ACCase)	Yes	Yes	Yes	Yes	Systemic herbicide which acts on grasses.
Glufosinate	Finale	10 (glutamine synthetase inhibitors	Yes	No	No	Yes	Nonselective, minimally translocated and act as contact and systemic herbicide.
Clethodium	Envoy Plus	1	Yes, but can cause injury	Yes	No	Yes	Selective, systemic herbicide commonly used to control grass weeds.
Paraquat	Gramaxzone	22 (cell wall synthesis inhibitors	No	Yes	No	Yes	Non-selective contact herbicide and can be safely used in greenhouse.
Diquat	Reward	22	Yes	No	No	Yes	Non-selective and contact herbicide.
Glyphosate	RoundUp Pro	9 (EPSPS ** inhibitor)	No	No	No	Yes	Systemic, non-selective and it can be used in empty greenhouse.
Flumioxazin	Sureguard	14 (PPO *** inhibitor)	No	Yes (on gravel/under benches) as a pre-emergent	Yes	Yes (for woody ornamentals)	It cannot be used on crops and control broadleaf weeds and grasses. It has both pre and post emergence activity. It can be used in empty greenhouses.
Hydrogen peroxide	-	Not applicable	Yes	Yes	Yes	Yes	Used for sanitation and algae control.
Bleaching	-	Not applicable	Yes (if Selective)	No	Yes	Yes	Bleach itself is non-selective in nature can cause potential harm to both plants and the environment. Instead, specialized bleaching herbicides are used for targeted weed control.
Baking soda	-	Not applicable	Yes	Yes (to control molds)	Yes	Yes	Used with acetic acid to control certain weeds [55].
Quaternary ammonium compunds	Greenshield	Not applicable	No	Yes (only on hard surfaces)	Yes	Yes	Those derived from 2,2′-thiodiacetic acid have shown selective herbicidal properties against sorrel, gallant-soldier, velvet leaf and barnyard grass [56,57]. Also used as disinfectants in nurseries to control liverwort, algae and moss.

* WSSA abbreviation of Weed Science Society of America. ** EPSPS abbreviation of enzyme 5-enolpyruvylshikimate-3-phosphate synthase. *** PPO abbreviation of the enzyme protoporphyrinogen oxidase.

Table 2. Commonly used synthetic post-emergence herbicides for effective weed control in ornamental production [38,40,52,53,54].

Active Ingredient	Trade Name	WSSA * Group and Mechanism of Action	Usage				Notes
			Greenhouse		Container	Field
			With Crop	In Soil
Fluazifop-butyl	Fusilade ll	1 (inhibit acetyl-CoA carboxylase (ACCase)	Yes	Yes	Yes	Yes	Systemic herbicide which acts on grasses.
Glufosinate	Finale	10 (glutamine synthetase inhibitors	Yes	No	No	Yes	Nonselective, minimally translocated and act as contact and systemic herbicide.
Clethodium	Envoy Plus	1	Yes, but can cause injury	Yes	No	Yes	Selective, systemic herbicide commonly used to control grass weeds.
Paraquat	Gramaxzone	22 (cell wall synthesis inhibitors	No	Yes	No	Yes	Non-selective contact herbicide and can be safely used in greenhouse.
Diquat	Reward	22	Yes	No	No	Yes	Non-selective and contact herbicide.
Glyphosate	RoundUp Pro	9 (EPSPS ** inhibitor)	No	No	No	Yes	Systemic, non-selective and it can be used in empty greenhouse.
Flumioxazin	Sureguard	14 (PPO *** inhibitor)	No	Yes (on gravel/under benches) as a pre-emergent	Yes	Yes (for woody ornamentals)	It cannot be used on crops and control broadleaf weeds and grasses. It has both pre and post emergence activity. It can be used in empty greenhouses.
Hydrogen peroxide	-	Not applicable	Yes	Yes	Yes	Yes	Used for sanitation and algae control.
Bleaching	-	Not applicable	Yes (if Selective)	No	Yes	Yes	Bleach itself is non-selective in nature can cause potential harm to both plants and the environment. Instead, specialized bleaching herbicides are used for targeted weed control.
Baking soda	-	Not applicable	Yes	Yes (to control molds)	Yes	Yes	Used with acetic acid to control certain weeds [55].
Quaternary ammonium compunds	Greenshield	Not applicable	No	Yes (only on hard surfaces)	Yes	Yes	Those derived from 2,2′-thiodiacetic acid have shown selective herbicidal properties against sorrel, gallant-soldier, velvet leaf and barnyard grass [56,57]. Also used as disinfectants in nurseries to control liverwort, algae and moss.

* WSSA abbreviation of Weed Science Society of America. ** EPSPS abbreviation of enzyme 5-enolpyruvylshikimate-3-phosphate synthase. *** PPO abbreviation of the enzyme protoporphyrinogen oxidase.

3.3. Organic Postemergence Herbicides

While these products show promise for use in sustainable growing systems, organic producers should first consult their organic certifying agency before applying them, as approval may vary. In terms of weed management and selectivity, organic herbicides are effective only on weeds that have already emerged and offer no residual control for future weed growth. Although they can scorch the foliage of perennial weeds, these plants often regrow quickly. These herbicides are most effective on young, small weeds but tend to lose efficacy as the weeds mature (Table 3). Organic herbicides act only on the plant tissue they come into direct contact with, making thorough spray coverage critical for effectiveness. These contact herbicides can damage any green plant material they touch, but are generally safe to use as directed sprays around woody stems and trunks. Their effectiveness depends on applying enough product at the right concentration to fully coat the weeds. However, the high cost—estimated between $988.42–$1482.63 per hectare for broadcast applications as of 2010—makes them potentially less economical than hand weeding for commercial growers. Additionally, since organic herbicides offer no residual control, repeated applications are required to manage perennial weeds or new waves of weed seedlings. In the future, their use in commercial agriculture may become more viable through precision technologies like camera-guided sprayers that target weeds specifically, reducing waste and crop exposure [58].

Pelargonic acid, marketed as Scythe, is a postemergence contact herbicide effective against small seedling weeds but only causing injury to larger annual and perennial weeds. Its efficacy decreases in colder temperatures, making it less effective than diquat under such conditions. Scythe is often viewed as a more natural alternative to conventional herbicides, appealing to customers who prefer to avoid synthetic pesticides. Some individuals who accept the use of insecticidal soaps may also be open to Scythe, commonly referred to as a “herbicidal soap.’’ A similar product, ammonium nonanoate (Axxe), is derived from natural sources and is OMRI-certified for organic use. Like diquat, spray drift from these herbicides can cause localized phytotoxic damages to desirable plants but does not translocate to kill the entire plant. Therefore, care should be taken to prevent contact with valuable vegetation. However, pelargonic acid-based herbicides come with certain drawbacks, including higher costs, slightly lower efficacy compared to diquat, and a strong, persistent odor that some find unpleasant. Additionally, spray drift can cause significant eye irritation [48].

D-Limonene: Citrus rinds contain volatile essential oils, such as orange, lemon, and grapefruit oil, with d-limonene being their primary chemical component. Known for its strong surfactant properties, d-limonene effectively breaks down fats, oils, and waxes, making it a key ingredient in many natural cleaning products. It is also widely used in industrial applications, such as degreasing automotive parts and serving as a safer alternative to mineral spirits. In herbicidal applications, d-limonene functions similarly to other strong detergents, such as ammonium nonanoate and sodium lauryl sulfate. Like all herbicidal soaps, it disrupts the waxy protective layer on plant cell walls, causing cells to lose water and ultimately leading to plant dehydration and death [59].

Acetic Acid (Vinegar): In commercial agriculture, concentrated vinegar (10–20% acetic acid) is required for effective weed control, whereas household white vinegar (5% acetic acid) is sufficient for smaller-scale gardening. Strong acids, like concentrated vinegar, and strong bases, such as sodium or potassium lye, kill weeds by breaking down cell walls. This process causes the plant to lose water rapidly, leading to dehydration and eventual death [59].

These non-selective herbicides act on contact, much like fatty acid-based herbicides. While they effectively control seedling annual broadleaf weeds, they only scorch the foliage of larger annuals, perennials, and grasses without eliminating the root system. For the best results, thorough spray coverage is essential. Their effects become visible quickly, often within an hour on warm, sunny days. However, users should not assume that “natural” equates to “safe.” In fact, many vinegar- and oil-based herbicides have higher dermal toxicity than synthetic alternatives and may be labeled with a “Danger” warning due to their potential hazards [60].

Table 3. Commonly used organic post-emergent herbicides for effective weed control in ornamentals production [47,53,61,62].

Active Ingredient	Trade Name	Usage				Notes
		Greenhouse		Container	Field
		With Crop	In Soil
Acetic acid	Weedpharm, other vinegar products	Yes	Yes	Yes	Yes	Repeated applications are needed. Wood vinegar is used for broadleaf weed control [63].
Pelargonic acid	Scythe	Yes (with care)	Yes	Yes (only for woody shrubs)	Yes	Effectively control barnyard and johnsongrass [64]. Used in integrated weed management systems.
Ammonium nonanoate	Axxe	Yes	Yes	Yes	Yes	Broad spectrum herbicide, effectively controls Indian chickweed, pigweed and crabgrass, but repeated applications needed [65].
Lemon grass oil	GreenMatch Ex	Yes	Yes	Yes	Yes	Mostly used with pelargonic acid in sustainable weed management.
D-limonene	Avenger weed killer	Yes	Yes	Yes	Yes	Non-selective. More efficient in younger smaller weeds than larger older leaves.
Cinnacure aldehyde	Cinnacure	Yes	Yes	Yes	Yes	Works as contact herbicide but may injure ornamental crops by sporadic injury.
Clove oil	Weed slayer	Yes	Yes	Yes	Yes	Nonselective, used for grassy and broadleaf weeds, offers better control with adjuvants like garlic extracts [65].

Table 3. Commonly used organic post-emergent herbicides for effective weed control in ornamentals production [47,53,61,62].

Active Ingredient	Trade Name	Usage				Notes
		Greenhouse		Container	Field
		With Crop	In Soil
Acetic acid	Weedpharm, other vinegar products	Yes	Yes	Yes	Yes	Repeated applications are needed. Wood vinegar is used for broadleaf weed control [63].
Pelargonic acid	Scythe	Yes (with care)	Yes	Yes (only for woody shrubs)	Yes	Effectively control barnyard and johnsongrass [64]. Used in integrated weed management systems.
Ammonium nonanoate	Axxe	Yes	Yes	Yes	Yes	Broad spectrum herbicide, effectively controls Indian chickweed, pigweed and crabgrass, but repeated applications needed [65].
Lemon grass oil	GreenMatch Ex	Yes	Yes	Yes	Yes	Mostly used with pelargonic acid in sustainable weed management.
D-limonene	Avenger weed killer	Yes	Yes	Yes	Yes	Non-selective. More efficient in younger smaller weeds than larger older leaves.
Cinnacure aldehyde	Cinnacure	Yes	Yes	Yes	Yes	Works as contact herbicide but may injure ornamental crops by sporadic injury.
Clove oil	Weed slayer	Yes	Yes	Yes	Yes	Nonselective, used for grassy and broadleaf weeds, offers better control with adjuvants like garlic extracts [65].

4. Herbicides for Greenhouses and Enclosed Structures

The application of herbicides within fully enclosed greenhouse structures is highly restricted due to concerns about herbicide volatilization and crop safety. Vaporized herbicides can become trapped within enclosed spaces, causing unintended damage to crops and posing risks to workers. Herbicide restrictions apply to traditional greenhouses, propagation houses, and overwintering houses, while open-sided shade structures do not fall under the same regulatory limitations. Because few herbicide options exist for greenhouse use, an integrated weed management strategy is critical, emphasizing prevention and sanitation alongside chemical control methods. Residual herbicides such as flumioxazin (SureGuard) and indaziflam (Marengo) may be applied to gravel floors, under benches, and around greenhouse foundations before placing crops inside. However, treated greenhouses must remain empty for at least 24 h before reintroducing plants to minimize risks of phytotoxicity [7].

While preemergence herbicides are typically not labeled for application inside greenhouse containers, some postemergence herbicides can be used to control emerging weeds under benches and around greenhouse. Outside greenhouse structures, weed management is equally critical to prevent the introduction of airborne weed seeds and minimize perennial weed infestations. The area surrounding greenhouses should be covered with ground cloth, plastic, or gravel to suppress weed growth. Preemergence herbicides such as prodiamine (Barricade), flumioxazin (SureGuard), and indaziflam (Marengo) provide residual control of weeds in these areas.

The availability of herbicides labeled for greenhouse use is quite limited. However, certain organic herbicides, such as ammonium nonanoate (Axxe) and pelargonic acid (Scythe), can be safely applied inside greenhouses even when crops are present (Table 3). These organic herbicides are non-selective, meaning they will harm or eliminate any plant tissue they contact. As a result, they must be applied as directed sprays to prevent unintended crop damage. Since these herbicides act only on the tissue they contact and do not translocate within the plant, their efficacy is higher against smaller, newly emerged weeds and lower against well-established or perennial weeds with extensive root or rhizome systems.

Khadduri investigated the efficacy of Sporatec, a formulation containing rosemary (Rosmarinus sp.), clove (Syzygium aromaticum), and thyme (Thymus vulgaris) oil. When tested on a western redcedar (Thuja plicata) crop infested with common liverwort and moss, the treatment achieved 91% control at 9 DAT. However, the redcedar seedlings suffered significant damage, and liverwort recolonization occurred within 14 days [66].

Most synthetic herbicides are intended for postemergence use, but some preemergence products with residual activity are also available, preventing the germination of new weeds. The feasibility of using synthetic herbicides in greenhouses while crops are present varies. For example, flumioxazin (SureGuard) is a preemergence herbicide, but crops cannot be present at the time of application. However, plants can be returned to the treated greenhouse 24 h after application and after the site has been irrigated [47]. Another example is indaziflam (Marengo), a relatively new preemergence herbicide that is labeled for enclosed greenhouse structures.

Regardless of whether herbicide is synthetic or organic, it is crucial to carefully follow the manufacturer’s label instructions to achieve optimal weed control while minimizing risks to crops, the environment, and human health. A partial list of preemergence, synthetic post-emergence, and organic post-emergence herbicides that can provide effective weed control inside greenhouse conditions have been discussed in Tables 1, 2 and 3, respectively.

5. Herbicide Resistance

According to the Weed Science Society of America herbicide resistance is “the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type.” The continuous application of a single herbicide or herbicides with the same mode of action can lead to the development of herbicide resistance in certain weed populations. Resistance in plants may occur naturally or be induced through genetic engineering, tissue culture selection, or mutagenesis [67]. Various mechanisms can contribute to the evolution of herbicide resistance in weeds. One widely accepted hypothesis suggests that resistant individuals are initially present in a population at very low frequencies. However, repeated herbicide applications selectively eliminate susceptible individuals, allowing resistant plants to survive, reproduce, and increase in abundance over time [68].

Another proposed mechanism for resistance development involves genetic mutations that arise following herbicide application, subsequently conferring resistance to that particular herbicide [69]. Resistant plants then pass these genes on to their progeny, further propagating herbicide resistance in the population. The emergence of herbicide resistant weed biotypes poses a significant challenge for agricultural and horticultural crop production worldwide. A biotype refers to a subgroup within a species that exhibits distinct traits not commonly found across the entire species population. Some weed biotypes may develop multiple resistance, meaning they exhibit resistance to herbicides from different chemical families that target diverse sites of action [68]. While herbicide resistance is not yet widespread in nursery production systems, nursery managers must understand the mechanisms of resistance development to mitigate potential threats and prevent the spread of resistant weed populations [69].

Herbicide-resistant weed populations have been documented globally. In Australia, Wimmera ryegrass (Lolium rigidum), a pervasive weed species, has developed resistance to numerous selective herbicides used in the region including clethodium [70]. More recently, multiple biotypes of Wimmera ryegrass have been reported to exhibit resistance to glyphosate, a widely used non-selective herbicide commonly applied in container nurseries [71]. Although reports of glyphosate resistance remain relatively uncommon, certain field bindweed (Convolvulus arvensis) biotypes have demonstrated variable responses to glyphosate applications [72]. Additionally, horseweed (Conyza canadensis) was recently identified as the first annual broadleaf weed to exhibit confirmed glyphosate resistance [73]. Currently, at least seven weed species have been officially documented as glyphosate-resistant [67], and given the continued widespread use of this herbicide, it is likely that more resistant species will emerge in the future.

Several weed species have developed resistance to fluazifop-butyl, including Rottboellia cochinchinensis, Sorghum halepense, Digitaria sanguinalis, and Eleusine indica, through mechanisms such as target-site mutations and enhanced metabolic detoxification [74,75,76,77,78]. Resistance to glufosinate has been confirmed in major weeds like Amaranthus palmeri and Eleusine indica, raising serious concerns for weed control. In A. palmeri, resistance is linked to increased expression of the glutamine synthetase gene, reducing herbicide effectiveness without changes to the target enzyme [79,80,81].

Resistance to paraquat and diquat has been reported in several important weed species, including Conyza canadensis (horseweed) in areas such as New York and Ontario, Conyza sumatrensis (Sumatran fleabane), and Conyza bonariensis (hairy fleabane), all showing cross-resistance to both herbicides [82,83,84,85,86,87]. Resistance has also emerged in Lolium multiflorum, Hordeum glaucum, and Plantago lanceolata, further complicating management efforts [88,89,90]. Flumioxazin remains effective against key weeds like Amaranthus tuberculatus (waterhemp) and Phalaris minor (littleseed canarygrass), even in populations resistant to other herbicide groups such as ALS, ACCase, and PSII inhibitors. To date, no widespread resistance to flumioxazin has been reported in major weed species [91,92,93,94,95].

Dimethenamid-P continues to control resistant weeds such as Amaranthus palmeri (Palmer amaranth) and Amaranthus tuberculatus (waterhemp), including populations resistant to ALS inhibitors and glyphosate. It is particularly effective when used in pre- and postemergence programs or as part of sequential strategies [96,97,98]. However, species like Galium aparine (catchweed bedstraw), Amaranthus retroflexus (redroot pigweed), and Abutilon theophrasti (velvetleaf) exhibit natural tolerance to isoxaben, while grasses also tend to be inherently more tolerant, limiting this herbicide’s effectiveness on these weeds [99,100]. Resistance to oryzalin has been confirmed in weeds such as Eleusine indica (goosegrass), Setaria viridis, and Lolium rigidum, complicating control measures [101,102,103]. Similarly, resistance to trifluralin is reported in major species like Lolium rigidum (annual ryegrass) and Setaria viridis (green foxtail), reducing the efficacy of dinitroaniline herbicides [103,104,105].

High resistance to pendimethalin has been observed in Solanum nigrum populations in Xinjiang cotton fields, linked to increased detoxifying enzyme activity [106]. In Lolium rigidum, resistance involves both target-site and non-target-site mechanisms, which can sometimes coexist in the same population [101]. Meanwhile, Rumex dentatus remains generally susceptible to pendimethalin, although its effectiveness can be diminished under certain field conditions, warranting resistance management [107,108]. Surveys in the southern United States and Tennessee found that 58% of annual bluegrass populations exhibited some degree of resistance to prodiamine, with some surviving at rates up to 32 times higher than the recommended field rate [96,109,110]. Resistance to indaziflam has also been confirmed in annual bluegrass (Poa annua) populations in the southeastern United States. These populations were not controlled by early post-emergence indaziflam applications and showed resistance to several other herbicides, suggesting possible multi-herbicide resistance [111].

6. Herbicide Resistance Management Strategies

6.1. Herbicide Mixtures and Rotations

Effective herbicide resistance management relies heavily on the principle of diversification. One of the key strategies is mixing herbicides with different modes of action (MOA). For instance, combining dimethenamid-P with other herbicides like pendimethalin or imazaquin, or using it sequentially in pre- and post-emergence (PRE and POST) applications, has been shown to not only broaden weed control but also delay resistance development [96,97,112]. Tank-mixing with pendimethalin or utilizing premixed formulations that incorporate multiple MOAs ensure extended residual activity and reduces the chances of resistance selection [112,113]. Similarly, pairing isoxaben with herbicides such as florasulam can enhance control across a wider range of species and is particularly beneficial in managing herbicide-resistant weed populations [114,115]. This combinational approach has become a cornerstone in resistance management, especially as single-site herbicides continue to face increasing resistance pressures.

Beyond simple mixing, rotating herbicides with different MOAs is also a widely recommended practice. However, studies have shown that while rotation can slow the evolution of resistance, using mixtures of soil-applied herbicides tends to be even more effective in reducing the risk of resistance development compared to using a single active ingredient or relying solely on rotation strategies [116]. That said, it is important to manage these rotations wisely. Improper use or over-reliance on chemical mixtures can still select for more generalist resistance mechanisms [117]. In the face of increasing resistance to commonly used herbicides, alternative chemistries are becoming more valuable. For instance, indaziflam and oxadiazon have demonstrated high efficacy against prodiamine-resistant weed populations. In addition, combining these with herbicides like simazine or S-metolachlor offers an extended spectrum of control and added resilience to resistance development [111,118,119,120].

These alternative options should be viewed not as replacements but as complementary tools that can be integrated strategically into a broader weed management plan. This highlights the need for balanced, informed use of chemical tools within a larger integrated framework.

6.2. The Role of Pre and Post Emergence Herbicides in IWM

Pre-emergence (PRE) herbicides remain a critical component of Integrated Weed Management (IWM), primarily because of their ability to prevent weed establishment at the seed or seedling stage, thereby reducing early-season competition and often providing season-long control when properly timed. In early-planted soybeans, for instance, field trials demonstrated that full-rate PRE applications, followed by timely post-emergence (POST) herbicides, resulted in the most consistent and effective weed control. The PRE herbicides delayed weed growth by two to three weeks, reduced early-season weed interference, and minimized yield loss. The combination of PRE and POST applications proved superior to single applications, as POST alone was less effective without a strong PRE foundation. Importantly, the use of pyroxasulfone in POST treatments did not consistently enhance late-season control, likely due to canopy closure suppressing later weeds, which underlines the value of a robust PRE strategy [121]. Similar benefits were observed in maize, where combining PRE and POST herbicides achieved high weed control efficiency (95–100%) and significantly improved yields and growth traits compared to weedy plots. Sequential PRE and POST applications proved to be an effective strategy to minimize weed competition and maximize productivity [122].

In camelina, combining PRE and POST herbicides was highly effective in controlling weeds while maintaining crop safety and productivity. Experiments showed that s-metolachlor (PRE) followed by clethodim, fluazifop-p, or clopyralid (POST) reduced weed biomass by 84–90%, with yields comparable to hand-weeding and no negative impact on seed oil quality [123]. Field trials in grain legumes further underscored the advantages of PRE herbicides, with flumioxazin demonstrating the best weed suppression (>70%), followed by benfluralin and terbuthylazine + pendimethalin. POST herbicides such as imazamox complemented PRE control, although certain weeds like Sinapis arvensis were less effectively controlled. PRE herbicides consistently improved yields, particularly in crops like vetch, which showed higher competitiveness than peas [124]. In sugarcane, integrating PRE herbicides like pendimethalin with brown manuring and POST hand hoeing resulted in superior weed control efficiency (79%), higher cane yield (100.5 t/ha), and the best benefit–cost ratio (2.72), outperforming other treatments [125].

Post-emergence herbicides also play an important complementary role by targeting weeds that escape PRE control or emerge later, adding flexibility to the IWM program. In camelina, for example, the combination of PRE and POST herbicides effectively controlled grasses and broadleaf weeds, maintaining yields and quality [124]. Similarly, in groundnut, POST herbicides such as sodium acifluorfen + clodinafop-propargyl and imazethapyr + imazamox significantly reduced weeds, resulting in yields close to hand-weeding and far better than unweeded control [126]. In soybean, POST application of imazethapyr + quizalofop-ethyl effectively controlled both monocot and dicot weeds, enhancing yields, returns, and nutrient uptake [127]. In aerobic rice, a sequential application of pyrazosulfuron (PRE) and bispyribac-sodium (POST) achieved weed control and yield levels nearly equivalent to hand-weeding, while preventing substantial yield losses from weeds [128]. Integration of chemical and non-chemical approaches further improved outcomes, as seen in citrus, where ecological mowing combined with PRE and POST herbicides achieved over 95% weed control, reduced biomass, and minimized herbicide resistance risk [129]. Similar integrated approaches in cereals and sugar beet demonstrated the effectiveness of combining PRE herbicides with mechanical methods, achieving high weed control and reducing herbicide use significantly [130,131].

Despite these advantages, the use of PRE herbicides is not without drawbacks. Phytotoxicity remains a notable risk, especially when herbicides are not properly selected or timed. In camelina, several herbicides caused severe crop injury and mortality, highlighting the need for careful selection of crop-safe products [123]. In grain legumes, while flumioxazin and benfluralin provided effective control, minor, transient phytotoxicity was still observed in sensitive crops like peas [124]. Furthermore, no single herbicide can control all weed species. In soybean, even effective POST applications such as imazethapyr + quizalofop-ethyl still left some weeds uncontrolled, demonstrating the limitations of relying solely on herbicides [127]. Over-reliance on herbicides can also lead to environmental concerns, such as persistence in soil, runoff, and effects on non-target organisms. In canola, repeated use of PRE and POST herbicides has led to herbicide-resistant weeds, shifts in weed communities, and risks of outcrossing with wild relatives, all of which threaten long-term sustainability [132]. Herbicide resistance, environmental and health risks, regulatory restrictions, and shifts in weed populations collectively emphasize the need for diversified weed management strategies [132].

The role of pre-emergence herbicides within IWM is therefore to provide a strong foundation for weed control while reducing early competition and enabling higher crop productivity. Their greatest value is realized when they are integrated with POST applications, cultural practices, and mechanical or biological methods, creating a diverse and adaptable weed management system. Rotating herbicides with different modes of action, integrating non-chemical methods, and tailoring strategies to local conditions ensure long-term effectiveness and sustainability. In summary, pre-emergence herbicides are indispensable tools in IWM, but their use must be judicious and complemented by other methods to mitigate risks and enhance sustainability.

While herbicides remain an important tool, they are increasingly less effective when used in isolation. The lack of new herbicide discoveries combined with rising resistance issues makes it clear that simply increasing herbicide use will not offer a sustainable solution. An Integrated Weed Management (IWM) approach—incorporating chemical, cultural, mechanical, and biological strategies—is critical for long-term success. For example, combining reduced rates of trifluralin with non-chemical methods such as mulching or hand-weeding has proven effective in both improving control and enhancing sustainability [133].

Cultural practices such as crop rotation, use of cover crops, and strategic planting dates help suppress weed growth naturally and reduce reliance on chemical inputs. Mechanical methods, including tillage, precision cultivation, and flame weeding, provide physical disruption of weeds and are particularly useful in organic or low-input systems. These approaches have been previously discussed in detail in our earlier work [6]. As this article is technically the second part and continuation of—Emerging perspectives on non-chemical weed management tactics in container ornamental production in the United States [6]. However, it is important to emphasize that no single method is sufficient on its own. The integration of these diverse tactics is what ultimately strengthens the system, reducing weed pressure while preserving long-term productivity.

Hence, a diverse toolbox of tactics is central to sustainable weed management. IWM promotes the integration of methods such as:

• Field sanitation and BMP’s (Best Management Strategies);
• Crop rotation to break weed life cycles and other cultural controls;
• Mechanical and physical control to disrupt weed growth;
• Biological control like grazing or use of bioherbicides;
• Targeted herbicide use based on weed population dynamics;
• Using herbicides with different modes of actions and different herbicide mixes.

These diverse strategies, when used together, reduce the selection pressure on any one method and help preserve the effectiveness of current tools.

Preventative strategies are the first line of defense. Preventing weed establishment through sanitation, clean seed practices, and field border management can drastically reduce the size and persistence of weed seed banks. Additionally, regular monitoring and scouting allow for early detection and timely interventions, which are key to preventing small infestations from becoming larger, more costly problems. As the authors, we believe that prevention and adaptability should be cornerstones of any weed management plan. We must move from reactive to proactive systems, and that starts with greater attention to early intervention strategies.

A one-size-fits-all approach rarely works in weed management. Instead, site-specific strategies—tailored to a field’s weed history, soil conditions, cropping system, and other local factors—offer the best outcomes. Equally important is the capacity to adapt over time. Weed populations evolve, environmental conditions shift, and market pressures change. Therefore, flexibility and regular reassessment of management plans are essential for long-term resilience.

Despite the availability of diverse weed control tools, widespread adoption of IWM remains limited. This is often due to socioeconomic constraints, including cost, labor availability, risk aversion, and lack of awareness or technical support. It is crucial that we not only develop new technologies but also understand the reasons behind farmer decision-making. Outreach, education, and policy incentives must play a greater role in encouraging adoption of sustainable practices. As researchers and practitioners, we must expand our focus to address the economic, behavioral, and structural challenges that prevent the transition to more diverse and sustainable weed management systems [134]. Equally, it is imperative that stakeholders across the agricultural sector—applicators, herbicide manufacturers, extension agents, and regulatory bodies—support these efforts in a coordinated and meaningful way.

6.3. Role of Organic Herbicides in Integrated Weed Management (IWM)

Organic herbicides play an important role as a sustainable and environmentally friendly component of Integrated Weed Management (IWM), particularly in systems aiming to reduce reliance on synthetic chemicals. When combined with cultural practices like cover crop mulches, organic herbicides help suppress weed pressure effectively while maintaining soil health and supporting beneficial insect populations, thus reducing the negative impacts of intensive tillage and chemical use on the ecosystem and biodiversity (1,5,8). Studies in organic vegetable systems have shown that organic herbicides, such as capric/caprylic acid, applied alongside cover crop mulches can significantly lower weed infestations compared to fallow controls, without reducing beneficial insect activity or weed seed biocontrol. This underscores their compatibility with IWM principles and their environmentally responsible nature [135]. Bio-herbicides, derived from allelopathic plants or microbes, have also emerged as promising alternatives to synthetic herbicides in sustainable agriculture. Although their real-world performance is sometimes limited by variability and a lack of research on their physiological effects on weeds, they fit well into low-input and organic systems when integrated with cultural and mechanical weed control methods [136]. Organic herbicides further align with the demands of consumers and markets for healthy, safe, and sustainably produced food by reducing environmental contamination and supporting soil and crop health [137].

Another notable advantage of organic herbicides in IWM is their compatibility with other non-chemical strategies such as mulching, intercropping, and biological control, enabling more robust and diverse weed management programs (1,5,8). Using organic mulches as herbicide carriers, for example, has been shown to maintain weed control efficacy while reducing phytotoxic effects on sensitive ornamental plants like ‘Hetz Midget’ arborvitae, and even enhancing plant biomass in species such as ‘Goldflame’ spirea, demonstrating their ability to enhance crop safety and growth within IWM frameworks [138].

However, organic herbicides also come with certain limitations that must be acknowledged within an IWM context. They generally offer lower and shorter-term weed control efficacy compared to synthetic herbicides and may require more frequent applications to maintain effectiveness, which can increase both labor and operational costs [136,139]. Moreover, organic herbicides often have a limited weed control spectrum and may fail to adequately suppress a broad range of weed species or control well-established weeds, which reduces their standalone effectiveness in conventional systems [136,139]. Bio-herbicides, while environmentally favorable, also face challenges due to inconsistent performance influenced by environmental factors and a limited number of commercially available products with well-understood modes of action [136].

In summary, organic herbicides are best viewed as one element of a multi-tactic IWM approach, where they complement cultural, mechanical, and biological methods to reduce chemical inputs, limit the development of herbicide resistance, and maintain ecological balance in cropping systems. In ornamental production, in particular, their integration with mulching and other non-chemical methods can achieve satisfactory weed control while minimizing both crop injury and environmental risks [135,136,137,138]. Although not a substitute for synthetic herbicides in all contexts, organic herbicides make a valuable contribution to sustainable weed management strategies when used judiciously as part of an integrated plan.

7. Conclusions

Major conclusions include:

• Significant research gaps remain in evaluating alternative chemical classes and non-traditional products (e.g., insecticides, miticides, essential oils) for liverwort suppression in container nursery and greenhouse systems, particularly under varying environmental and regional conditions.
• Preemergence herbicides such as indaziflam and dimethenamid-P exhibit potential for greenhouse weed control, but comprehensive studies on their crop selectivity, residual activity, application timing, and long-term efficacy under greenhouse conditions are essential.
• Current herbicide portfolios for enclosed production environments are limited, underscoring the urgency for screening novel active ingredients, assessing phytotoxicity on diverse ornamental species, and optimizing formulations and delivery systems.
• Integrated Weed Management (IWM) frameworks must be advanced, emphasizing herbicide rotation, sanitation protocols, substrate management, and the integration of organic and cultural tactics to delay resistance evolution and enhance overall system resilience.

Weed management in nursery ornamental container production remains a significant challenge due to the economic burden, limited herbicide options, and the potential for herbicide resistance development. While chemical control is an essential component of integrated weed management, reliance on a narrow range of herbicides increases the risk of resistance evolution. Some attempts have been made previously to control common liverwort (one of the most problematic weed species in northeastern and pacific northwestern region of United Sates in greenhouse and nursery conditions) with both synthetic and organic chemicals/herbicides, but more research is required in this area because, in many cases, the results varied from region to region and with environmental conditions. Indaziflam and dimethenamid-P have emerged as potential preemergence herbicides that could be effective even in greenhouse environments; however, further research is needed to assess their long-term efficacy and crop safety. The limited availability of preemergence herbicides suitable for use in enclosed greenhouse settings emphasizes the need for alternative approaches. Identifying the group of chemicals and determining their phytotoxic effects on ornamentals, costs involved in their application, and application rates and timing that can provide effective weed control s will help the billion-dollar green industry in the United States to improve productivity and profit margins. Hence, there is a need to conduct further research on both nonchemical and chemical methods for controlling weeds such as common liverwort in container nurseries and greenhouse operations.

A critical knowledge gap exists in testing alternative pesticides, including insecticides and miticides such as Zerotol and Tetra Curb Max, as well as organic products for liverwort control. Essential oils and plant-derived extracts have demonstrated some degree of liverwort suppression, but their practical application requires further investigation. Complete liverwort control with post or pre- emergence herbicide alone may not be achieved. Integrating proper cultural and sanitation practices is required for a long-term success and to prevent the spread of herbicide-resistant weed populations, integrated weed management (IWM) strategies must be prioritized. IWM involves the combination of cultural, mechanical, biological, and chemical control methods to reduce selection pressure on herbicides and delay the evolution of resistance. Sanitation practices, substrate management, and strategic herbicide rotation should be incorporated to enhance long-term weed control while minimizing environmental impact.

Additionally, continued research on novel herbicides, biopesticides, and precision weed management technologies will be essential in ensuring the sustainability of weed control in nursery ornamental crop production. Ultimately, developing a comprehensive weed management framework that integrates chemical and non-chemical strategies will be crucial in mitigating economic losses, maintaining plant health, and supporting the overall profitability of the ornamental nursery industry. Further collaboration between researchers, growers, and regulatory agencies will help address existing knowledge gaps and drive innovation in weed control methodologies.