Mulching for Weed Management in Medicinal and Aromatic Cropping Systems

Abstract

Weeds are one of the main problems in cultivation of medicinal and aromatic plants (MAPs); they negatively affect yield (herba and essential oil), and the overall quantity and quality of biomass, flowers, roots, seeds, and secondary metabolites. This review evaluates mulching as a sustainable, non-chemical method for weed management in the cultivation of MAPs and examines how effectively organic, synthetic, and living mulches reduce weeds and increase yields. Regarding different mulch materials such as straw, sawdust, bark, needles, compost, polyethylene, and biodegradable films, the basic processes of mulch activity, including light interception, physical suppression, and microclimate adjustment, are examined. The review further analyzes the impact of mulching on soil parameters (moisture, temperature, pH, chlorophyll content) and the biosynthesis of secondary metabolites. The findings consistently indicate that mulching substantially reduces weed biomass, improves crop performance, and supports organic farming practices. However, there are still issues with cost, material availability, and possible soil changes, and the efficacy is affected by variables including cultivated plant species, mulch type, and application thickness. The review highlights the importance of further research to optimize the selection of mulch and MAPs and their application across various agroecological conditions, and indicates that mulching is a potential, environmentally friendly technique for weed control in MAP cultivations.

1. Introduction to Mulching: Principles and Agroecological Function

Mulching, whether with organic or synthetic materials, has been extensively documented as an effective practice for suppressing weed growth [1,2,3,4,5,6,7]. In addition to its weed-suppressive properties, mulch also influences the incidence of plant pathogens and pests [8] and improves key soil parameters such as moisture retention [9], temperature regulation, and pH stability [10,11,12]. Moreover, the application of mulch has been shown to enhance environmental conditions in agroecosystems, promoting increased arthropod biodiversity [13] and providing effective protection against soil erosion under conditions of intense rainfall or wind [14].

Medicinal and aromatic plants (MAPs) constitute a significant component of native flora and encompass a diverse group of species that produce valuable secondary metabolites. These bioactive compounds are of growing economic interest in the pharmaceutical, food, and cosmetic industries [15]. Moreover, the World Health Organization (WHO) reports that herbal medicines serve as the primary healthcare source for 75–80% of the global population, and with the ongoing growth of the world’s population, the demand and market value of MAPs are projected to keep increasing [16]. However, the increasing global demand has placed considerable pressure on wild populations, many of which now face serious ecological challenges. Furthermore, the inefficiencies and inconsistencies associated with wild harvesting underscore the limitations of relying solely on natural stands. Consequently, the cultivation of MAPs is increasingly recognized as a sustainable and economically viable alternative. Economically important types of plants such as Lavandula angustifolia (lavander), Mentha spp. (mint), Melissa officinalis (lemon balm), and Chamomilla recutita (chamomile) are cultivated on approximately 77 million ha worldwide, producing an estimated 330 million tons of raw material each year [17]. Species diversity is also remarkable: India alone hosts roughly 7500 aromatic plant species, China about 6000, Africa around 5000, and Europe approximately 2000, reflecting broad global distribution and cultivation practices [18]. Nevertheless, MAP cultivation presents unique agronomic challenges, among which weed infestation remains a primary constraint [5]. Weeds can significantly reduce biomass yield, complicate mechanical harvesting, and negatively impact the quality of harvested raw materials [15].

In this context, mulching plays a critical role in improving both the quantity and quality of MAP production by effectively reducing weed pressure and enhancing hydrothermal conditions in the rhizosphere. Positive effects of mulch application on MAP yield and oil quality have been demonstrated in numerous species, including Mentha piperita [5,6,7,19,20] (Figure 1D), Melissa officinalis [21] (Figure 1F), Angelica archangelica [6] (Figure 1E), Mentha arvensis [1], Foeniculum vulgare [2,3], Coriandrum sativum and Plantago psyllium [2], Ocimum basilicum [22,23], Rosmarinus officinalis [22,24], Thymus serpyllum, Lavandula officinalis [24], Arnica montana [25] (Figure 1B), Satureja montana L. [26] (Figure 1A), and Gentiana lutea [27] (Figure 1C).

This review emphasizes the role of mulching as a non-chemical weed management strategy in the cultivation of MAPs, positioning it as an effective and environmentally sustainable practice that aligns with the principles of organic agriculture. By examining the capacity of mulch to suppress weed growth, improve soil conditions, and reduce reliance on synthetic herbicides, we highlight its potential to contribute to the ecological sustainability of MAP production systems. Moreover, mulching can lower labor requirements and production costs, making it a viable alternative for both conventional and organic growers. Its compatibility with sustainable farming frameworks underscores its value as a practical tool for enhancing the agronomic performance and environmental stability of MAP cropping systems.

2. The Impacts of Weeds on Medicinal Crop Production

2.1. Growing and Contents of Target Chemical Compounds

Weeds cause a significant negative impact on cultivated plants, making their control essential across all cropping systems, including medicinal crops. According to Upadhyay et al. [28], in annual medicinal plant productions, yield losses are primarily induced by weed infestation (up to 45%), followed by insects (up to 30%), pathogens (up to 20%), and other pests (approximately 5%). Weeds compete directly with cultivated plants for essential resources, exhibit high reproductive potential, and spread rapidly. As a result, a strong negative correlation has been observed between weed presence and the yield of MAPs [15,28].

Additionally, weeds might introduce dangerous or toxic species into the harvested biomass. Several toxic weed species pose contamination risks to MAPs by introducing harmful secondary metabolites. Pyrrolizidine alkaloids (PAs), mainly from Senecio spp. (S. vulgaris and S. vernalis) and Boraginaceae weeds like Myosotis arvensis, could contaminated herbs MAP [29]. In addition, weeds such as Convolvulus arvensis produce tropane alkaloids, which also contaminate collected biomass [30]. Also, raw materials present teas pose a serious threat to human health, primarily through hepatotoxic effects, including liver veno-occlusive disease and chronic liver damage [31]. Jiao et al. [32] demonstrated that PAs such as intermedine and intermedine-N-oxide originating from PA-producing weeds can migrate through the weed–soil–tea pathway, accumulating in fresh Camellia sinensis leaves and subsequently in dried tea products.

In MAPs, weed control is difficult, especially when it comes to toxic weeds. Chemical weed control in MAPs is often limited due to a lack of registered herbicides [30], high crop sensitivity, risk of herbicide residues in harvested products, and potential negative impacts on active compound content and ecological safety, which is particularly critical for pharmaceutical-grade and residue-sensitive markets [33]. Therefore, given these detrimental effects, weed management is a crucial component of medicinal plant cultivation, and should be aligned with approved practices suitable for these crops—non-chemical methods, typically resembling principles of organic agriculture [33,34]. The selection of weed control methods should provide prolonged crop protection and preserve the quality of harvested raw material [15,25]. Depending on the species and production system, the target raw material may consist of aerial biomass, leaves, seeds, essential oil, roots, or other plant parts.

2.2. Yield Loses

Previous studies have confirmed that weeds are among the primary factors contributing to reduced yield, aboveground biomass, leaf production, and underground parts of MAPs [1,5,6,7,20,34,35,36,37,38]. Matković et al. [20] reported an 88% reduction in aerial biomass of peppermint (Mentha piperita) in weed-infested fields in the South-East Banat region of Serbia, while other research on the same region showed that weed competition reduces the biomass yield of peppermint by up to 66% [7] and 54.6% [5], demonstrating the significant negative impact of weeds on plant growth. Similarly, Darre et al. [35] recorded yield losses ranging from 35.7% to 50.9% in the same species under production with no weed removals in Cordoba, Argentina. Singh and Saini [1] observed an average biomass loss of 40.3% in Mentha arvensis (field mint) due to weed competition in India. In Coriandrum sativum (coriander), weed interference led to an average biomass reduction of 77.3% over four years in Sicily [2], and 53.6% in India [32]. For Foeniculum vulgare (fennel), Carrubba and Militello [2] reported biomass losses of 80.6%, while Yousefi and Rahimi [32] documented a 91.7% reduction under weedy conditions. It is widely known that parasitic plants cause significant damage too, and scientific research has also confirmed that peppermint and chamomile plants parasitized by field dodder accumulate considerably less dry matter, approximately 25% and 63% less, respectively, than their non-infested plants [36].

The impact of weeds is also reflected in MAPs grown for roots and seed. Weed competition significantly reduced the root yield of angelica (Angelica archangelica), where the root yield in the un-mulched control plots was 7.63 t ha⁻¹, compared to 14.22 t ha⁻¹ in the treatment with black polyethylene mulch, representing a 46.34% decrease relative to the best-performing treatment [6]. Similarly, the root yield of one other medicinal plant, yellow gentian (Gentiana lutea L.) was significantly influenced by cultivation practices, with the highest yield reaching 3.39 t ha⁻¹ under specific fertilization and planting density conditions on black polyethylene mulch, highlighting the importance of optimized agronomic practices even in dry-farming conditions [39]. Moreover, beyond the previously mentioned effects, weeds also negatively impact seed yield by disrupting plant development, as well as interfering with key physiological and metabolic processes [2,40]. This interference can lead to reduced flowering, impaired nutrient allocation, and ultimately lower seed production [2], especially in medicinal plant species where reproductive success is closely tied to overall plant vigor [4]. A previous study by Carrubba and Militello [2] reported a 67.6% seed yield reduction in coriander in Italy due to weed pressure, while losses of 61.9% and 98.3% were recorded in India [37] and Iran [3], respectively. Additionally, Carrubba and Militello [2] observed a 79.6% reduction in fennel seed yield under similar conditions.

2.3. Chemical Contamination and Quality of MAPs

The presence of weeds can interfere with the plant’s metabolic processes and influence the synthesis of secondary metabolites such as essential oils [4,7]. In addition to reducing yield, weeds may lower the final market value of the harvested medicinal raw material [28]. The effects of weed infestation on essential oil yield appear to be variable. Generally, the essential oil yield was also negatively affected in weedy control plots, with reductions reaching 41.5% compared to mulched treatments, indicating that weed pressure significantly compromises both quantity and quality of peppermint essential oil [5]. In Serbia, the research on peppermint showed that weed presence in plots without any treatments led to a reduction in essential oil yield by approximately 50% and negatively affected oil composition by lowering menthol content and increasing unwanted components such as menthone [7]. Likewise, in India, Mentha species showed a 12.3% increase in essential oil content under weed pressure compared to weed-free plots [1]. In the study, where it was shown that weeds affect the root yield of angelica, it was also shown that effective weed control is critical not only for improving root biomass but also for maximizing essential oil productivity [6]. This study clearly demonstrates that weed competition has a negative impact on essential oil yield. In un-mulched control plots, where weed biomass was highest and angelica plants grew with weeds, the essential oil yield was significantly lower, measured at 0.76 kg ha⁻¹, compared to 2.11 kg ha⁻¹ in the plots treated with black polyethylene mulch [41]. This represents a 64% reduction in oil yield due to weed interference in cultivated angelica in Serbia [41]. On some other MAPs, such as basil (Ocimum basilicum), essential oil yield was 15.1% higher in weedy plots [23], and milk thistle (Silybum marianum) exhibited a 33.5% increase in oil yield in the presence of weeds [42]. Sarić-Krsmanović et al. [37] reported that infestation by field dodder (Cuscuta campestris) significantly reduced essential oil yield in chamomile (Chamomilla recutita), with an average decrease by 0.2%, whereas in peppermint (Mentha piperita), essential oil yield showed a marginal increase by 3.87%, indicating species-specific responses to parasitic weed stress. However, contrasting results have been reported. Singh and Saini [1] documented a 35.7% decrease in essential oil content of Mentha arvensis under weed competition, while Kothari et al. [43] recorded reductions of 74.4% and 70.0% in the first and second harvests, respectively, in the same species.

3. Mulching: Types and Mechanisms of Action

Mulches can be classified according to various criteria, with one of the most widely used approaches based on their origin. Broadly, mulching materials are divided into natural and synthetic types. Natural mulches include both living mulches—such as cover crops—and mulch mats or covers, which may be derived from organic matter (e.g., straw, wood chips, compost). In contrast, synthetic mulches are typically composed of materials such as polyethylene, polypropylene, or biodegradable polymers. Bond and Grundy [44] proposed a widely accepted classification system in which mulches are grouped as either living mulches or mulch mats, with mulch mats further categorized as organic (e.g., bark, plant residues) or inorganic (e.g., plastic films, gravel). Within this framework, commonly used mulch films are considered a form of inorganic mats, often employed for their durability and effectiveness in long-term weed suppression.

3.1. Living Mulch

These types of mulches are composed of cover crops, either single species or species mixtures, selected for their favorable biological and ecological traits that contribute to suppressing weed emergence and development. There is often a notable negative correlation between the biomass of cultivated crops and that of weeds: as weed biomass decreases, crop yield tends to increase, and vice versa. Teasdale et al. [45] observed that an average weed suppression rate of approximately 70% achieved through living mulches resulted in significant yield increases across various cropping systems. Didehbaz et al. [46] demonstrated that species such as spring and winter wheat, barley, and mixtures of winter rye and barley could be effectively used as living mulches in peppermint cultivation to reduce weed density and biomass while also influencing weed flora composition. Furthermore, winter wheat mulch not only suppressed weeds but also enhanced the yield of essential oil in peppermint leaves. Similarly, Joogh et al. [47] reported that winter wheat mulch increased the number of branches, fresh and dry leaf biomass in peppermint, while winter rye provided more effective weed biomass reduction by the first harvest. In coriander, fenugreek (Trigonella foenum-graecum L.) significantly reduced weed biomass, although the effect was not uniform throughout the growing season, while the authors of ref. [48] recommended combining living mulch with mechanical weed control for optimal results.

Numerous studies have examined the two advantages of using Fabaceae species as living mulches: their capacity to suppress weeds, which also consume nitrogen and may, in some situations, outcompete crops, as well as their ability to improve soil fertility through biological nitrogen fixation [49,50]. By increasing the amount of nitrogen available, these leguminous species give the primary crops a competitive edge against weeds. Among the most frequently studied species are white clover (Trifolium repens), used successfully as a living mulch in row crops such as maize [51], sunflower [52], soybean [53], cabbage [54], broccoli [55], and strawberries [56].

In addition to weed suppression, living mulches contribute to soil structure improvement [57], erosion control [50,58], reduction in nitrate leaching, and enhanced wastewater purification [50], while also affecting soilborne pathogens, insect pests, and beneficial soil organisms.

However, one of the key challenges in using living mulches is competition for essential resources—light, water, and nutrients—between the main crop and the cover crop. If this balance shifts in favor of the mulch species, it can adversely affect crop development. Competitive interactions have been documented in soybean intercropped with Medicago species [53] and cabbage grown with white clover [54], where despite weed suppression, yield reductions were recorded.

Nevertheless, living mulches remain a promising non-chemical strategy within integrated weed management (IWM) frameworks, particularly in sustainable farming systems [44,45,59,60]. To maximize their benefits, careful consideration must be given to the life cycle and phenological stages of weeds prevalent in the target region. Teasdale et al. [45] emphasized that an effective living mulch should minimize competition for light, water, and nutrients relative to the crop. When confined to inter-row spaces, living mulches can prevent or delay weed emergence and significantly reduce weed seed production [45,61]. Achieving a functional balance between crop and cover crop species is critical to avoiding adverse competition [62]. This can be attained by selecting species with different growth cycles, staggered emergence timing [63], or different planting schedules [61]. Ideally, living mulch species should have a prostrate growth habit and remain shorter than the crop.

3.2. Mulch Mats

Mulch mats, composed of either organic or synthetic materials, are laid on the soil surface. Organic ones are mostly made from dried plant residues, which may either remain on the field post-harvest or be applied after planting. This method is typically suitable for widely spaced crops.

These mulches, when destroyed pre-planting, are referred to as cover crops, and can be easily incorporated into the soil through mechanical means. Unlike synthetic mulches, they can be returned to the soil after fulfilling their surface role, thereby improving soil fertility and structure. During their presence on the surface, they offer numerous advantages such as moisture and temperature conservation [12,64,65,66] and light interception, which delays weed emergence and promotes early crop development [9,67,68,69,70].

The effectiveness of mulch mats largely depends on the type and persistence of plant material, as well as its thickness and coverage [5,69,70,71,72,73]. Mulch thickness affects the amount of light reaching the soil, which in turn influences weed germination [45]. For instance, Pupalienė et al. [73] found that a 10 cm-thick organic mulch layer was more effective at reducing weed density than a 5 cm layer. However, Matković et al. [20] demonstrated that different kind of organic mulch materials, even at the same thickness, vary in suppression efficiency. Coarsely chopped and rigid straw mulches tend to leave gaps that allow light penetration and facilitate weed emergence [20], whereas synthetic films, such as polyethylene (PE), provide more complete coverage. Lightweight organic mulches (e.g., shredded straw, corn stover, sawdust) can be displaced by wind or washed away on slopes, making them unsuitable for windy or steep terrains [74]. On the other hand, straw mulches enhance soil aeration, which is vital for microbial activity and healthy root development.

Mulch mats may be made from biodegradable natural materials or non-biodegradable synthetics, with PE films being the most commonly used synthetic option [75] (

Table 1. Types of mulches used in the cultivation of medicinal and aromatic plants (MAPs).

Type of Mulch	Examples	Advantages	Disadvantages	Application in MAPs	References
Non-biodegradable (synthetic)	Polyethylene (PE) and polypropylene (PP) films	High weed suppression, durable, effective moisture retention	Must be removed after harvest, risk of overheating, poor aeration, microplastic pollution	Often used for perennial MAP crops (e.g., lavender, rosemary) where long-term weed control is needed	[75,76]
Biodegradable (natural)	Straw, compost, sawdust, pine needles	Environmentally friendly, improve soil structure and microbial activity, low cost	Degrade quickly under rain/snow, shorter weed suppression period (one season)	Suitable for annual MAP crops (e.g., chamomile, basil, coriander) to reduce costs and improve soil health	[76,77]
Biodegradable (bio-based synthetic)	PLA (polyactic acid) films, strach-based films	High weed suppression, biodegradable, less labor for removal	Higher initial cost, degradation rate depends on climate and soil microbiology	Tested in perennial MAPs such as thyme and oregano as sustainable alternative to PE	[75,78,79]
Post-harvest MAP residues (fresh)	Residues of chamomile, peppermint, or sage	Weed suppression, potential pest deterrence	Possible phytotoxicity from allelopathic compounds if used fresh	Applied in annual MAP production for integrated weed and pest control	[80,81]
Composted MAP residues	Composted lavender stalks, mint waste	Improves soil fertility, reduces allelopathic effects, long-lasting organic matter input	Requires composting facilities and time	Suitable for high-value MAPs like lemon balm and mint	[80,81]

). While organic mulches are generally cheaper, they are often less effective than synthetics in suppressing weeds [76]. However, under conditions of excessive rainfall or strong winds, synthetic films can damage sensitive crops such as cucurbits by causing overheating or poor aeration [75]. Unlike organic mulches, synthetic films must be removed post-harvest to avoid environmental pollution, whereas biodegradable materials decompose naturally through microbial or atmospheric activity [75,76] (Table 1). Improper disposal or degradation of these materials can contribute to the accumulation of microplastics in soil ecosystems, posing long-term risks to soil health, microbial diversity, and potentially entering the food chain. These concerns have prompted increased interest in biodegradable alternatives and stricter regulations on the use of conventional plastic mulches in sustainable agriculture systems. On the other hand, natural mulches such as straw, compost, or sawdust degrade more quickly, particularly under the influence of rain and snow [76,77]. Although their weed-suppressive effect typically lasts only through a single growing season (spring to autumn), they significantly reduce weed management costs and can improve soil structure and microbial activity.

4. Effects of Mulching in Weed Suppression in the Medicinal Crops

The effectiveness of mulches in weed control is primarily attributed to the mechanical pressure exerted by the mulch layer [44,45]. Another important factor is the reduced light penetration to the soil surface, which delays weed seed germination and emergence. By blocking sunlight, mulch prevents weed seedlings from being established [9,10,73,77,78], thereby providing the crop with a competitive advantage in early stages of growth [69].

However, the overall efficacy of mulch as a weed barrier depends significantly on several factors, including the type of material used (synthetic vs. organic), the application thickness [73], its coverage capacity [69], and its durability, particularly in perennial systems [79]. The effectiveness also varies depending on whether the mulch is applied in annual or perennial cropping systems.

Numerous authors [3,5,6,7,20,69,82] have emphasized the beneficial effects of mulches on crop growth and development, noting improvements in both the quantity and quality of yields. Another major advantage of mulching over chemical herbicides lies in its longer-lasting effects. Skroch et al. [79], for example, reported that a 9 cm thick mulch layer composed of long pine needles remained intact for 230 days under field conditions, while pine bark mulch retained its structure for over 630 days, despite losing about 17% of its initial volume through natural decomposition.

4.1. Straw Mulch

The application of straw mulch in scientific research dates to 1978, when its effects on soil temperature, plant growth, and barley yield were first investigated [10]. Since then, a substantial number of studies have explored straw mulch use in vegetable production, given its wide availability and cost-effectiveness [14,69,83,84].

Considering that the term “straw” encompasses a wide range of post-harvest crop residues, research has also focused on identifying the most used types and assessing whether their effectiveness varies depending on crop origin. The findings indicate that wheat straw is among the most frequently applied mulching materials [9,10,85,86,87,88]. In Southeast Europe, maize stover mulch is also increasingly common [70,84,87], whereas rapeseed and rice residues are more frequently used in Asia and some parts of Europe [1,87].

These types of mulch generally consist of shredded, untreated crop residues, free of pesticide contamination. Their positive effects include reductions in weed density [69,71,89,90], shifts in weed species composition [20], and decreases in total dry weed biomass [1,64,89,90].

The effectiveness of straw mulch as a biodegradable ground cover largely depends on its application thickness [71,73] and the dominant weed flora—whether annual or perennial species prevail [91]. For instance, Jodaugienė et al. [69] demonstrated that a 10 cm-thick straw layer was more effective in suppressing perennials than a 5 cm layer. Conversely, Massucati and Köpke [91] found that a 4 t ha⁻¹ straw application more effectively controlled annual weeds. Sinkevičienė et al. [83] observed that straw mulch reduced total weed numbers by an average of 75.1% over four consecutive years, using mulch layers between 5 and 10 cm. Similarly, Matković et al. [88] achieved a 30.0% reduction in weed biomass with a 5 cm wheat straw mulch in cultivated peppermint. Pupalienė et al. [73] reported that straw mulch decreased weed germination rates by 3.5 to 14.1 times. This reduction in weed pressure also translated into higher crop yields. For example, Matković et al. [20] reported yield increases in peppermint (Mentha piperita) from 1.5 to 2.9-time higher yield than in weedy peppermint, in the first and second harvests, respectively. Similarly, Singh and Saini [1] found that straw mulch led to a 20% increase in field mint (Mentha arvensis) yield. Khera et al. [92] observed a 10% yield increase at a straw application rate of 5 t ha⁻¹, while Patra et al. [93] documented 17% and 31% increases with rice straw residues.

Beyond weed control and yield benefits, straw mulch improves soil moisture retention, temperature regulation [10], and soil structure [87]. Another practical advantage is its flowability into the soil after use.

However, straw mulch does present challenges. It can harbor rodents and other pests, and its lightweight and portability make it a potential vector for weed seed dispersal between fields. As noted by Peachey et al. [94], this may lead to field contamination with new weed species. Moreover, straw mulch is generally less effective than plastic mulch film or living mulches in suppressing weeds [20,95,96].

4.2. Use of Medicinal Plant Residues as Compost Mulches

Post-harvest residues of MAPs, when processed into compost, can also serve as effective mulching mats. Both their allelopathic qualities and the potential phytotoxicity that could result from the release of new chemicals during the breakdown of their organic matter are the main factors in choosing appropriate medicinal species for compost production [80,81]. Using old, decomposed materials can help reduce these possible allelopathic effects.

A study using compost derived from MAPs residues produced by the Institute for Medicinal Plant Research “Dr. Josif Pančić” found contradictory effects when applied as a 5 cm thick mulch in peppermint cultivation. Specifically, it resulted in an increase in both the dry biomass and species richness of weeds [88]. One likely explanation for this outcome is the presence of viable weed seeds within the composted material [97]. Conversely, Kamariari et al. [98] observed a reduction in weed biomass, particularly broadleaf species, following the application of compost made from Sideritis scardica and Echinacea purpurea in a maize cropping system. However, while both composts were effective in weed suppression, echinacea-based compost exhibited phytotoxic effects on the crop itself. Although not evaluated specifically for weed control, a biosolid-based compost—comprising treated sewage sludge mixed with tree bark residues, agronomic waste, and dried leaves—was found to enhance nitrogen availability and thus stimulate peppermint growth and yield [99]. Similarly, Ram and Kumar [100] reported a 10% increase in soil inorganic nitrogen content and subsequent improvement in both yield and quality of field mint (Mentha arvensis) when composted distillation residues of lemongrass (Cymbopogon sativum) were combined with pea (Pisum sativum) residues and applied between crop rows.

The positive effects of compost mulch are typically due to its physical smothering action, even when applied at relatively low thicknesses (e.g., 3 cm), which limits light penetration to the soil surface and consequently suppresses weed seed germination—provided that the compost material is not contaminated with viable weed seeds [101]. Furthermore, as composted mulch decomposes and is incorporated into the soil, it contributes to nutrient enrichment, thereby offering potential benefits for crop development.

4.3. Natural Wood-Derived Organic Mulches

A considerable body of research has examined the use of organic mulches derived from wood processing, including shredded bark, sawdust, and coniferous residues, in agricultural and horticultural systems. The efficacy of these mulches in weed suppression depends primarily on the type, size, and shape of wood mulch (e.g., sawdust, shredded bark, coniferous residues), as well as its thickness and applied amount. The most frequently studied mats include pine bark [11,71,88,102,103,104], teak bark [12], hardwood chips [104], and black locust bark [88]. Saha et al., 2019 [104], showed that hardwood chips applied at 5 cm provided 64% less garden spurge (Euphorbia hirta) than in non-mulched control, while the effect of pine bark mulch on the same weed species was 44%. Thies mulches had very small effects on eclipta (Eclipta prostrata) and large crabgrass (Digitaria sanguinalis) [105,106]. Cochran et al. [103] showed that 2.5 cm of pine bark mulch reduced garden spurge and eclipta fresh weights as well as weed counts by more than 80% compared with a non-mulched control.

By applying a 5 cm layer of pine bark mulch to peppermint cultivation, the authors of ref. [88] were able to reduce weed biomass by 47.5%. Similarly, Shahriari et al. [107] completely suppressed weeds (100%) in peppermint fields by applying shredded wood bark particles (5–8 cm in size) at a depth of 5 cm. In another study, Greenly and Rakow [71] found that applying pine bark mulch at a thickness of 25 cm reduced weed cover by 96.5%. Additionally, their results suggested that as mulch thickness increased (7.5 cm, 15 cm, and 25 cm), weed species’ coverage and presence gradually decreased. Broschat [108] further confirmed the impact of pine bark mulch (Pinus elliottii), finely shredded eucalyptus (Eucalyptus spp.) wood mulch, and finely shredded cypress (Taxodium distichum) wood chip mulch on weed density. Author discovered that, compared to plots mulched with any type of wood mulch, the number of dicots (mainly large flower pusley (Richardia grandiflora) and total weeds per plot were significantly higher in non-mulched plots. However, because torpedo grass (Panicum repens), the most common grass weed, spreads vegetatively through underground stolons rather than seedlings, which mulches can suppress, the same author [108] demonstrated that the number of grass weeds did not differ between mulched and non-mulched plots. Teak bark mulch (Tectona grandis L.) demonstrated high efficacy: Awodoyin et al. [12] and Olabode et al. [98] observed 56% and 67.8% reductions in weed biomass, respectively, using 2 cm and 3 cm mulch layers. Additionally, Olabode et al. [105] noted that teak mulch was very successful in suppressing broadleaf weed species.

In addition to weed suppression, these mulches significantly enhanced crop yield. Matković et al. [20] found that peppermint yields increased 3.1-fold in the first harvest and 4.6-fold in the second harvest under pine bark mulch treatments. Likewise, Shahriari et al. [82] observed a 2.6-fold increase in dry peppermint yield by the end of the growing season with the use of 5 cm of shredded bark under irrigation. In another report, Shahriari [107] noted that bark mulch positively influenced peppermint growth parameters, including fresh and dry biomass, leaf surface area and count, and the number of branches, although the specific mulch characteristics used in that study were not detailed.

4.4. Sawdust Mulches

Sawdust is a by-product of milling the woody core of various tree species, and when applied as mulch it can substantially limit weed development. A 5 cm layer of black-locust (Robinia pseudoacacia) sawdust placed between peppermint rows reduced total weed biomass by 57.1% over an entire growing season (~210 days) [88]. Using a 4 cm layer of mixed-species sawdust, Abouziena and Radwan [90] reported overall biomass reductions of 28.5% after 45 days and 41.7% after 75 days. Over a five-year trial, Pupalienė et al. [73] observed that total weed biomass declined 2.6–6.9-fold in sawdust-mulched plots.

When weed functional groups were analyzed separately, Abouziena and Radwan [90] found that sawdust lowered broadleaf- and grass-weed biomass by 17% and 37.1%, respectively, at 45 days; by 75 days the reductions were 43.4% (broadleaf) and 32.3% (grass weed). Long-term studies likewise showed strong effects on weed density: authors [83] recorded an average 70.8% decrease in weed numbers across four consecutive years with 5–10 cm of sawdust, while in other study [90] authors measured density reductions of 45% at 45 days and 48.6% at 75 days. Pupalienė et al. [73] further noted that, early in summer, sawdust cut weed counts 5.4–11.4-fold relative to un-mulched controls and offered a longer-lasting suppression of germination than straw mulch. Yield benefits accompany this weed control: in peppermint, Matković et al. [20] documented a 1.5-fold increase in fresh-mass yield at the first harvest and a 2.9-fold increase at the second harvest under sawdust mulching.

Collectively, these findings highlight sawdust’s capacity to deliver durable, multi-season weed suppression and concomitant yield gains, especially when applied in sufficiently thick layers (≥5 cm) and maintained throughout the cropping cycle.

4.5. Pine Needles

Dried pine needles have also demonstrated notable weed suppression effects. In peppermint cultivation, a 5 cm layer of pine needle mulch applied between rows resulted in a 32.0% reduction in total weed biomass [88]. Broschat [108] further showed that weed density was reduced by 43.5% on neutral soil (pH 7.2), and by 92.7% on more acidic soil (pH 6.8), highlighting the pH-dependent efficacy of this mulch type. Skroch et al. [78] also confirmed that both long and short pine needles, when applied in a 9 cm layer, reduced total weed density by 50%. The agronomic benefits of pine needle mulch extended to significant yield increases. Matković et al. [20] reported that peppermint yields were 4.2-times higher in the first harvest and 5.3-times higher in the second harvest under pine needle mulch compared to the control with weeds.

More broadly, bark and sawdust mulches are considered among the most effective weed control options, especially for species with varying germination timing [69]. Additionally, these mulches often alter soil pH, which can improve nutrient availability [109], leading to increased flower numbers, higher biomass, and better-developed plants [110,111,112]. Their capacity to conserve soil moisture is particularly useful when applied in early spring (March–April) during the rainy season [74].

However, challenges remain. These mulches are not always easily accessible, their cost can be high, and methods for safe post-use disposal are insufficiently studied. Incorporation into the soil may alter soil structure, pH, and microbiota [74].

4.6. Paper-Based Mulch Materials: Newspaper, Cardboard, and Black Paper

Shredded newspaper, cardboard, and paper-based mulches offer additional alternatives, particularly when derived from recycled materials. These materials are often by-products and, particularly when free of ink, can be incorporated into the soil after use [86]. When applied as mulch, newspaper, cardboard, and black paper form a physical barrier that inhibits weed emergence while improving soil moisture retention and thermal regulation [9,113].

However, the use of printed materials carries potential risks, such as lead (Pb) contamination and carcinogenic carbon compounds like graphite [113,114]. The efficacy of paper mulches depends on multiple factors: the amount applied, agroecological conditions, crop density, and management practices [76,86,115]. For instance, Sánchez et al. [86] reported 50% weed suppression using shredded newspaper. Two layers of shredded newspaper mulch applied in late May effectively prevented weed emergence for the first two weeks after application, according to Pellett and Heleba [116]. By September, weed populations had decreased by 98.0% with a 10 cm mulch layer and by 99.3% with a 15 cm mulch layer, indicating that thicker mulch layers offer more effective and long-lasting weed growth suppression. Six weeks after applying a 5 cm mulch layer in the pumpkin crop interrow, Splawski et al. [117] reported a 91.3% weed number reduction and a 97.4% biomass reduction. It is important to note that this result followed glyphosate treatment in May and June and direct seeding in June.

Cardboard mulch laid continuously in peppermint fields resulted in complete weed suppression (100%), leading to fourfold and 7.2-fold yield increases in the first and second harvests, respectively [88]. Likewise, Ustuner and Ustuner [118] reported 99.7% effectiveness using 1.5 mm thick cardboard. Grundy and Bond [119] also highlighted cardboard as a highly effective physical barrier throughout the growing season, though it must be weighed down with heavy objects (e.g., bricks) to prevent wind displacement.

A practical limitation of cardboard is its susceptibility to wind lift if not adequately secured, which can damage crops during strong weather conditions.

4.7. Synthetic and Biodegradable Mulch Films

Synthetic mulches are mostly manufactured in the form of films, with polyethylene (PE) and polypropylene (PP) being the dominant materials. Their primary role is to suppress weed growth. A key distinction between these materials lies in their durability under environmental stress: PP films are generally less resistant to ultraviolet radiation and temperature extremes than PE films. Fontana et al. [24] highlighted the effectiveness of black PP films in the cultivation of medicinal herbs such as rosemary, lavender, and thyme, suggesting their use as a viable alternative to manual weeding and hoeing.

The weed-suppressive properties of black PE films have been extensively documented. For example, in peppermint fields, complete weed control (100%) was achieved using PE films of varying thickness and type: black perforated (30 µm), gray black (20 µm), solid black (30 µm), and water-permeable black agrotextile films (1 mm) [82,88]. Similarly, water-permeable black PE films (1 mm) were equally effective in lavender and rosemary cultivation [120], and total weed suppression was reported in pumpkin fields treated with black PE film [117]. Several other studies have reported slightly lower, yet substantial, weed suppression effects. Fontana et al. [24] reported 90% weed-free surface in rosemary, lavender, and thyme plots using black PP water-permeable films (0.35 mm). Anzalone et al. [89] found that black PE film (15 µm) reduced weed coverage by 65.1%, and dry biomass by 94.3%. Grassbaugh et al. [85] observed an 80% reduction in total weed biomass using the same film, while Cirujeda et al. [121] documented 99.3% weed suppression over three consecutive years.

In addition to weed control, these films significantly enhanced crop yield. Matković et al. [20] reported a 6.2-fold increase in peppermint yield across two harvests when using perforated black PE film (30 µm), while solid black PE (30 µm) and water-permeable PE (1 mm) films increased yield by 7.1 and 6.9 times (Figure 1D), respectively. Radanović et al. [25] observed a 2.5-fold increase in flower yield of mountain arnica when black PE film (30 µm) was applied, and a 2.2-fold increase with silver PE film (25 µm) (Figure 1B). Cultivation of rosemary, thyme, and lavender on black water-permeable PE films (0.35 mm) resulted in more vigorous shoot development and greater plant height [24]. Similar findings were observed for root development in yellow gentian grown on black water-permeable PE film (1 mm) under high-altitude conditions [27] (Figure 1C).

The beneficial effects of PE mulches are also attributed to their capacity to create favorable conditions for soil microfauna and microflora [119,120,121,122], which indirectly enhance soil structure and promote root system development. In addition, reflective films (especially silver) can repel insect pests, including disease vectors, thus reducing crop infection risks [8]. These films also convert solar radiation into heat, warming the soil surface [8].

The effectiveness of mulching films depends on both their color and thickness. Commercially available variants include black, brown, red, gray, and silver films, as well as combinations (e.g., silver–brown, black–brown). According to previous research, colored mulches such as clear, violet, and light green increased soil temperature by 2.5–2.9 °C, respectively, compared to the un-mulched control. On the other hand, dark-colored mulches, including black, dark green, and red, resulted in a soil temperature increase of 1.4–2.1 °C relative to the un-mulched control [120]. A notable limitation of plastic films is their commonly used thicknesses (15, 20, 25, 30 µm, and 1 mm), which may allow certain aggressive weed species to penetrate them. For instance, Setaria viridis and Echinochloa crus-galli were reported to have pierced black PP water-permeable films (0.35 mm) [24,120]. Furthermore, the need for proper post-harvest removal of plastic films is a major environmental concern, as improper disposal can lead to long-term pollution [75].

Biodegradable mulch films represent a more recent development in agricultural weed control, with research interest emerging primarily in the 1960s and 1970s [70]. Matković et al. [88] reported complete (100%) suppression of weed biomass using black biodegradable film (30 µm). Similarly, two types of black biodegradable films were shown to have high weed control efficiency by Cirujeda et al. [121]; a 15 µm thick film decreased weeds by an average of 97.0% over three years, while a 17 µm film obtained an average reduction of 95.4%. However, using the identical 15 µm film, Anzalone et al. [89] saw a lesser reduction in weed coverage (61.5%), even though dry biomass was still reduced by 90.5%. Additionally, biodegradable mulches have shown promise in increasing output. Matković et al. [20] reported peppermint yield increases of 4.6 times in the first harvest and 6.6 times in the second. However, Carrubba and Militello [2] found that black biodegradable mulch was not sufficiently effective in controlling weeds in coriander and fennel plots, despite positively influencing seed yield.

Despite their environmental advantages, biodegradable films are not without limitations. Perennial grass weed species with rhizomatous growth habits, such as Agropyrum repens, can penetrate these films [74]. Nevertheless, their primary benefits include environmental sustainability and waste reduction, as they degrade naturally under the influence of sunlight and precipitation [75]. These films also allow rainwater infiltration, reduce evapotranspiration from the topsoil, help maintain chemical soil balance, reduce erosion, and suppress soil-borne pathogens and pests [122,123,124,125]. In doing so, they foster a moist, well-structured soil environment conducive to microbial activity and robust root system development in cultivated plants.

5. Influence of Mulching on the Content and Chemical Composition of Essential Oils

Essential oils (EOs) are secondary metabolites of medicinal plants with well-established physiological and ecological roles. They actively participate in plant metabolism and contribute significantly to plant adaptation under various environmental conditions [126]. These compounds are believed to enhance the host plant’s resilience to environmental fluctuations and may function as natural repellents [127]. Under abiotic stress, essential oils may reduce transpiration or help stabilize plant temperature regimes [128]. Furthermore, environmental stressors are often associated with increased biosynthesis of essential oils [4,23].

The yield and composition of essential oils are influenced by numerous biotic and abiotic factors. In peppermint, variables such as nitrogen and phosphorus fertilization [129], precipitation [82,130], soil salinity [131,132], heavy metals [133,134], crop age [135], harvest timing and row spacing [135], UV radiation [136,137], sunlight exposure [129], drying methods of MAPs [129], pesticide application [35], and mulching practices [7,82] all play a significant role.

Species of the genus Mentha have high water requirements, and irrigation is essential in arid regions to achieve optimal EO content [82,130]. However, irrigation efficiency depends on climatic factors, soil type, and physicochemical soil properties [138]. Mulching reduces irrigation needs by conserving soil moisture in the upper soil layer [82]. In peppermint cultivation, both shredded bark mulch and black PE film significantly increased EO content, with a 2.8-fold increase observed with bark mulch combined with irrigation compared to the non-mulched control [82]. Mulching with synthetic materials such as silver-brown film and black agrotextile significantly enhanced the peppermint yield, while EO content ranged from 5.88% to 7.50%, compared to 4.64% and 7.13% in non-weeded and weeded controls, respectively [7]. Additionally, organic mulches like dry pine needles also contributed to improved EO yield and quality, demonstrating their potential as effective, non-chemical weed management tools in peppermint cultivation [7].

Similarly, straw mulch increased essential oil content in field mint (Mentha arvensis) [139], while sugarcane bagasse mulch improved plant development and enhanced EO content by 8% in the first harvest and 10% in the second [138]. Patra et al. [93] reported a 10% increase in EO yield in M. arvensis under rice straw mulch, and a 24% increase using distillation waste mulch from citronella (Cymbopogon winterianus). These organic mulches also enhanced nitrogen uptake, increasing it by 18% (rice straw) and 25% (citronella residue), thus further supporting EO biosynthesis.

Beyond yield, mulching also influences chemical composition of EOs. In M. arvensis, sugarcane bagasse mulch increased menthol content to 78.7% and 86.1% in the first and second harvests, respectively [138]. Saxena and Singh [130] similarly reported higher menthol levels in the first harvest (78.8%) compared to the second (75.2%) under the same mulch treatment. Application of synthetic and organic mulches influenced the chemical profile of peppermint EO, leading to notable variations in the relative content of major constituents such as menthol and menthone [7]. In contrast, Brar [140] found that straw mulch did not affect menthol content or other EO constituents in M. arvensis.

6. Effects of Mulching on Relative Chlorophyll Content in Leaves

Chlorophyll content, often estimated by SPAD values measured using a chlorophyll meter, is an important indicator of plant physiological status. This non-destructive method allows for rapid assessment of leaf greenness without inducing stress in the measured plants [141]. Higher SPAD readings are generally associated with increased chlorophyll concentrations and deeper green leaf coloration, as the WHO monograph defines its dried leaves as being green to greenish brown for some MAPs [142].

The presence of weeds, through competitive interactions, can negatively affect plant greenness and thus chlorophyll levels [35]. Since mulching is effective in reducing weed pressure, it can indirectly influence the relative chlorophyll content in cultivated plants. Karkanis et al. [36] reported a negative correlation between chlorophyll content in peppermint leaves and both weed density and biomass. As weed pressure increased, leaf chlorophyll content decreased accordingly. Beyond biotic stressors, chlorophyll synthesis is sensitive to abiotic factors such as heat, drought, and soil salinity, as well as chemical stressors like herbicide application and elevated CO₂ concentrations [143]. Interestingly, herbicide application in MAPs was found to increase chlorophyll content [35], potentially due to reduced interspecific competition.

Soil nutrient dynamics under mulched conditions can be complex. For instance, bark mulch reduced chlorophyll content in the first year due to nitrogen immobilization, i.e., conversion of inorganic nitrogen into an organic form unavailable to plants. However, in the second year, as the mulch decomposed, nitrogen was released back into the soil in plant-available forms, resulting in increased chlorophyll levels. These authors also noted higher SPAD values during the early vegetative phase under bark mulch application. Treatments with synthetic mulches resulted in minimal reductions in the relative chlorophyll content of peppermint leaves compared to the hand-weeded control, with differences ranging from −1.93% to +4.53%, highlighting their effectiveness in preserving leaf physiological quality relevant for herbal drug use [5]. Contrasting results were reported by Shahriari [107], who found no significant change in chlorophyll content when bark mulch and black PE film were combined with irrigation in peppermint cultivation. Mulching generally promotes vegetative growth in early plant stages, but during later developmental phases, plants tend to shift from vegetative to reproductive growth, reducing leaf biomass and chlorophyll production [20]. Supporting this, Yang et al. [144] found that peppermint plants grown on white PE mulch (0.6 mm) had higher chlorophyll content in April than in May, and also higher than those grown on corn straw mulch.

All these findings suggest that in appropriate agronomic systems, the use of various mulch types can positively influence chlorophyll content, especially when integrated with nitrogen fertilization and adequate irrigation. Optimizing these combined agronomic practices can improve photosynthetic efficiency and overall plant performance in horticultural production systems.

7. Effects of Mulching on Soil Properties

Mulching significantly influences various soil characteristics that directly or indirectly shape the growing conditions for plants. Key effects of mulches include moisture conservation, soil temperature regulation, and changes in soil chemical properties such as pH. Together, these factors contribute to improved plant growth and development, enhanced soil fertility, and reduced weed competition. The application of various mulching materials helps preserve soil moisture [78,145], moderates soil temperature fluctuations [8,12,77,78,79,117], and can alter soil pH [109]. Mulches typically keep soil cooler in summer and warmer in winter compared to bare soil, also helping prevent freezing damage [145]. These modifications protect beneficial soil microflora [122], improve soil structure, support root development, and enhance nutrient uptake [139].

7.1. Influence of Mulching on Soil Temperature

Soil temperature critically influences plant development by regulating processes like seed germination, microbial activity, and nutrient availability. In many cases, it has a greater effect on plant growth than ambient air temperature [146], particularly during early developmental stages. Mulching modifies thermal conditions in the root zone, influencing plant development rates and stress tolerance. By altering soil temperature, mulching also impacts microbial activity, nutrient transformations, and nutrient availability, thus affecting plant growth both directly and indirectly [109,144].

Soil temperature plays a key role in seed germination and early root zone development, particularly during the early growth stages [47,147,148].

Mulch films can raise rhizosphere soil temperature by 3–4 °C, which stimulates faster root growth [149]. However, as the plant canopy develops, it reduces heat accumulation in the soil during summer [150]. Mulches also buffer diurnal temperature fluctuations by reducing daytime maxima and raising nighttime minima [72,78,79]. This effect is particularly beneficial during cold spells, helping prevent frost damage to soil and perennial plant reproductive organs [151].

The extent of soil temperature modification depends on mulch type, thickness, material, as well as soil moisture and crop canopy density. Organic mulches generally reduce soil temperature, while synthetic plastic films tend to increase it [71,72,152]. For instance, straw mulch at 10 t ha⁻¹ reduced soil temperature due to reduced solar radiation penetration [153]. Prihar et al. [154] found that 6 t ha⁻¹ of straw reduced maximum soil temperature at 10 cm depth by 3–6 °C, and Munn [77] reported a 2.5 °C reduction under the same conditions.

Bragagnolo and Mielniczuk [155] demonstrated that 7.5 t ha⁻¹ of wheat straw decreased maximum soil temperature at 5 cm depth by 8.5 °C and simultaneously increased soil moisture by 10%. However, as plant canopy coverage increased, surface soil temperature decreased due to shading [156]. In contrast, Awodoyin et al. [12] reported that straw mulch had minimal impact on soil temperature at 15 cm depth. However, a slight increase of 1.2 °C was observed at 5 cm depth during afternoon hours. Newspaper mulch reduced soil temperature at 10 cm depth by 4.2 °C [77]. Interestingly, Splawski et al. [117] found that a 5 cm layer of the same mulch increased temperature at 23 cm depth by 0.3 °C, while temperatures at 5 cm remained unchanged.

Organic mulches such as pine bark and pine needles maintained more stable surface temperatures compared to bare soil [78]. Teak bark mulch slightly altered temperatures at 5 and 15 cm depth, with the largest difference (0.9 °C) observed at 15 cm in the morning [12]. Wood bark mulches increased soil temperature by 3.2 °C at 5 cm and by 0.3 °C at 15 cm depth relative to bare soil [117].

Various PE films also influence soil thermal dynamics. Ponjičan and Bajkin [157] found that nine types of PE film increased soil temperature at 10 cm depth by 2.5–4.5 °C. Ramakrishna et al. [67] observed that black PE film (9 µm) increased soil temperature by 3.4 °C at 5 cm and 2.9 °C at 10 cm depth, 90 days post-application. Similarly, Splawski et al. [110] reported that a 32 µm black PE film raised temperature by 5.1 °C at 5 cm and by 0.4 °C at 23 cm depth. Ponjičan and Bajkin [157] noted a 3.4 °C increase at 10 cm depth using a 30 µm black PE film, while Skroch et al. [79] recorded a 0.8 °C increase in surface soil with a thicker 6 mm PE film. Gray PE film (22 µm) elevated soil temperature by 2.6 °C at 10 cm depth [157]. Awodoyin et al. [12] observed higher morning temperatures under gray-black PE film (25 µm) compared to weed-covered soil—by 1 °C at 5 cm and 1.5 °C at 15 cm depth. The afternoon differences were even greater: 4.9 °C at 5 cm and 2.6 °C at 15 cm.

7.2. Influence of Mulching on Soil pH

Soil pH is a key factor that influences nutrient availability and microbial activity, both of which are essential for healthy plant growth. It is a dynamic property that can fluctuate seasonally and be affected by agricultural practices such as mulching. Soils are generally classified as acidic, neutral, or alkaline based on their pH values, and different plant species, including both crops and weeds, have specific optimal pH ranges for growth [158].

The effect of mulching on soil pH depends largely on the chemical composition and the characteristics of the soil on which it is applied. For example, Broschat [108] evaluated the effects of pine bark mulch (pH 3.6), eucalyptus mulch (pH 4.6), and pine needle mulch (pH 4.4), and found no significant change in soil pH six weeks after application. Similarly, Greenly and Rakow [71] observed no change in soil pH following the application of a 25 cm layer of pine bark mulch. Pakdel et al. [109] hypothesized that pine bark and plane tree sawdust might reduce nitrogen and lime availability, potentially increasing soil pH, but no significant effect was observed.

In contrast, other studies have documented pH reduction associated with mulching. Billeaud and Zajicek [159] demonstrated that pine bark mulch reduced surface soil pH proportionally with mulch thickness, with a 15 cm layer decreasing pH by 0.4. Duryea et al. [106] similarly reported a 0.6 unit pH decrease under pine needle mulch. Tindall et al. [160] observed a 0.6 unit pH reduction over three years in fields mulched with straw, which was partly attributed to annual fertilization with urea-ammonium nitrate. Alharbi [161] found that mixed mulches composed of 5 cm palm leaves and 10 cm gravel caused greater pH reductions in the topsoil (0–30 cm), with pH values 0.7 units lower than in non-mulched soil; at deeper layers (30–60 cm and 60–90 cm), reductions were smaller (0.3 and 0.2 units, respectively).

Regarding synthetic mulches, PE film alone typically does not alter soil pH [160]. However, Sharma and Bhardway [162] reported that PE mulching accelerates decomposition of organic residues beneath the film, leading to increased production of organic acid and a subsequent decrease in soil pH. This acidification can enhance the availability of micronutrient, such as manganese (Mn), zinc (Zn), copper (Cu), and iron (Fe). Wang et al. [163] similarly documented a 0.4 unit pH decrease at 15 cm soil depth after five years of PE mulch application.

Conversely, organic compost mulches have shown slight increase in soil pH; Chan et al. [164] documented a 0.02 unit rise following the application of 7.5 cm thick compost mulch.

These findings suggest that mulching may be a valuable long-term strategy to subtly modify soil pH and nutrient availability. However, due to the typically gradual nature of pH changes, mulching is not suitable for rapid soil pH correction within a single growing season. Moreover, mulch thickness plays a crucial role in determining the extent of its impact on soil pH.

8. Limitations and Challenges of Mulching in Medicinal Plant Systems

Mulching is well known for its ability to reduce weeds and improve soil quality in medicinal and aromatic plant (MAP) production. However, there are a number of agronomic, financial, and environmental obstacles to its actual application. The cost and accessibility of appropriate mulching materials are two of the biggest obstacles. It is challenging for producers to implement mulching on a wide scale in many areas due to limited access to organic leftovers or biodegradable films. Even though synthetic mulches, such as polyethylene (PE) films, are very effective at suppressing weeds and increasing production [24,82,85,88,89], small-scale producers sometimes cannot afford them due to their high cost. Furthermore, using synthetic mulches has significant environmental issues, especially regarding how they should be disposed of after use. Improper management of plastic films can lead to soil contamination and long-term ecological degradation, which runs counter to the principles of organic and sustainable agriculture.

Organic mulches are more eco-friendly, bring their own set of challenges [81,89]. Their effectiveness in weed suppression is often lower than that of synthetic alternatives [83,90,105,108], especially under heavy weed pressure or in the presence of perennial species. Additionally, the physical properties of organic mulches, such as their lightweight or texture, might result in non-uniform soil coverage, susceptibility to wind displacement, and may serve as a source of inoculum of pathogens [8]. Organic materials like straw or compost may also retain viable weed seeds, thus introducing additional challenges into the cultivations [81].

Additional complexity is introduced by the use of cover crops or live mulches. These mulches can compete with MAPs for essential resources such as light, water, and nutrients. Particularly in low-input systems, improperly managed living mulches may inhibit crop growth instead of weeds. Therefore, the type of mulch and application thickness must be carefully matched to the specific crop species and agroecological conditions in the area. Long-term mulching may also cause unexpected alterations to the characteristics of the soil. For example, during early decomposition, organic mulches may immobilize nitrogen, limit the availability of nutrients and decrease the amount of chlorophyll in crops during crucial growth phases.

Mulching has a lot of potential as a non-chemical method of weed suppression and improving soil in MAP production, but ecological, financial, and technological considerations limit its widespread use. In order to overcome these obstacles and enhance the general performance and sustainability of MAP production systems, future studies should concentrate on mulch selection, application schedule, and integration with other non-chemical control techniques.

9. Conclusions

Mulching is a practical, effective, and eco-friendly method of controlling weeds in MAPs. Organic and synthetic mulches, as well as biodegradable ones, have shown promise in suppressing weed development, improving soil moisture retention and temperature regulation, and increasing the quantity and quality of essential oil and herbal row material. The physical barrier created by mulch improves the rhizosphere’s microclimate and biological conditions while lowering the amount of light available for weed germination. Among commonly used materials, black polyethylene films and organic mulches such as straw, bark, sawdust, and pine needles have consistently produced favorable agronomic outcomes. However, a number of aspects, such as the type and thickness of the mulch, specific crop species, and regional agroecological conditions, affect how beneficial mulching is overall. Despite these benefits, certain limitations persist, such as the cost of suitable mulching materials, non-naturally biodegradation of PE films, and challenges related to long-term soil management. Additionally, significant research gaps remain regarding MAPs’ responses to mulching practices.

In summary, mulching improves the production of sustainable MAPs by suppressing weeds and reducing the need for herbicides and labor costs. More interdisciplinary research is needed to develop site-specific agronomic guidelines that optimize mulch selection and application across different MAP species and agroecological areas in order to fully realize these benefits.