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Article

Evaluation of Substrate Stratification, Fertilizer Placement, and Mulching on Growth of Common Nursery Weed Species and Container-Grown Ornamental Species

by
Yuvraj Khamare
1,*,
Stephen C. Marble
1,
Brian J. Pearson
1,
Jianjun Chen
1 and
Pratap Devkota
2
1
Department of Environmental Horticulture, Mid-Florida Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, Apopka, FL 32703, USA
2
Agronomy Department, West Florida Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, Jay, FL 32565, USA
*
Author to whom correspondence should be addressed.
Horticulturae 2023, 9(7), 747; https://doi.org/10.3390/horticulturae9070747
Submission received: 12 May 2023 / Revised: 19 June 2023 / Accepted: 23 June 2023 / Published: 27 June 2023
(This article belongs to the Section Floriculture, Nursery and Landscape, and Turf)
Browse Figures
Review Reports Versions Notes

Abstract

:
The objective of this study was to determine how topdressing or incorporating fertilizer with stratified or mulched substrates could affect the growth of Hibiscus rosa-sinensis ‘Snow Queen’, a popular ornamental plant, and the growth of liverwort (Marchantia polymorpha) and bittercress (Cardamine flexuosa), two common nursery weed species. Five different substrate treatments were evaluated, which included three stratified substrates composed of pine bark screened to a small (0.63–1.27 cm), medium (≤1.90 cm), and large (0.96–1.90 cm) particle size and two industry-standard substrates that were either mulched with rice hulls or remained unmulched. All treatments were then fertilized via either topdressing or incorporating a controlled-release fertilizer (CRF). Bittercress control was highest in mulched containers, followed by those stratified using the medium pine bark, and its growth increased overall in topdressed vs. incorporated containers regardless of substrate or mulch treatment. All the stratification treatments resulted in a decrease in liverwort coverage compared to the industry standard treatment, but topdressing generally increased liverwort coverage compared with incorporating fertilizer. In conclusion, both topdressing and incorporation appear to be compatible with fertilizer placement methods with substrate stratification from a crop production standpoint; however, weed growth may increase if fertilizer is topdressed.

1. Introduction

Container nursery operations use adequate fertilizer along with frequent irrigation to produce marketable plants. While ample fertilizer use and frequent irrigation improve ornamental plant quality and growth, it also creates a favorable environment for weed growth. Weeds can outcompete ornamental plants for light, water, nutrients, and space, reducing their growth and desired quality. Additionally, the market for ornamental crops requires weed-free container-grown plants [1]. Hence, weed control is extremely critical for nursery growers, but the cost to manage weeds with current practices can exceed $25,000 per hectare per year [2,3].
The primary method of weed control in container production is achieved with preemergence herbicides along with supplemental hand weeding. However, the excessive utilization of herbicides has caused environmental, safety, and water quality fears due to runoff or leaching into groundwater [4]. Furthermore, the number of herbicides and modes of action specific to ornamental plant production is considerably low. There has also been a lack of discovery of herbicides with new modes of action in recent years [5]. Moreover, the high diversity of ornamental species growing in nursery production makes it difficult to use herbicides. Other problems with herbicide use in container production involve improper calibration, herbicides falling between container gaps [6], and the need for multiple applications [7]. Furthermore, many popular ornamental plants are highly sensitive to herbicides. As a result, hand weeding has become a common practice. Mathers [8] reported that hand weeding could cost nurseries up to $10,000 per hectare per year. Hand weeding is not only expensive and time-consuming but also hard to implement, given labor shortages [9]. These challenges have stimulated the need for the development of more integrated and sustainable weed management strategies that can be implemented in a wide variety of ornamental plants.
Substrate stratification, a new substrate management approach, has been shown to be a tool for weed management [10,11]. Substrate stratification involves creating layers of diverse substrates or textures of the same substrate within a container [12]. Substrate stratification was initially designed to improve water and fertilizer efficiency [13,14,15], and then in other work, the original principle was modified to evaluate its use to control weeds [10,11]. The method of substrate stratification for weed management involves using larger, coarse, and easily draining particles as the top strata without any fertilizer and a substrate that is highly capable of retaining moisture, finely textured, with fertilizer incorporated as the bottom strata [10]. As a result, the top stratum of the substrate lacks nutrients or moisture for weed seeds to establish. In previous studies, substrate stratification has been shown to decrease the growth of spotted spurge (Euphorbia maculata) by 14 to 55%, bittercress (Cardamine flexuosa) by 80 to 97%, and liverwort (Marchantia polymorpha) coverage by 95 to 99% [11].
Strategic fertilizer placement and mulching are two of the most effective nonchemical tools to manage weeds in container nursery production [16,17,18,19,20,21,22]. Substrate stratification combines these two methods by strategically placing fertilizer only in the bottom stratum and creating a mulch-like layer with the use of coarse particles in the top strata [10]. While acceptable weed control was noted in previous studies with substrate stratification [11], it was not evident if fertilizer placement or the use of coarse particles as the top stratum had the most substantial impact on weed control or if the weed management advantage would be mitigated if fertilizer was topdressed rather than incorporated. In previous work, we hypothesized that fertilizer placement was likely the primary factor for weed suppression [10]. With this assumption, further investigation was needed to evaluate the effect of fertilizer placement along with stratified substrates on the growth of ornamental and weed species. Furthermore, many ornamental species are grown for one or more years and topdressed with fertilizer to supply additional nutrition after the initial nutrient dose is depleted, but the effect this topdressing has on stratification as a weed management tool is unknown. The main goal of this study was to evaluate the effect of fertilizer placement on ornamental plants grown using stratified substrates and the resulting effect on the suppression of common weed species.

2. Materials and Methods

Aged pine bark (screened to <2.54 cm) was purchased from a local supplier. It was further screened to three different size ranges, including 0.63 to 1.27 cm, 0.96 to 1.90 cm, and ≤1.90 cm (with fines) pine bark. To create an industry-standard substrate, pine bark was screened to pass through a 1.27 cm screen and included all fines (≤1.27 cm). Stratified substrate treatments consisted of either 0.63 to 1.27 cm, 0.96 to 1.90 cm, or ≤1.90 cm pine bark applied at a depth of 2.5 cm as the top strata and ≤1.27 cm pine bark as the bottom strata. The pine bark sizes used as the top strata were further classified into small (0.63–1.27 cm), medium (≤1.90 cm), and large (0.96–1.90 cm) pine bark. A non-stratified treatment was also created to act as an industry-standard treatment. The standard treatment comprised only the ≤1.27 cm pine bark throughout the entire container. A controlled-release fertilizer (CRF) [Osmocote Blend 17N–2.2P–9.1K (8 to 9 months); ICL Specialty Fertilizers, Dublin, OH, USA] at a rate of 9.2 g per liter was applied in all the treatments. CRF was either incorporated in the bottom strata of the stratified treatments or topdressed after potting. The fertilizer in the non-stratified standard treatment was also either incorporated in the substrate or topdressed on the substrate surface. An additional treatment was constructed with rice hulls applied as a mulch on top of the substrate surface at a depth of 2.5 cm with ≤1.27 cm pine bark as the substrate. The mulch treatment utilized a substrate comprised solely of ≤1.27 cm pine bark, similar to the standard treatment. The fertilizer in the rice hull treatment was also either incorporated in the substrate or topdressed on top of the mulch after potting. This treatment structure resulted in a 5 × 2 factorial with five substrate or mulch treatments (standard, small, medium, large, or mulched) and two fertilizer placements (topdressed or incorporated). All the Trials with weed species were conducted in a shade house, whereas the trial with the ornamental plant was conducted on a full sun nursery pad at the University of Florida’s Mid-Florida Research and Education Center in Apopka, FL, USA, beginning in December 2021.

2.1. Experiment 1: Influence of Substrate Stratification and Fertilizer Placement on the Growth of Hibiscus

Uniform 5 cm plug tray liners of hibiscus (Hibiscus rosa-sinensis ‘Snow Queen’) liners were planted into a 3.8 L (18 cm height, 20 cm diameter) nursery container using the above-mentioned substrates in June 2022. While transplanting in stratified treatments, the rootballs were covered approximately half in the top strata and half in the bottom strata as the rootball was 5 cm deep. The liners in the mulch treatment were planted in the ≤1.27 cm pine bark substrate, followed by mulching with rice hulls at a 2.5 cm depth. For all the treatments, the top strata were at a depth of 2.5 cm, whereas the depth of the bottom strata was 16.5 cm. The aforementioned fertilizer was either incorporated prior to potting or topdressed on the substrate or mulch surface after potting (topdressing) at the above-mentioned rate. After potting, all the plants were placed on a full-sun nursery pad. Plants were irrigated 1.3 cm per day (as determined using rain gauges) with overhead irrigation (Xcel-Wobbler™; Senninger Irrigation, Clermont, FL, USA) via two irrigation cycles (7:00 A.M. and 2:45 P.M.) throughout the trial. The plants were assessed for 24 weeks after planting (WAP), and data collection included plant growth index [(height + width at widest point + perpendicular width) ÷ 3] measured every 4 WAP, and root and shoot dry weights at the study’s conclusion (16 WAP). In addition to growth data, leachate samples were collected monthly to determine nutrient leaching through the various substrate and fertilizer treatments. The samples of leachate were collected from 30 containers (3 random replicates per treatment from each experimental run or n = 6) every 4 weeks for 16 weeks. The leachate samples were collected in a 50 mL plastic vial and analyzed by UF/IFAS analytical services laboratories (Gainesville, FL, USA). The measurements included pH, electrical conductivity (EC), ammonium (NH4-N), nitrate nitrogen (NO3-N), orthophosphate (OrthoP), and potassium (K). EC and pH were measured with a dual probe meter (LabFit AS-3010D dual probe pH robot, Bayswater, WA, Australia). NH4-N, NO3-N, and OrthoP were analyzed by flow injection analysis (Flow solution 3700 automated chemistry analyzer, O-I-Analytical a xylem brand, College Station, TX, USA) and K by inductively coupled plasma spectrophotometer (Spectro Analytical Instruments, Mahwah, NJ, USA). The experiment evaluating these variables on hibiscus was a completely randomized design with six single container replications for each treatment and was repeated in time following the same methodology.

2.2. Experiment 2: Influence of Substrate Stratification and Fertilizer Placement on the Growth of Common Nursery Weed Species

In a different set of trials, bittercress and liverwort were used to assess the influence of substrate stratification and fertilizer placement on nursery weed species. Twenty-five seeds of bittercress were surface sown onto 3.8 L containers using the above-mentioned substrates in December 2021. The containers of this trial were kept in a shade house with 60% shade. All the containers received 0.8 cm irrigation per day through overhead irrigation (Xcel- Wobbler™; Senninger Irrigation, Clermont, FL, USA) for the remainder of the experiment. Weed counts were conducted of the emerged bittercress at 4 WAP and counts of plants with one or more true leaves at 10 WAP. At the conclusion of the trial, i.e., at 10 WAP, the shoot dry weight of the harvested plant material was obtained by cutting it at the soil level and then drying it in a forced-air oven at a temperature of 60 °C for seven days until it reached a stable weight. The experiment was a completely randomized design with six single container replications for each treatment and was repeated in time following the same methodology.
To assess the growth of liverwort on stratified substrates and fertilizer placement, a different set of containers were used in December 2021. Before beginning the trial for liverwort, 1.68 L (13.30 cm height, 13.30 cm diameter) square nursery containers were filled with a substrate made up of pine bark and peat (80:20 v:v) mixed with the same CRF via incorporation. The surface of the square nursery containers was covered with five to six pieces of liverwort. Containers were kept in the shade house described earlier and watered 0.8 cm per day via overhead irrigation (Xcel-Wobbler™). The containers remained in the shade house till the surface was completely covered by liverwort with gemmae cups formed and no noticeable substrate. These containers were fully covered with liverwort in approximately 10 weeks and were used to naturally induce sporulation in the treatments. A similar method was successfully used in previous studies to sporulate liverwort in containers [10]. A separate set of 1.68 L square nursery containers were filled and fertilized with the same treatments mentioned previously and placed in the same shade house. For all the treatments, the top strata were at a depth of 2.5 cm, whereas the depth of the bottom strata was 11.5 cm. The inoculum containers were placed at a distance of 0.5 cm from each replication, and each replication was surrounded on all four sides. Liverwort surface coverage was evaluated at 16 WAP by taking digital photos of each treatment using a smartphone (iPhone 8 Plus; Apple, Cupertino, CA, USA) from a height of 0.9 m. Images were cropped using a software program (Microsoft Paint; Microsoft Corp, Redmond, WA, USA) so that only the surface of the substrate and liverwort was detectable in the image [10]. Liverwort coverage was then determined using the color threshold tool (hue, saturation, and brightness) in Image processing software ([23]; ImageJ; U.S. National Institutes of Health, Bethesda, MD, USA) to provide a quantitative value of liverwort coverage. The experiment included a completely randomized design with six single-container replications per treatment and was repeated in time following the same methodology.

2.3. Statistical Analysis

All experiments and experimental repetitions were constructed as a 5 × 2 factorial treatment arrangement with five substrate treatments and two fertilizer placements in a completely randomized design with six single-container replications per treatment. Data were subjected to mixed model analysis of variance (ANOVA, F test) using statistical software (JMP® Pro ver. 14; SAS Institute, Cary, NC, USA) to determine the effects of substrate, fertilizer placement, and the interaction of these variables on experimental parameters collected. Before analysis, all data were inspected to ensure the assumptions of ANOVA were met. Experimental run and replication were considered random effects, while substrate treatments and fertilizer placement were considered fixed effects. When ANOVA results revealed significant effects, means comparisons for fertilizer placement were performed using the Student t-test, and means comparisons for substrate treatments were performed using Tukey’s honest significant differences (HSD) test. In all cases, effects were considered significant at α = 0.05. There was no treatment by experimental run interactions; therefore, results were combined across both experimental runs.

3. Results and Discussions

3.1. Influence of Substrate Stratification and Fertilizer Placement on the Growth of Hibiscus

The effect of substrate stratification was not significant for any parameters collected on hibiscus growth (Table 1).
However, fertilizer placement significantly affected the growth index as well as both shoot and root dry weights, with topdressed plants being slightly larger than plants fertilized by incorporation (Figure 1 and Figure 2).
At early evaluation dates, differences in growth index were minimal and not observed visually, however significant, with topdressed plants being slightly larger than plants fertilized via incorporation. At trial conclusion at 16 WAP, hibiscus that were topdressed had a 12% higher growth index than plants with incorporated fertilizer. Similarly, the shoot and root dry weight of all plants grown in substrate treatments with fertilizer incorporated was reduced by 13% and 15%, respectively, compared to the substrate treatments that were topdressed (Figure 2). While some slight growth differences were observed, all plants would have been considered marketable and salable [24]. In a previous stratified substrate study, a similar substrate of 0.63–1.27 cm pine bark as top strata with fertilizer incorporated in bottom strata reduced the shoot and root dry weight of ligustrum (Ligustrum japonicum) by 17% and 27%, respectively, but did not decrease the growth of blue plumbago (Plumbago auriculata); however, topdressing was not evaluated [11]. In several other experiments, topdressing has been shown to increase plant growth more than other methods. For example, Broschat [25] reported that the dry weight of hibiscus shoots decreased in treatments when fertilizer was incorporated compared to when it was topdressed or layered with fertilizer just below the bottom of the liner during transplanting. The authors concluded that the optimal fertilizer varied among different species, but overall, none of the species grew best with incorporated fertilizer. A similar study reported that dibbling and topdressing fertilizers resulted in better plant quality compared to the ones that were incorporated [26]. This finding could be caused by the locality of CRFs applied. It is known that nutrient release from CRFs depends on their formulation, container substrate components, production temperature, and irrigation volume and frequency [27]. The incorporation places CRFs in a location with a higher level of substrate moisture and a more constant temperature, which will result in a relatively quicker release of nutrients. With frequent irrigation, the released nutrients could be largely leached from the container, resulting in a limited supply of nutrients to plant roots. On the other hand, topdressed CRFs are not subjected to having most of their surface area in contact with a higher moisture substrate; thus, nutrient release could be slower and more available for plant growth, especially as the study progressed at later weeks. In the case of this study, both shoot and root dry weights of plants topdressed with CRF were significantly higher than those incorporated, suggesting greater nutrient availability in the topdressed substrate. As discussed previously, in these stratified substrates, the top strata do not contain nutrients. For ornamentals such as hibiscus, which require high nutrient levels for optimal growth, the topdressed treatment likely provided greater availability of nutrients as compared with incorporation, as a large amount of nutrients in incorporated fertilizers are often lost or unavailable at potting until root growth matures [15].

3.2. Influence of Substrate Stratification and Fertilizer Placement on Emergence and Shoot Biomass of Bittercress and Liverwort Establishment

At both 4 WAP and 10 WAP, there was a significant substrate × fertilizer placement interaction on bittercress emergence (Table 2).
At 4 WAP, all the stratified substrate treatments with fertilizer topdressed had a similar bittercress emergence to the industry standard treatment, and the only substrate treatment that decreased bittercress emergence was the rice hull mulch treatment. Interestingly, when fertilizer was incorporated, bittercress emergence was lower in the industry standard treatment, with the highest emergence in the small and large stratified substrates. A similar trend was observed at 10 WAP with fertilizer incorporated, where the large, stratified treatment had higher bittercress emergence compared to medium stratified treatment, mulch treatment, and industry-standard treatment (Table 2). AT 10 WAP, bittercress emergence when fertilizer was topdressed was similar in all the substrate treatments, with only the mulch treatment providing a reduction in bittercress emergence relative to the industry standard. When fertilizer was incorporated, all treatments had bittercress counts equal to or higher than the industry standard, indicating that stratification and mulch did not provide any reduction in bittercress emergence.
While stratification had little to no impact on bittercress emergence, several treatments significantly decreased bittercress biomass. For biomass, the effects of the substrate (p ≤ 0.0001) and fertilizer placement (p = 0.0012) were significant (Table 2) with the mulch treatment and the medium stratified substrate resulting in a significant reduction (84% and 50%, respectively) in biomass relative to the industry standard. Various substrate and/or fertilizer placement weed reduction strategies have previously shown no significant effect on weed germination or emergence but have caused reductions in overall biomass and thus provided a weed management benefit. For example, Saha [22] showed that fertilizer placement had no impact on the germination of eclipta (Eclipta prostrata) or large crabgrass (Digitaria sanguinalis) but reduced seed production by over 90% and biomass by over 80% by limiting weed growth following emergence. It is also not surprising that rice hulls significantly reduced bittercress growth as several studies examining rice hulls at depths similar to the current study have reported they are a very effective weed management option for container-grown plants [16,28]. In terms of fertilizer placement, topdressing containers resulted in higher bittercress growth than incorporating fertilizers across all substrates, increasing shoot weight by 72% (Figure 3).
This result was similar to previous reports where topdressing resulted in significantly higher growth of spotted spurge and large crabgrass compared to sub-dressing, a fertilizer placement method similar to stratification [29]. In another container study, subdressing fertilizer at a depth of 2.5 cm did not reduce the germination of eclipta compared to a topdressed container, but a decrease in shoot dry weight of eclipta was observed compared with topdressing [20]. This response can be species-specific, and a depth of 5 cm or more has shown to be more effective in consistently controlling several common container nurseries weed species [20,22] compared with the 2.5 cm depth that was evaluated in this study.
While the effect of the substrate was significant for liverwort coverage (p ≤ 0.0001) and was, in general, 30% coverage or less for mulched and stratified treatments compared with over 60% for the industry standard, results were confounded by a substrate × placement interaction.
Regardless of placement, the use of stratified substrates or mulch decreased liverwort coverage significantly compared to the industry standard treatment, ranging from an approximately 50% to over 90% reduction (Figure 4).
For containers with fertilizer incorporated, the lowest liverwort coverage was found in pots that were mulched, followed by the medium stratified substrate. In topdressed pots, mulch again provided the greatest reduction in liverwort coverage, but the next highest reductions were observed in the small and large stratified substrates, and in general, liverwort coverage was higher in all topdressed stratified and mulched treatments compared to when fertilizer was incorporated. Overall, these results suggest that while topdressing may diminish the effects of using rice hulls or stratified substrates, these methods would still be expected to provide a considerable benefit compared with using an industry-standard, non-mulched substrate. These results with liverwort are similar to a previous study with stratified substrates that resulted in a 90% to 99% reduction in liverwort establishment [10,11]. As liverwort growth is highly influenced by both moisture and fertility levels [30], stratified substrates or the use of mulch have been consistently shown to provide significant benefits for liverwort management. Sarkka [31] reported that sphagnum moss mulch and blackcurrant stem pieces as mulch controlled liverwort growth by 90% to 100%.

3.3. Nutrient Leaching Analysis

Leachates collected every 4 weeks over the course of the 16-week study were analyzed across all sampling dates due to a few differences among sample dates, which showed the overall trend of nutrient leaching over the course of the study. For NH4, the main effect of substrate was significant (p = 0.0214), while placement nor substrate × placement was significant (Table 3).
Across all sampling dates, all treatments had similar ammonium leaching compared to the industry standard, but treatments that were mulched had notably lower ammonium leaching compared with the medium and large stratified treatments. In contrast, NO3 leaching was influenced only by fertilizer placement (p < 0.0001), with topdressing resulting in higher NO3 in the leachate compared to when fertilizer was incorporated. This same trend was also observed with K and EC, but no differences were observed in orthophosphate. For substrate pH, few differences were noted, and all fell within the desired range for most crops (5.4 to 6.5) [32]. Overall, in this study, topdressing had higher leachate of NO3 and K compared to the treatments with fertilizer incorporated. The increase in the growth index of hibiscus plants by 16 WAP with fertilizer topdressed could potentially be due to the higher amount of NO3 and K available for the plants (Table 3).
Few studies have previously examined nutrient movement through various mulch materials and none through stratified substrates, as we evaluated here. In the case of rice hulls, [16] showed only a slight decrease in plant nutrient status when a liquid fertilizer was applied through a rice hull mulch layer, but there was no detrimental effect on the growth of sunflower (Helianthus annuus). In work by Glenn [33], the growth of petunias (Petunia floribunda) did not differ when a CRF was placed either above or below a wastepaper mulch. In contrast, the growth of hydrangea (Hydrangea macrophylla) consistently increased when a CRF was placed below multiple mulch materials compared to growth when the CRF was placed on top of the mulch, which was attributed to, at least in part, by the slow N and P release from the CRF placed on top of the mulch early in the growing season [34]. Nutrient release from CRF is dependent on the type of coating materials, coating thickness, soil moisture, and soil temperature [35,36]. The nutrient release is primarily dictated by the diffusion mechanism, and the diffusion of nutrients is influenced by temperature [35,37]. In the present study, topdressing resulted in consistently higher levels of N, K, and EC in leachates and also increased hibiscus growth. These results are different from those reported by Altland [34], which could be attributed to differences in fertilizers. Although we did not measure the substrate temperature or moisture retention throughout the study, research has shown that mulches act as a physical barrier, reduce substrate water evaporation and protect the bottom layer of the substrate from extreme heat conditions [8,38,39]. Our leachate results revealed that NO3-N and K were leached less when the fertilizer was incorporated and had a layer of either mulch or top strata of pine bark in stratified substrate treatments, which may have acted to cool the container and slowed the release of the CRF. However, this would contradict other studies on fertilizer placement and nutrient leaching, such as Cabrera [36], who reported that fertilizer incorporated in the substrates had a higher N leaching than the fertilizer topdressed. One additional study compared the leaching pattern of CRF between topdressing, incorporated, and dibbled fertilizer placement and reported that incorporated and dibble leached a higher volume of nutrient load compared to topdressed fertilizer [40]. We hypothesize that the incorporated treatments might have actually leached a higher amount of nutrients more rapidly compared to the topdressed treatments, but our leachate collection missed the release peak due to more infrequent nutrient analysis in the present study (monthly) compared to others (weekly), such as Cabrera [41]. More rapid nutrient release and leaching would have caused the incorporated treatment to be less efficient at delivering nutrients resulting in hibiscus with a lower growth index and biomass. On the other hand, N and K in the leachate of topdressed treatment were higher, which contributed to greater hibiscus growth and, subsequently, higher weed growth. Further research is needed to provide a greater understanding of how both water and nutrients move through stratified substrates and how this movement influences plant growth and weed control.
Results from this study agree with previous reports, which illustrate that stratifying substrates for weed management purposes will likely have minimal to no impacts on crop growth or marketability, as evidenced by hibiscus growth over a 16-week period. In addition, all stratified treatments reduced liverwort establishment and growth, which is also in line with previous reports. The stratified treatments with medium particles and rice hull mulch with fertilizer incorporated reduced the growth of bittercress compared to the industry standard treatment. This indicates that more research is needed to refine both the substrate particle size used as the top strata as well as the depth of the top strata. One of the primary concerns by many nursery growers with using stratified substrates or mulch, such as rice hulls, is that they will impede nutrient movement to the ornamental crop plant. For instance, CRF placed below the mulch resulted in larger growth of container-grown hydrangea (Hydrangea macrophylla) compared to the CRF placed above the mulch [34]. In another study, [16] reported that rice hull mulch resulted in a slight decrease in NO3 and NH4+ concentration that was temporary and did not cause any negative effect on plant growth. Here, results showed this was not the case, at least when considering crop growth, as hibiscus growth was highest in topdressed treatments based on all data collected. From a negative standpoint, topdressing also resulted in the most bittercress growth across all substrates and the highest liverwort establishment in the large stratified and mulched treatments.

4. Conclusions

The results of these experiments indicate that substrate stratification can be used as a weed management tool with potentially no impact on ornamental plant growth. The study revealed that while the growth of hibiscus plants that were topdressed had higher growth than incorporated fertilizer, the overall growth in both topdressed and incorporated treatments were considered to be of marketable size and quality. In the case of weed growth, the biomass of bittercress was considerably higher when the fertilizer was topdressed compared to when it was incorporated. Liverwort growth was substantially reduced in all the stratified substrates and in mulched compared to the industry standard substrate regardless of fertilizer placement. It is challenging to make broad recommendations for ornamental plants since the effect of fertilizer placement on crop growth is highly species-specific [42]. However, the effect of fertilizer placement on weed growth is more consistent [19,20,22,29,43]. Considering all available research in the area, both topdressing and incorporation appear to be compatible with substrate stratification or the use of mulch, but effects are likely species-dependent. More ornamental and weed species should be evaluated to determine optimal stratification techniques that provide the most crop growth and weed-suppressive benefits. Commercially, stratified substrates can be implemented if nurseries have adequate infrastructure, such as with the use of two soil hoppers that could fill containers with different substrates for the bottom and top layers. In addition to work needed on other weed and ornamental species, further study is required to analyze the cost associated with stratified substrates and compare it with other weed management methods.

Author Contributions

Conceptualization, all authors; methodology, S.C.M., Y.K., B.J.P. and J.C.; data curation, S.C.M., P.D. and Y.K.; writing—original draft preparation, Y.K. and S.C.M.; writing—review and editing, all authors; visualization, S.C.M. and Y.K.; supervision, S.C.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded in part by the FDACS Specialty Crop Block Grant Program. Grant no 27437.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to acknowledge Annette Chandler for technical assistance with this research. No conflicts of interest have been declared.

Conflicts of Interest

The authors declare no conflict of interest.

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Horticulturae 09 00747 g001 550
Figure 1. The difference in growth index for hibiscus grown in a substrate with fertilizer placement either topdressed or incorporated in the bottom strata. Measures were reported from 4 to 16 weeks after planting (WAP). Error bars represent standard errors from the mean. The same letters on each WAP are not significantly different according to the student-t test at p < 0.05.
Figure 1. The difference in growth index for hibiscus grown in a substrate with fertilizer placement either topdressed or incorporated in the bottom strata. Measures were reported from 4 to 16 weeks after planting (WAP). Error bars represent standard errors from the mean. The same letters on each WAP are not significantly different according to the student-t test at p < 0.05.
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Figure 2. Root and shoot dry weight of hibiscus grown in a substrate with fertilizer placement either topdressed or incorporated in the bottom strata. Error bars represent standard errors from the mean. The same letters on each fertilizer placement are not significantly different according to the student-t test at p < 0.05.
Figure 2. Root and shoot dry weight of hibiscus grown in a substrate with fertilizer placement either topdressed or incorporated in the bottom strata. Error bars represent standard errors from the mean. The same letters on each fertilizer placement are not significantly different according to the student-t test at p < 0.05.
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Figure 3. Shoot dry weight of bittercress grown in substrate with fertilizer placement either topdressed or incorporated in the bottom strata. Error bars represent standard errors from the mean. The same letters on each fertilizer placement are not significantly different according to student-t test at p < 0.05.
Figure 3. Shoot dry weight of bittercress grown in substrate with fertilizer placement either topdressed or incorporated in the bottom strata. Error bars represent standard errors from the mean. The same letters on each fertilizer placement are not significantly different according to student-t test at p < 0.05.
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Figure 4. The difference in surface area coverage of liverwort grown in substrate with fertilizer placement either topdressed or incorporated in the bottom strata. Error bars represent standard errors from the mean. The same letters on each substrate size are not significantly different according to student-t test at p < 0.05.
Figure 4. The difference in surface area coverage of liverwort grown in substrate with fertilizer placement either topdressed or incorporated in the bottom strata. Error bars represent standard errors from the mean. The same letters on each substrate size are not significantly different according to student-t test at p < 0.05.
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Table
Table 1. Influence of stratified substrate and fertilizer placement on the growth of hibiscus.
Table 1. Influence of stratified substrate and fertilizer placement on the growth of hibiscus.
Growth Index (cm) xBiomass w
ANOVA v4 WAP8 WAP12 WAP16 WAPShoot wt (g)Root wt (g)
Substrate zNS uNSNSNSNSNS
Placement y0.00130.01530.00310.0001<0.00010.0298
Substrate × PlacementNSNSNSNSNSNS
z Substrate consisted of small (0.63–1.27 cm), medium (≤1.90 cm), and large (0.96–1.90 cm) pine bark sizes as the top strata at a depth of 2.5 cm with the bottom strata made of ≤1.27 cm pine bark. A mulch treatment consisted of rice hulls at a depth of 2.5 cm, and an industry-standard substrate consisted of ≤1.27 cm pine bark used throughout the container. y The fertilizer was either topdressed or incorporated in the bottom strata in all the substrate treatments. x Growth index was determined by calculating [(height + width at widest point + perpendicular width) ÷ 3] from 0 to 16 weeks after planting (WAP). w Shoot and root dry wt was taken at the trial conclusion at 16 WAP. v Analysis of variance was performed using a mixed model in JMP to test for the significance of main effects and interactions. Effects are considered significant at p < 0.05. u NS stands for the non-significant difference.
Table
Table 2. Influence of stratified substrates and fertilizer placement on emergence and biomass of bittercress.
Table 2. Influence of stratified substrates and fertilizer placement on emergence and biomass of bittercress.
Substrate zWeed (No.) yBiomass x
4 WAP10 WAP
TopdressedIncorporatedTopdressedIncorporated
Small2.7 ab v4.6 ab2.7 ab3.9 ab2.3 ab
Medium2.7 ab2.7 bc1.8 ab2.4 bc1.6 bc
Large3.3 a5.4 a2.7 ab4.9 a2.6 ab
Mulch0.1 b2.5 bc1.5 b2.1 bc0.5 c
Standard5.1 a1.9 c4.1 a2.0 c3.2 a
ANOVA w
Substrate<0.00010.0011<0.0001
PlacementNSNS0.0012
Substrate × placement<0.00010.0024NS
z Substrate consisted of small (0.63–1.27 cm), medium (≤1.90 cm), and large (0.96–1.90 cm) pine bark sizes as the top strata at a depth of 2.5 cm with the bottom strata made of ≤1.27 cm pine bark. A mulch treatment consisted of rice hulls at a depth of 2.5 cm, and an industry-standard substrate consisted of ≤1.27 cm pine bark used throughout the container. The fertilizer was either topdressed or incorporated in the bottom strata in all the substrate treatments. y Weed count was assessed by surface sowing 25 seeds of bittercress to each container and counting germinated seedlings at 4 weeks and 10 weeks after planting (WAP). x Shoot dry wt was taken at trial conclusion at 10 WAP. w Analysis of variance was performed using a mixed model in JMP to test for significance of main effects and interactions. Effects are considered significant at p < 0.05. NS stands for non-significant difference. v Mean effect means of substrate within a column and followed by the same lower-case letter are not considered significantly different based on Tukey’s HSD test at p < 0.05.
Table
Table 3. Leachate sample analysis. The samples were collected through the pour-through method and analyzed for ammonium (NH4N), nitrate nitrogen (NO3N), orthophosphate (OrthoP), potassium (K), pH, and electrical conductivity (EC).
Table 3. Leachate sample analysis. The samples were collected through the pour-through method and analyzed for ammonium (NH4N), nitrate nitrogen (NO3N), orthophosphate (OrthoP), potassium (K), pH, and electrical conductivity (EC).
Substrate yNH4N zNO3NOrthoPKpHEC
(ppm)(ppm)(ppm)(ppm)
Small15 ab v19.9 3.4 25.1 5.9 b0.6
Medium16.9 a19.3 3.6 26.6 6.1 ab0.6
Large17.3 a19.8 3.7 29.0 6.1 ab0.6
Mulch8.7 b12.3 2.7 20.2 6.6 a0.5
Standard14.6 ab20.4 3.5 23.5 5.9 b0.6
Fertilizer placement x
Topdressed15.6 23.4 a3.6 29.6 a6.1 0.6 a
Incorporated13.4 13.3 b3.2 20.2 b6.2 0.4 b
ANOVA w
Substrate0.0214NSNSNS0.0209NS
Fertilizer placementNS<0.0001 xNS<0.0001 xNS<0.0001 x
Substrate × Fertilizer placementNSNSNSNSNSNS
z Leachate samples were collected from 3 random replicates per treatment every 4 weeks for 16 weeks using the pour-through method. The leachate samples were analyzed for ammonium (NH4N), nitrate nitrogen (NO3N), orthophosphate (OrthoP), potassium (K), pH, and electrical conductivity (EC). y Substrate consisted of small (0.63–1.27 cm), medium (≤1.90 cm), and large (0.96–1.90 cm) pine bark sizes as the top strata at a depth of 2.5 cm with the bottom strata made of ≤1.27 cm pine bark. A mulch treatment consisted of rice hulls at a depth of 2.5 cm, and an industry-standard substrate consisted of ≤1.27 cm pine bark used throughout the container. x The fertilizer was either topdressed or incorporated in the bottom strata in all the substrate treatments. w Analysis of variance was performed using a mixed model in JMP to test for significance of main effects and interactions. Effects are considered significant at p < 0.05. NS stands for non-significant differences. v Means followed by the same letter within a column are not significantly different.
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Khamare, Y.; Marble, S.C.; Pearson, B.J.; Chen, J.; Devkota, P. Evaluation of Substrate Stratification, Fertilizer Placement, and Mulching on Growth of Common Nursery Weed Species and Container-Grown Ornamental Species. Horticulturae 2023, 9, 747. https://doi.org/10.3390/horticulturae9070747

AMA Style

Khamare Y, Marble SC, Pearson BJ, Chen J, Devkota P. Evaluation of Substrate Stratification, Fertilizer Placement, and Mulching on Growth of Common Nursery Weed Species and Container-Grown Ornamental Species. Horticulturae. 2023; 9(7):747. https://doi.org/10.3390/horticulturae9070747

Chicago/Turabian Style

Khamare, Yuvraj, Stephen C. Marble, Brian J. Pearson, Jianjun Chen, and Pratap Devkota. 2023. "Evaluation of Substrate Stratification, Fertilizer Placement, and Mulching on Growth of Common Nursery Weed Species and Container-Grown Ornamental Species" Horticulturae 9, no. 7: 747. https://doi.org/10.3390/horticulturae9070747

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