Abstract
Preemergence herbicide application under plastic mulch is an effective way to manage weeds in tomato (Solanum lycopersicum L.) production. Nonetheless, applying herbicides beneath plastic mulch in raised beds is associated with the inherent risk of crop phytotoxicity. This highlights the need to explore crop-safe methods for herbicide application in plastic mulch beds. The research aimed to evaluate the effectiveness of preemergence herbicides S-metolachlor and flumioxazin at the labeled rate (X) and reduced rate (0.5X, half of the labeled rate) either alone or in combination with a super absorbent polymer, soil binding agent, or compost to determine their ability to control weeds effectively in plastic mulched beds without causing negative effects on tomato crops. Two different experimental trials, trial 1 (Mar–Jun 2021) and trial 2 (Oct 2021–Jan 2022) were performed in the fields in Immokalee, FL, USA. The experimental design was a randomized complete block with five replications. S-metolachlor (X) plus soil binding agent and S-metolachlor (X) plus compost mix suppressed purple nutsedge density by >85% and approximately 68%, respectively, during trial II. Similarly, S-metolachlor (X), S-metolachlor (0.5X) plus super absorbent polymer, S-metolachlor (X) plus super absorbent polymer, S-metolachlor (X) plus soil binding agent, and S-metolachlor (X) plus compost caused reductions in purple nutsedge biomass by >50% compared with that of the nontreated control during trial II. Treatments did not significantly impact tomato crop vigor and chlorophyll contents during trials I and II. Moreover, treatments did not significantly affect tomato crop biomass and yield during both trials. In summary, using preemergence S-metolachlor (X) combined with soil binding agent or compost could be a viable option for suppressing purple nutsedge in plastic mulched tomato beds.
Tomato (Solanum lycopersicum L.) is one of the largest agricultural commodities grown in Florida and contributes $426 million to the state economy (Chanda et al. 2021). In Florida, fresh market tomatoes were harvested from 29,900 acres, making the state a leading producer in 2020 (US Department of Agriculture, National Agricultural Statistics Service 2020). Tomato crops are commercially grown in a plasticulture production system (applying a polyethylene mulch over a raised bed) throughout the year in Florida (Freeman et al. 2022; Lament 1993; Yu et al. 2020). Immokalee, a key tomato production area in South Florida, features very sandy or gravelly loam soil originating from limestone bedrock. The soil has low organic matter (<2%) and a pH between 6.5 and 7; this region frequently experiences heavy rainfall with an average more than 1000 mm from June to October (Hasanuzzaman et al. 2020; Wang et al. 2002; Zhang et al. 2017).
Weed control is a major production challenge for tomato growers in Florida (Boyd 2015; Tiwari et al. 2022). Purple nutsedge (Cyperus rotundus L.), a perennial weed species, is problematic in Florida’s tomato plasticulture (Tiwari et al. 2024; Van Wychen 2016). The use of plastic mulch does not adequately control this weed because of its pointed shoot tips that can pierce through the mulches (Stoller and Sweet 1987; Wills 1987). The nutsedge rhizomes develop multiple shoots propagating rapidly under the plastic mulch within a few weeks (Adcock et al. 2008; Yu et al. 2020). For example, a study by Webster (2005) reported the development of 3440 purple nutsedge shoots in a 22.1-m2 patch from a single purple nutsedge tuber within 60 weeks. Tomato yield can be influenced by multiple factors, such as the nutsedge density (Morales-Payan et al. 1997) and the timing of competition (Knezevic et al. 2002; Weaver and Tan 1983). Morales-Payan et al. (1997) observed a 44% yield reduction in tomatoes from 200 purple nutsedge shoots/m2 under greenhouse studies.
The loss of methyl bromide in 2005 led the Florida vegetable growers to adopt alternative fumigants that provided inconsistent weed control. Since then, growers have transitioned to the use of preemergence herbicides for weed management (Santos et al. 2006; Yu et al. 2019). One of the major problems with the extensive use of preemergence herbicides, particularly at high rates, is the potential for crop phytotoxicity (Boyd 2014; Dhouib et al. 2016; Le Bellec et al. 2015).
S-metolachlor (Dual Magnum®; Syngenta Crop Protection, Greensboro, NC, USA), a preemergence herbicide that belongs to the chloroacetamide family, is applied as preemergence to the preformed beds before laying plastic mulch (Bangarwa et al. 2009; Devkota et al. 2013; Kanissery et al. 2021). S-metolachlor is a very-long-chain fatty acid inhibitor that effectively controls annual monocots, small-seeded dicots, and perennial yellow nutsedge species (Bach and Faure 2010; Böger 2003; Fuerst 1987). This group of herbicides is absorbed by both the roots and shoots of emerging seedlings. The roots and shoots of the affected plants tend to have the inhibited growth (Pillai et al. 1979). It has been reported that soil organic matter significantly enhances the degradation of S-metolachlor. Because the application rates influence both atmospheric volatilization and soil degradation of chemicals, S-metolachlor and other herbicides used in the field should be applied at the minimum rate necessary to achieve effective weed control (Long et al. 2014).
Flumioxazin (Chateau®; Valent, Walnut Creek, CA, USA) is a broad-spectrum preemergence herbicide that belongs to the N-phenyl phthalimide family (Matringe et al. 1989). This herbicide is applied between the raised beds in vegetables (Boyd 2016); however, it is also used under the plastic mulch in tomatoes and small fruit crops, such as strawberries [(Fragaria ×ananassa (Wetson) Duchesne ex Rozier (pro sp.) [chiloensis × virginiana)] because it suppresses yellow nutsedge (Cyperus esulentus L.) (Boyd and Sandhu 2021; Tiwari et al. 2022). It is a protoporphyrinogen oxidase (PPO) that inhibits herbicides and acts by inhibiting the PPO enzyme (Duke et al. 1991; Lee and Duke 1994). This herbicide is absorbed by the roots and shoots of the plants with limited symplastic movement because of the rapid foliar desiccation and burn-down (Boyd 2014). Microbial degradation is considered the primary factor that affects flumioxazin persistence in Tifton and Greenville soils, where only 2% mineralization has been reported in both soil types (Ferrell and Vencill 2003).
The soil characteristics of Immokalee include fewer binding sites for preemergence herbicides, ultimately causing herbicide leaching following a significant rainfall event. This leaching may adversely affect the herbicide efficacy, impacting crop establishment with inadequate weed suppression (Heap 2014; Rhue and Everett 1987). To overcome these challenges and gain increased herbicidal efficacy and crop safety, crop-safe applications of these preemergence herbicides under plastic mulch for tomatoes are needed.
Superabsorbent polymers are hydrophilic in nature (Li et al. 2023). They are extensively used in agriculture as water-holding reservoirs, nutrient mobilizers, seed-coating agents, and carriers for fertilizers or pesticides to facilitate seed germination and enhance crop yield (Palanivelu et al. 2022; Peppas et al. 2012). Herbicide applications using superabsorbent polymers involve mixing polymer granules with the herbicide solution and allowing the granules to absorb the herbicide. Applying these herbicide-soaked granules facilitates the slow release of the active ingredient (a.i.) and protects them from degradation by light, moisture, or soil microorganisms (Marimuthu et al. 2022). A superabsorbent-based delivery system provides a slow-release profile for herbicides into the soil over time (Campos et al. 2015). In these systems, solute release primarily occurs through diffusion. As water is absorbed into the matrix, it simultaneously facilitates the diffusion of solutes, such as the a.i. of herbicides. These polymer granules, also referred to as hydrogels, have a high swelling capacity and release the trapped a.i. slowly and in a controlled manner when mixed with herbicide solutions and broadcasted over the soil (Campos et al. 2015). Although superabsorbent polymers have been studied to determine their ability to regulate the release of agricultural chemicals and reduce leaching (Marimuthu et al. 2022), their use as a slow-release carrier for preemergence herbicides aimed at improving effectiveness and safety in tomato production has not yet been explored.
Soil binding agents are adjuvants that enhance the effectiveness of herbicides applied to soil (Tiwari et al. 2022). When combined with herbicides before spraying, these adjuvants improve the herbicide’s binding to the soil, thus helping it remain longer in the “weed germination zone”—the 4- to 8-inch-deep soil layer where most weed seeds are found. To effectively suppress weed seed germination, the preemergence herbicide must stay within this zone. Additionally, this improved retention reduces the risk of herbicide leaching into the crop root zone, where it could potentially harm the crops (Hauser 1962; Martins et al. 2016). Including a suitable adjuvant can improve herbicide effectiveness, thus reducing the total amount of herbicide required to achieve desired results and lowering weed management expenses (Green 1992). Previous studies have demonstrated that incorporating the oil-based soil-binding adjuvant Grounded® (Helena Agri Enterprises, LLC, Collierville, TN, USA) into the herbicide tank resulted in a runoff loss reduction of 17% to 40% across all tested preemergence herbicides when applied to bare soil (Fillols and Davis 2020). Nevertheless, no research conducted to date has investigated the impact of these adjuvants on improving the retention of preemergence herbicides in sandy soils during vegetable production.
Compost application is a soil amendment approach in vegetable production that is used to stimulate plant growth and suppress soil-borne diseases (Duong 2013; St. Martin and Brathwaite 2012). Composts are often used as organic mulch in horticultural practices and contribute to an ecological approach to weed management (Altieri 1988). For instance, compost extracts rich in biological agents can help reduce weed seed banks in the soil (Jerkins 2018). The maturity of compost and its application depth can significantly impact weed and sensitive crop germination and emergence because of the production of phytotoxic compounds (Niggli et al. 1990; Ozores-Hampton et al. 1998). Additionally, compost application often reduces herbicide mineralization and helps stabilize herbicide residues (Barriuso et al. 1997).
The utilization of compost in conjunction with plastic mulching has been assessed in vegetable plasticulture production systems and has demonstrated positive effects on soil quality and crop yield (Wang et al. 2010). Studies of tomatoes in Europe and North America reported an increase in soil organic matter, improvements in crop biomass, yield, quality, microbial biomass, and a reduction in the incidence of soil-borne disease (Fusarium wilt) from compost application (Morra et al. 2021; Ozores-Hampton et al. 1998; Rani and Tripura 2021; Wilson et al. 2018; Zhang et al. 2002).
Conversely, adding compost to soil can have negative effects, such as the buildup of harmful substances like ammonia from animal manure, soluble salts, and pathogenic microorganisms. These problems can reduce seed germination and stunt plant growth (Jones and Huang 2003). Additionally, compost application is linked to increased weed emergence, as observed during studies of tomatoes (Cheimona et al. 2016) and citrus (Albrecht et al. 2023). The authors attributed this to the gradual release of nutrients from the compost, which benefits weeds and their soil seed bank.
It has been well-established that soil type and organic matter content significantly influence herbicide behavior in soil, including adsorption or binding, desorption, degradation, and leaching (Kanissery et al. 2020; Weber 1990), as well as its efficacy (Blumhorst et al. 1990). For example, herbicide adsorption can be enhanced when dissolved organic matter derived from compost applications creates additional sites for herbicide adsorption in the soil (Businelli et al. 1998). Therefore, adjusting the application procedure may be necessary for the herbicide–compost combinations to enhance the effectiveness. Consequently, additional research is imperative to evaluate the integration of preemergence herbicides into the integration of compost and plastic mulching to effectively mitigate the risk of weed infestations in the plastic-mulched beds.
The objectives of this study were to evaluate the ability of preemergence herbicides S-metolachlor and flumioxazin at the labeled rate (X) and half of the labeled rate (0.5X) alone as blanket sprays, which involve applying herbicides uniformly across the raised bed to achieve full coverage, or in combination with a super absorbent polymer, soil binding agent, or compost to improve weed control efficacy and to determine crop safety in tomato plasticulture production in the Immokalee region in southwest Florida.
Materials and Methods
Study location.
Two field trials were conducted in the Spring and Fall 2021 at the University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) Southwest Florida Research and Education Center (SWFREC) (26°28′7′′N, 81°26′22′′W), Immokalee, FL, USA. The experimental field had an even distribution of purple nutsedge throughout each production season. However, the site has a history of fluctuating weed pressure from season to season. The soil at the experimental site is classified as Spodosol fine sand with >90% sand and a of pH of 6.1, and it has a subtropical climate in the region (Gairhe et al. 2021; websoilsurvey.sc.egov.usda.gov). The monthly total rainfall and the mean air temperature data from Mar 2021 to Jan 2022 (Fig. 1) were obtained from the Florida Automated Weather Network (FAWN) located at the UF/IFAS-SWFREC in Immokalee, FL, USA. Before bed preparation, the field was cultivated and irrigated to ensure tilth and adequate soil moisture. The entire field was irrigated before bed preparation using a Model T/E 30 × 980 AG-RAIN Water-Reel (Schrader Avenue, Havana, IL, USA). When the beds were formed, using one bed super bedder (Kennco Manufacturing Inc., Ruskin, FL, USA), a CASE IH Farmall Compact C Series II Tractor (Indio, CA, USA) with a bed shaper (Kennco Manufacturing Inc.) was used to shape and press the bed. At the beginning of the experiment during bedding, fertilizer containing NH4, P2O5, and K2O at 226–168–252 kg/ha (the recommended rates) were placed in a narrow strip below the soil surface, thus exposing it on the bed with CASE IH Farmall Compact C Series II Tractor (Indio, CA, USA) using a stainless 1-bed fertilizer distributor (Kennco Manufacturing Inc.). Drip tape (Jain Irrigation Inc., Haines City, FL, USA) was installed just beneath the soil surface of each bed at a depth of 4 cm in two rows for bed irrigation. The tape consisted of the emitters every 30 cm with a flow rate of 0.95 L/min/30 m.
Monthly total rainfall (mm) and average air temperature (°C) recorded from Mar 2021 to Jan 2022. The research duration was from Mar to Jun 2021 for trial I and Oct 2021 to Jan 2022 for trial II. Data were generated from the Florida Automated Weather Network (FAWN) located at the Southwest Florida Research and Education Center, Immokalee, FL, USA, where the field experiments occurred.
Citation: HortScience 59, 12; 10.21273/HORTSCI18095-24
Experimental design and treatment application.
The experiment was performed using a randomized complete block design with full factorial combinations with five replications. The size of the individual plot was 3 m long with a 0.75-m-wide raised bed and an alley of 1 m between the neighboring plots. Treatments included the application of preemergence S-metolachlor (Dual Magnum®; Syngenta Crop Protection, LLC, Greensboro, NC, USA) at rates of 0.5 kg a.i./ha (0.5X) and 1 kg a.i./ha (for coarse soils less than 3% organic matter) (X) and flumioxazin (Chateau®; Valent, Walnut Creek, CA, USA) at rates of 0.14 kg a.i./ha (0.5X) and 0.27 kg a.i./ha (X) alone as blanket sprays (Freeman et al. 2022) or in combination with superabsorbent polymer (Soil Vigor®; Eco-Novelty Corp., Troy, MI, USA), soil binding agent (Grounded®; Helena Agri-Enterprises, LLC), or compost (Florida Soil Builders Inc., Immokalee, FL, USA). This type of compost was used because it is representative of the type of compost produced in Florida at the time when the study was conducted. It uses a mixture of yard waste with a carbon-to-nitrogen ratio of 24 to 30:1; no animal byproducts are involved in the process (Ozores-Hampton 2017). A nontreated control was also included for comparison with the treatments. Soil fumigants were not used during this study because they suppress nutsedge, and one of the objectives of this study was to evaluate the treatments for purple nutsedge control. The superabsorbent polymer, also known as water retention granules, was thoroughly mixed with preemergence herbicide solution at a 20:1 ratio (i.e., 20 mL herbicide solution to 1 g of the absorbent medium). Treatments were mixed in a 5-gallon plastic bucket (Northern Tool and Equipment Company Inc., Burnsville, MN, USA) following safety precautions. To ensure the homogeneity in the herbicides and superabsorbent polymer mix gel, the mixture was placed inside a Klutch Portable Electric Cement Mixer in a 4.25-feet3 drum (Northern Tool and Equipment Company Inc., Burnsville, MN, USA) and rotated for approximately 30 min. The herbicides combined with the superabsorbent medium were spread by hand over the top of the beds to ensure even coverage. Safety precautions were followed, including wearing safety boots, glasses, gloves, and protective clothing (Tambe et al. 2019). Each plot received 136.20 g of the superabsorbent herbicide mixture. Preemergence herbicides combined with soil binding agent were sprayed over the bed tops in two passes with 121.55 L·ha−1 of water using a backpack sprayer (Bellspray Inc., Opelousa, LA, USA) equipped with a single ATR 80 nozzle (Teejet Technologies, Wheaton, IL, USA) at a pressure of 0.24 MPa. Preemergence herbicide with the compost was thoroughly mixed using the cement mixer (the same process as that described for superabsorbent polymer) and was applied to the top of the bed by hand, covering the bed’s entire area. Each plot received 1563.36 g of compost–herbicide mixture. The treatments were applied on 3 Mar 2021 for trial I and on 30 Sep 2021 for trial II. The planting beds were covered with 1.25-mil plastic mulch (Intergro, Berry Plastics Corporation, Evansville, IN, USA) that was mechanically installed using raised bed plastic mulch layer equipment. Holes were punched in a single row with 20-cm spacing using a mechanical hole puncher. All plots were transplanted with tomato HM 1823, a widely used commercial cultivar in the production region. Tomato seedlings were transplanted in a single row with 20-cm spacing between plants on 16 Mar 2021 for trial I and on 28 Oct 2021 for trial II. The longer gap between the treatment application and transplanting in trial II was attributable to the need to re-transplant all the tomato seedlings in the experimental plots because they initially performed poorly. The irrigation practices throughout the cropping season in tomato transplants followed the industry standards (Freeman et al. 2022).
Data Collection
Weed counts and biomass.
The total number of purple nutsedge shoots that penetrated the plastic mulch over the entire bed was counted at 4, 8, and 12 weeks after transplanting (WAT) for trials I and II.
Purple nutsedge biomass was hand-harvested using FELCO F-2 Classic Manual Hand Pruner (Pygar USA Inc., Seattle, WA, USA) from the entire plot at the end of the season. The weed biomass sample of this species was placed in paper bags and oven-dried for 2 d, and the dry weight was recorded.
Crop vigor.
Crop vigor was recorded by choosing three representative plants from the center of each plot at 4 and 8 WAT. The visual estimation scale was 0 to 10, where 0 is a completely desiccated or dead tomato plant and 10 is a completely healthy tomato plant. Chlorosis, necrosis, stunting, and deformed plants were visually assessed to estimate the crop vigor.
Chlorophyll content.
Leaves were chosen from three tomato plants in the middle of each plot to determine the chlorophyll content at 4 and 8 WAT. A soil plant analysis development (SPAD) 502DL chlorophyll meter (Konica-Minolta Corporation, Wayne, NJ, USA) and a nondestructive optical technique were used to measure the chlorophyll content of the leaves. Before performing measurements, the device was calibrated using a reading checker provided by the manufacturer (Jiang et al. 2017).
Tomato yield and biomass.
When the tomato fruit reached the breaker stage, three tomato plants from the center of each plot were chosen for the fruit harvest. Tomatoes were hand-picked on 21 May and 7 Jun 2021 for trial I, and on 13 and 31 Jan 2022 for trial II. Fruits were graded as small (diameter, <5.5 cm), medium (diameter, between 5.5 and 6.5 cm), large (diameter, between 6.5 and 7 cm), or extra-large (diameter, >7 cm) when recording the numbers of the harvested fruits (US Department of Agriculture, Agricultural Marketing Service 2005). Then, all the graded fruits were combined and weighed to record the marketable yield.
Finally, the aboveground tomato biomass was hand-harvested using the FELCO F-2 Classic Manual Hand Pruner (Pygar USA Inc., Seattle, WA, USA) by choosing the same three plants from which fruits were previously harvested for yield. The crop was harvested on 10 Jun 2021 for trial I and on 1 Feb 2022 for trial II. The harvested tomato plant samples were oven-dried at 65 °C for approximately 2 d to record the dry weight.
Statistical analysis.
Data were subjected to an analysis of variance using R programming (version 2022.07.01). Replications were considered as random effects, whereas the treatments were considered fixed effects. The purple nutsedge counts, crop vigor, and chlorophyll content data were obtained multiple times during the season. Therefore, they were analyzed using a repeated-measure analysis. Data assumptions were checked for normality and homogeneity of variance before the analysis. Treatment means were separated using Tukey’s honestly significant difference test at α = 0.05. There was a significant interaction between the two trials for most variables (P < 0.001) (Table 1); therefore, the data for each trial were analyzed and presented separately.
Analysis of variance of the effects of trials, time point, and treatments and the interactions between trials and time point, trials and treatments, time points and treatments, and trials, time points and treatments on purple nutsedge density, crop vigor, chlorophyll content, and yield at the Southwest Florida Research and Education Center, Immokalee, FL, USA.
Results
Effect of treatments on weed density and biomass.
Table 2 presents the treatment effects at different data collection dates for the purple nutsedge densities during trials I and II. For trial I, the purple nutsedge density was relatively lower than that of trial II. None of the treatments significantly affected weed density compared with that of the nontreated control in trial I (Table 2). In both trials, S-metolachlor or flumioxazin alone at either rate was not different from the nontreated control for purple nutsedge density. In trial II, S-metolachlor (X) plus soil binding agent significantly suppressed purple nutsedge density by >85% compared with the that of nontreated control at 4, 8, and 12 WAT. Similarly, S-metolachlor (X) plus compost significantly reduced purple nutsedge density by >65% compared with the nontreated control at 12 WAT in trial II (Table 2). However, in trial II, the combination of S-metolachlor (0.5X) plus compost resulted in a significant increase in purple nutsedge density of more than 61% compared with that of S-metolachlor (0.5X) alone at 4 and 8 WAT. Conversely, applying flumioxazin (X) plus compost led to a reduction in purple nutsedge density of approximately 65% compared with that of flumioxazin (X) alone at 8 WAT in trial II.
Purple nutsedge density in beds with tomato cultivar HM 1823 at the Southwest Florida Research and Education Center Immokalee, FL, USAi.
Treatments containing S-metolachlor (X), S-metolachlor (0.5X) plus super absorbent polymer, S-metolachlor (X) plus super absorbent polymer, S-metolachlor (X) plus soil binding agent, and S-metolachlor (X) plus compost significantly reduced purple nutsedge biomass by >50% compared with that of the nontreated control in trial II (Table 3). Flumioxazin alone as a blanket spray at both rates or in combination with super absorbent polymer, soil binding agent, and compost did not have a significant effect on purple nutsedge density (Table 3). However, in trial II, flumioxazin (0.5X) plus soil binding agent reduced purple nutsedge biomass by 41% compared with that of flumioxazin (0.5X) alone.
Purple nutsedge biomass during trials I and II at the Southwest Florida Research and Education Center, Immokalee, FL, USAi.
Effect of treatments on crop vigor and the chlorophyll content of the leaf.
In trials I and II, none of the treatments affected crop vigor (P > 0.05) compared with that of the nontreated control (Table 4). Treatments did not show any significant effect on the chlorophyll content of the leaf compared with that of the nontreated control in trials I (P = 0.15) and II (P = 0.54) (Table 5).
Visual estimate of crop vigor of HM 1823 (from 0 to 10, where 0 is complete kill and 10 is complete healthy plant) at the Southwest Florida Research and Education Center, Immokalee, FL, USAi.
Relative chlorophyll content of tomato crops at the Southwest Florida Research and Education Center, Immokalee, FL, USAi.
Effect of treatments on crop yield and biomass.
Yield and biomass of tomatoes were not significantly affected (P > 0.05) by any of the treatments compared with those of the nontreated control in trials I and II (Table 6).
Aboveground tomato biomass following treatment applications at the Southwest Florida Research and Education Center, Immokalee, FL, USA. Tomato yields averaged across harvests on 21 May and 7 Jun for trial I and on 13 and 31 Jan for trial II following the treatment application under plastic mulch at the Southwest Florida Research and Education Center, Immokalee, FL, USAi.
Discussion
Purple nutsedge interference has been a major threat to successful vegetable production worldwide (Wallace et al. 2013). Purple nutsedge control is challenging because of its extensive tuber production, rapid vegetative propagation, adaptability to various environmental conditions, and efficient C4 photosynthetic pathway (Miller and Dittmar 2014; Wills 1987). In this study, S-metolachlor (X) plus soil binding agent controlled purple nutsedge density by more than 85% compared with that of the nontreated control at 4, 8, and 12 WAT in trial II (Table 2). Similarly, flumioxazin (0.5X) plus soil binding agent reduced purple nutsedge biomass by 41% when compared with that of flumioxazin (0.5X) alone in trial II (Table 2). The soil binding agent used in our study was an oil-based adjuvant that limited the efficacy and adsorption of soil-applied herbicides, improved drift by reducing bouncing and subsequent off-target scattering of spray droplets, and enhanced herbicide application efficiency (Tiwari et al. 2022). When combined with preemergence herbicide, the soil binding agent binds free ions in the soil and pulls them together, keeping the herbicide’s a.i. in the soil where new weed seeds germinate (Fillols and Davis 2020). It has been reported that some adjuvants have exceptional soil binding properties, mostly regarding the reduction of leaching. For example, Atpolan® Soil Maxx (Agromix, Busko-Zdrój, Poland) reduced the leaching and increased the efficacy of preemergence herbicides such as metazachlor and clomazone (Woznica et al. 2016). Kočárek et al. (2018) reported that using the adjuvant Grounded® increased the soil adsorption of the herbicide pendimethalin. It has been reported that the use of Grounded® led to promising results in freshly tilled bare soil by reducing herbicide concentration in runoff while maintaining or slightly improving weed control (Fillols and Davis 2020). The 0.5X rate of S-metolachlor with a soil binding agent had no impact on weed suppression in any of the trials in our study. However, Schans (2001) and Borone et al. (2003) examined the effects of various plant oil derivative adjuvants, such as Renol (Filtersystemen, Leimuiden, Netherlands) and Trend 90 (Agri-Trend LLC, Twin Falls, ID, USA), with low doses of herbicides such as bentazone (0.125–0.5 L/ha), pyridate (0.25–1.0 L/ha), sulcotrion (0.125–0.25 L/ha), and nicosulfuron (0.25-0.5 L/ha). They found that herbicide effectiveness increased by 5% to 50%, whereas the application rate was reduced by 25%.
The X rate of S-metolachlor, when combined with the compost, reduced purple nutsedge density by approximately 69% and biomass by >50% at 12 WAT during trial II when compared with that of the nontreated control (Tables 2 and 3). Similarly, in trial II, the combination of flumioxazin (X) and compost reduced purple nutsedge by approximately 65% compared with using flumioxazin (X) alone at 8 weeks after transplanting. This effect may be attributable to the organic mulches suppressing weeds through their physical presence on the soil surface. For example, weed seeds and tubers that germinate close to the soil may struggle to grow effectively under compost layers, which can restrict their ability to photosynthesize (Crutchfield et al. 1986; Facelli and Pickett 1991). Nutsedge suppression may also be attributed to the allelopathic effects of compost on its growth. The release of phytotoxic compounds by microbes during the composting process has been shown to inhibit plant growth and development (Foshee et al. 1996; Niggli et al. 1990; Ozores-Hampton et al. 1998). Additionally, incorporating organic amendments like compost with the preemergence herbicide can facilitate a slow release of the herbicide’s a.i., thus reducing leaching (Zsolnay 1992) and enhancing weed suppression. However, in trial II, purple nutsedge density increased by more than 61% with the combination of S-metolachlor (0.5X) and compost compared with that associated with S-metolachlor (0.5X) alone at both 4 WAT and 8 WAT. The emergence of weeds could be influenced by specific environmental factors, with the type of weed determined by the environmental conditions in which a crop is grown. Typically, the growth media provides little to no support for weed proliferation (Osadebe et al. 2023).
S-metolachlor and flumioxazin alone, either at the 0.5X rate or at the X rate, had no significant impact on purple nutsedge density compared with the nontreated control for trials I and II. However, in trial II, S-metolachlor at X reduced purple nutsedge biomass by more than 50% compared with the nontreated control (Table 3). This reduction in biomass is significant for long-term suppression because lower biomass is directly linked to a decrease in the reproductive and spreading potentials of nutsedge (Dor and Hershenhorn 2013). A study by Bangarwa et al. (2009) indicated that the application of S-metolachlor as a preemergence herbicide provided control of yellow nutsedge; however, it was inconsistent for purple nutsedge control. The half-life of S-metolachlor ranges between 15 to 25 d, whereas flumioxazin has moderate persistence and a reported half-life of 21 d in southern US soils (Mueller et al. 2014; Tiwari et al. 2022). The short half-life of these preemergence herbicides may not allow for the longer persistence of the residues in the soil, thereby limiting their ability to suppress the growth of purple nutsedge (Boyd and Reed 2016; Tiwari et al. 2022). Flumioxazin, when applied without compost at both rates and in combination with compost under plastic mulch, did not demonstrate a notable effect on the suppression of purple nutsedge compared with that of the nontreated control. Microbial populations are the main agents of its degradation when in solution (Ferrell and Vencill 2003). In a study by Boyd (2014), combining flumioxazin with burndown products such as paraquat or carfentrazone resulted in 81% to 90% broadleaf weed suppression compared with that of the nontreated control in the row middle areas between plastic-mulched vegetable beds.
S-metolachlor (X) alone and both rates of S-metolachlor when combined with superabsorbent polymer reduced the purple nutsedge biomass by more than 50% compared with that of the nontreated control during trial II (Table 3). A study by Marimuthu et al. (2022) reported that hydrogel-based herbicide applications stabilize the a.i. and prevent its degradation from light, moisture, and soil microorganisms. A considerable reduction in the environmental degradation of the preemergence norflurazon was obtained by using a cellulose-based polymer as its slow-release carrier (Sopeña et al. 2011).
The sedges grow best under conditions with high soil moisture (Kaur et al. 2018). Soil moisture and rainfall influence weed seedling emergence, with annual bluegrass (Poa annua L.) being one of the primary factors that affects weed seedling density (Holm et al. 1977; Roberts and Potter 1980). The plots received comparatively higher rainfall during Fall 2021 than that in spring (Fig. 1), which could have triggered more purple nutsedge emergence during trial II.
In trials I and II, none of the treatments caused significant damage to the tomato crops compared with that of the nontreated control (Table 4). Even when applied at rates of 530 g a.i./ha and 1070 g a.i./ha, S-metolachlor did not cause significant damage to the strawberry in plasticulture systems, as reported by Boyd and Reed (2016). The chlorophyll content, as measured by the SPAD index, is a dimensionless quantity that acts as a diagnostic health indicator of plants (Peng et al. 1993). None of the herbicides alone or combined with a superabsorbent polymer, soil binding agent, or compost mixtures significantly affected the chlorophyll content of the leaf (Table 5). A similar study by Boyd (2015) reported no significant damage to the Charger and Florida 47 tomato cultivars when S-metolachlor was applied under plastic mulch before their transplanting in Balm, FL, USA. The results from this study suggest that the treatments could likely be safely adopted by tomato growers.
None of the herbicides or the herbicide mixture with superabsorbent polymer, soil binding agent, and compost influenced tomato HM1823 yield and biomass (Table 6). Similarly, Samtani et al. (2012) reported that flumioxazin, at rates as high as 210 g a.i./ha, did not consistently affect strawberry yields. Additionally, applying flumioxazin at rates four-times higher than that recommended on the label did not harm strawberry plants, lead to increased plant mortality, or decrease berry yield (Samtani et al. 2012). Tomato yields were comparatively lower during trial II than during trial I, which could be associated with the variations in climate (i.e., the rainfall and temperature events) and field conditions at crop transplanting (Fig. 1). The lack of significant herbicide effects on tomato yield is not surprising because these results are consistent with those of other experiments involving tomatoes during which the researchers found that preemergence herbicides such as S-metolachlor, fomesafen (Reflex; Syngenta, Greensboro, NC, USA), and oxyfluorfen (Goal; Dow AgroSciences LLC, Indianapolis, IN, USA) did not affect yield (Adcock et al. 2008; Boyd 2015). Morales-Payen et al. (1997) found that increasing the density of purple nutsedge to 200 plants/m2 reduced tomato yield. However, in our study, the density of purple nutsedge may have been too low to significantly affect tomato yield under field conditions. Additionally, Chaudhari et al. (2016) and Garvey et al. (2013) highlighted that the critical weed-free period for tomatoes typically lasts 3 to 6 weeks after transplanting. Therefore, any emergence of purple nutsedge after this period may not have posed a significant competitive threat, which aligns with our findings. However, from a management standpoint, purple nutsedge that emerges late in the season is difficult to control because its tubers can persist in the soil for several seasons and reproduce quickly (Neeser et al. 1997). Despite these findings, a limitation of our study was that it did not include a control to evaluate the effects of superabsorbent polymers, soil binding agents, or compost individually.
Conclusion
Based on the two trials, the approach of using preemergence herbicides alone or in combination with superabsorbent polymer, soil binding agent, and compost can be safely used under plastic mulch without compromising tomato crop safety and overall yield. S-metolachlor and flumioxazin treatments applied alone at full recommended rates (X) offered the same level of crop safety as that of half the labeled rates (0.5X). Future research is also needed to investigate the fate and persistence of the tested herbicides under plastic mulch in the sandy soils of southwest Florida to derive valuable insights regarding its effectiveness and the potential impact on weed suppression, crop establishment, and growth.
Our study suggests that the herbicides tested at both rates are safe for use in raised bed tomato plasticulture. No prior studies have specifically investigated the combined use of preemergence herbicides with superabsorbent polymer, soil binding agents, or compost in tomato plasticulture in terms of herbicide effectiveness and crop safety. Therefore, the findings of our study could provide a foundation for developing recommendations for herbicide application practices in tomato plasticulture.
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