Abstract
Pennsylvania bittercress (Cardamine pensylvanica) and other bittercress (Cardamine) species are among the most common and difficult-to-control weed species in container nurseries, and they have been vouched in most counties in Florida. Preemergence herbicides can provide control, but concerns over potential resistance development, environmental issues, and crop injury problems associated with herbicide use create the need for alternative weed control methods to be explored. Previous studies have shown the potential of mulch materials for controlling weeds in nurseries, but their use along with preemergence herbicides has not been extensively investigated. To compare the effects of different mulch materials and herbicides on Pennsylvania bittercress control, a full factorial designed greenhouse study was conducted. Three mulch treatments including no mulch, pine (Pinus sp.) bark, and rice (Oryza sativa) hulls were evaluated with three herbicide treatments, including water (i.e., no herbicide), isoxaben, and prodiamine applied at label rates. Twenty-five seeds of Pennsylvania bittercress were sown on the surface of each container and emergence (percent), coverage (square centimeters), seedhead number, and biomass (grams) were measured. The results showed that Pennsylvania bittercress in containers mulched with rice hulls had the lowest emergence throughout the experiment. For coverage, seedhead, and biomass parameters, Pennsylvania bittercress seeded in rice hulls treatments had significantly lower coverage, fewer seedheads, and lower biomass compared with those in nonmulched or pine bark treatments, regardless of herbicide treatment. With isoxaben and the water check, nonmulched treatments had the highest coverage/seedhead/biomass, whereas with prodiamine, Pennsylvania bittercress in pine bark mulched containers had the highest coverage/seedhead/biomass. In conclusion, applying rice hulls alone can provide better Pennsylvania bittercress control compared with isoxaben or prodiamine applied alone.
Weeds present one of the biggest challenges in the nursery industry because they can significantly affect nursery crop values by reducing their growth and salability (Amoroso et al., 2009). For instance, redroot pigweed (Amaranthus retroflexus) and large crabgrass (Digitaria sanguinalis) reduced Japanese holly (Ilex crenata) growth by 47% and 60%, respectively, making it less or not salable (Fretz, 1972). Similarly, eclipta (Eclipta prostrata) caused a 43% reduction in ‘Fashion’ azalea (Rhododendron hybrid) growth after 90 d, largely reducing its marketability (Berchielli-Robertson et al., 1990).
Bittercress (Cardamine sp.) are cool-season annual weeds but grow year-round in nursery and greenhouse production conditions (Crone and Taylor, 1996). It is one of the most common and difficult weed species in container nurseries because it reproduces rapidly, transports easily, and may harbor pests (Cross and Skroch, 1992; Gallitano and Skroch, 1993; Yu and Marble, 2022). For example, Pennsylvania bittercress (Cardamine pensylvanica) can forcibly dehisce seeds up to 5 m and produce up to 5000 seeds that germinate within 2 weeks (Altland et al., 2016; Bachman and Whitwell, 1994; Vaughn et al., 2011). Also, because bittercress seeds are small and sticky when wet, they easily adhere to nursery employees’ shoes or clothes and nursery containers’ sides, resulting in spread throughout production areas (Saha et al., 2018).
Many commonly used herbicides control Pennsylvania bittercress in containers, including flumioxazin, oxyfluorfen + pendimethalin, isoxaben + trifluralin, and dithiopyr, among others, providing more than 90% control in many instances (Altland et al., 2000; Judge and Neal, 2006; Saha et al., 2018). Moore et al. (1989) reported oxyfluorfen applied at 2.2 and 4.5 kg·ha−1 a.i. controlled Pennsylvania bittercress for 4 and 14 weeks without phytotoxicity to ‘Gloria’ azalea (Rhododendron obtusum) or ‘Merritt Supreme’ hydrangea (Hydrangea macrophylla). However, the wide use of herbicides can create problems, such as herbicide-resistant weeds, environmental concerns, and economic losses (Briggs et al., 2002; Case and Mathers, 2006; Derr et al., 2020; Powles and Yu, 2010; Riley, 2003). Thus, nonchemical (mainly mulches) and integrated weed control method (herbicide + mulches) need to be further evaluated (Yu and Marble, 2022).
In recent years, researchers have again been exploring using nonchemical or integrated weed control methods for weed control (Giaccone et al., 2018; Masilamany et al., 2017; Shen and Zheng, 2017; Somireddy, 2011; Witcher and Poudel, 2020). Some of the most widely evaluated mulch materials have included pine (Pinus sp.) bark, rice (Oryza sativa) hulls, Douglas fir (Pseudotsuga menziesii) bark, coconut (Cocos nucifera) coir, newspaper pellets, and wastepaper (Bartley et al., 2017; Burrows, 2017; Marble, 2015; Marble et al., 2019; Masilamany et al., 2017; Massa et al., 2019; Mathers and Case, 2010). Richardson et al. (2008) reported that applying 3 inches of pine bark mini-nuggets controlled hairy bittercress (Cardamine hirsuta) in large (3–7 gal) containers up to 150 d. Additionally, Altland and Krause (2014) reported rice hulls mulch at a depth of 0.5 to 1 inch provided excellent control of flexuous bittercress (Cardamine flexuosa). Poudel and Witcher (2022) showed that rice hull application reduced hairy bittercress germination by more than 25% compared with a nontreated (no mulch) control at week 2 after sowing. Mulch has also been investigated with the addition of preemergence herbicides in several studies, often showing superior control with a mulch + herbicide combination in comparison with the use of herbicides alone. For example, applying flumioxazin at a labeled rate of 0.4 kg·ha−1 a.i. with the addition of pine bark mini-nuggets at 3 inches led to 100% bittercress control, a higher control level compared with applying herbicide alone, which resulted in 92% control (Richardson et al., 2008). However, many of these studies have focused on mulch materials more suitable for a landscape planting bed as opposed to materials suited for container nurseries (Saha et al., 2019a, 2019b).
Although there have been promising results from nonchemical and integrated weed control studies, the comparison between them is less commonly reported. Additionally, for certain mulch materials and herbicides, it is unclear how the interactions affect weed control when combined or whether herbicides are even warranted when mulch is applied at an adequate depth. Thus, this experiment was designed to compare the effects of different herbicides and mulch materials on Pennsylvania bittercress control and determine the interactions effect of common herbicides and mulch materials used in nursery settings.
Material and methods
Self-collected Pennsylvania bittercress seeds were counted and put in a transparent glass tube (25 seeds/tube) before sowing. On 6 Oct 2021, trade gallon plastic nursery containers (top diameter 16.4 cm, bottom diameter 12.5 cm, depth 17.5 cm, volume 3 qt) were filled with substrates composed of pine bark (Southeast Soils Inc., Okahumpka, FL, USA) mixed with peat and sand at a 9:1:1 (pine bark:Florida peat:sand v:v:v) ratio. Two types of mulch materials rice hulls (Riceland Foods Inc. Stuttgart, AR, USA) and pine bark nuggets (Pacific Organics, Henderson, NC, USA) were applied at 1 inch on the soil surface. On 7 Oct 2021 (9:30 am), two preemergence herbicides isoxaben (Gallery SC; Corteva AgriSciences, Indianapolis, IN, USA) and prodiamine (Barricade 4L; Syngenta Corp., Greensboro, NC, USA) were applied at 1 and 1.5 lb/acre a.i., respectively, using a carbon dioxide (CO2) backpack sprayer (Bellspray R&D Sprayer, Opelousas, LA, USA) calibrated to deliver 100 gal/acre water using a flat-fan nozzle (8004; TeeJet Technologies, Wheaton, IL, USA) at a pressure of 30 psi. Twenty-five seeds were sown on the surface of each container on 11 Oct 2021. Controlled-release 17N–2.2P–9.1K fertilizer (Osmocote Blend 17–5–11, 8 to 9 month; ICL Specialty Fertilizers, Dublin, OH, USA) was added to each container at 11 g/container rate, representing the manufacturer’s recommended low rate, 4 weeks after sowing (WAS).
The experiment was arranged as a 3 × 3 factorial design with two factors (mulch and herbicide) and three levels (no mulch, pine bark, rice hulls; and water/no herbicide, isoxaben, and prodiamine). The data from two experiments were pooled because there was no treatment by experimental run interactions or differences. All the containers were placed in a shade house at the Mid-Florida Research and Education Center in Apopka, FL. Plants were irrigated 0.5 inch/d with overhead irrigation (Xcel-Wobbler; Senninger Irrigation, Clermont, FL, USA) via two cycles (7:00 am and 2:45 pm) throughout the experiment. The study was repeated following the same methodology and timeline on 7 Nov 2021.
Mulch particle size has been shown to have a significant impact on weed growth (Saha et al., 2020) but is rarely reported in mulch weed control evaluations. To determine particle size, mulch materials used in this study were air dried for 7 d before particle size analysis, which was performed by passing 100-g samples through 12.5-, 9.5-, 6.3-, 2.8-, 1.4-, and 1.0-mm soil sieves (Humboldt Mfg. Co., Elgin, IL, USA). Particles ≤1.0 mm were collected in a pan. The sieves and pans were shaken for 3 min using a sieve shaker (Gilson Company, Inc., Lewis Center, OH, USA), and the residues of each sieve were collected, weighed, and recorded. Six replicate samples for each substrate were analyzed.
Weed counts in each container were recorded weekly starting at 2 WAS. Emergence rates were calculated by the following formula: emergence rate (percent) = weed counts/25 × 100%. Plant pictures were taken weekly starting 7 WAS. ImageJ software (ver. 1.53a; National Institutes of Health, Bethesda, MD, USA) was used to calculated plant coverage area (square centimeters). Seedhead counts were recorded weekly starting 8 WAS. When studies were ended at 11 WAS, shoot biomass (dry weight) were measured after clipping plants at the soil line and placing shoots in a forced-air oven at 60 °C for 7 d and reaching a constant weight.
Data analysis
All the data were analyzed with the two-way analysis of variance using R program software (ver. 3.5.1; RStudio, Boston, MA, USA). All the means were separated by using the least significant difference when treatments were significantly different from each other at P ≤ 0.05. For parameters where there were interactions between mulch and herbicide, differences in mulch type were compared within each herbicide treatment.
Results and discussion
Particle size analysis
For pine bark, 94% of all particles were larger than 6.3 mm with only 2.2% of particles larger than 12.5 mm (Table 1). For rice hulls, 88.9% of all particles were retained on the 2.8- and 1.4-mm screens. The potting soil was typical of pine bark:peat:sand potting substrates with 21% retained in the 12.5-mm sieve, ∼60% of particles ranging from 9.5 to 1 mm, and 20.15% of particles being retained in the pan.
Particle size distribution of pine bark, rice hulls, and peatmoss mixed soil evaluated for Pennsylvania bittercress growth.
Effect of mulch type and herbicide on Pennsylvania bittercress emergence
Both main effects of mulch type and herbicide had a significant effect on emergence rate on almost all evaluation dates but were confounded by a significant mulch × herbicide interaction on emergence data collected in all weeks except for 4 and 11 WAS (Table 2). In addition, there was a significant mulch × herbicide interaction on weed coverage, seedhead counts, and biomass on all dates where this data were collected.
A summary of statistical analysis for mulch and herbicide (herb.) effects on Pennsylvania bittercress emergence [E (percent)] based on 25 surface sown seeds, coverage [C (square centimeters)], seedhead number (S) throughout the experiment (weeks 2 to 11 after sowing) and biomass (grams).
Within the no herbicide group (Fig. 1A) both pine bark and rice hulls mulch treatments significantly reduced Pennsylvania bittercress emergence compared with the nonmulched control, but rice hulls provided the lowest emergence rate on all evaluation dates. This trend was also observed in containers treated with isoxaben or prodiamine where the presence of mulch provided additional weed control benefits compared with use of herbicide alone, but a greater benefit was observed with rice hulls compared with pine bark on almost all evaluation dates. Within the isoxaben group (Fig. 1B), rice hulls treatments had significantly lower emergence in comparison with containers mulched with pine bark except for emergence rates at 4, 5, and 11 WAS, when nonsignificant difference was detected. In the prodiamine group (Fig. 1C), rice hulls treatments had similar emergence with pine bark at the beginning (2–5 WAS) and at the end (11 WAS). For all the treatments, emergence presented an increase–decrease–stable–increase trend regardless of herbicide application, although prodiamine group had a sharper increase at the end (Fig. 1).
The emergence rate (percent) of Pennsylvania bittercress grown in different mulch treatments (no mulch, pine bark, and rice hulls) applied with water (A, no herbicide), isoxaben (B), and prodiamine (C) 2 to 11 weeks after sowing (WAS). Different letters above the lines within the specific WAS indicate significant difference among mulch treatments according to least significant difference test.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
The emergence rate (percent) of Pennsylvania bittercress grown in different mulch treatments (no mulch, pine bark, and rice hulls) applied with water (A, no herbicide), isoxaben (B), and prodiamine (C) 2 to 11 weeks after sowing (WAS). Different letters above the lines within the specific WAS indicate significant difference among mulch treatments according to least significant difference test.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
The emergence rate (percent) of Pennsylvania bittercress grown in different mulch treatments (no mulch, pine bark, and rice hulls) applied with water (A, no herbicide), isoxaben (B), and prodiamine (C) 2 to 11 weeks after sowing (WAS). Different letters above the lines within the specific WAS indicate significant difference among mulch treatments according to least significant difference test.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
Mulch and herbicide effects on Pennsylvania bittercress coverage
Pennsylvania bittercress seeded in containers mulched with rice hulls had lowest level of bittercress coverage regardless of herbicide application (Figs. 2 and 3). In the no herbicide and isoxaben groups (Fig. 2A and B), pine bark resulted in significantly lower coverage area compared with Pennsylvania bittercress in containers containing no mulch. However, in prodiamine-treated containers (Fig. 2C), there was no difference in Pennsylvania bittercress coverage in containers mulched with pine bark compared with containers containing no mulch, especially noted at later evaluation dates. Coverage was overall lower in prodiamine (all mulch treatments) compared with isoxaben and no herbicide.
The coverage of Pennsylvania bittercress grown in different mulch treatments (no mulch, pine bark, and rice hulls) applied with water [A (no herbicide)], isoxaben (B), and prodiamine (C) 7 to 11 weeks after sowing (WAS). Different letters above the lines within the specific WAS indicates significant difference among mulch treatments according to least significant difference (LSD) test; 1 cm2 = 0.1550 inch2.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
The coverage of Pennsylvania bittercress grown in different mulch treatments (no mulch, pine bark, and rice hulls) applied with water [A (no herbicide)], isoxaben (B), and prodiamine (C) 7 to 11 weeks after sowing (WAS). Different letters above the lines within the specific WAS indicates significant difference among mulch treatments according to least significant difference (LSD) test; 1 cm2 = 0.1550 inch2.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
The coverage of Pennsylvania bittercress grown in different mulch treatments (no mulch, pine bark, and rice hulls) applied with water [A (no herbicide)], isoxaben (B), and prodiamine (C) 7 to 11 weeks after sowing (WAS). Different letters above the lines within the specific WAS indicates significant difference among mulch treatments according to least significant difference (LSD) test; 1 cm2 = 0.1550 inch2.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
The coverage and growth of Pennsylvania bittercress grown in different treatments (rice hulls, pine bark alone, isoxaben, prodiamin alone, the combination of rice hulls or pine bark with isoxaben or prodiamin) after 11 weeks of sowing. Treatment without mulch or herbicide (no mulch and water) was the control.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
The coverage and growth of Pennsylvania bittercress grown in different treatments (rice hulls, pine bark alone, isoxaben, prodiamin alone, the combination of rice hulls or pine bark with isoxaben or prodiamin) after 11 weeks of sowing. Treatment without mulch or herbicide (no mulch and water) was the control.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
The coverage and growth of Pennsylvania bittercress grown in different treatments (rice hulls, pine bark alone, isoxaben, prodiamin alone, the combination of rice hulls or pine bark with isoxaben or prodiamin) after 11 weeks of sowing. Treatment without mulch or herbicide (no mulch and water) was the control.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
Mulch and herbicide effects on seedhead numbers and biomass
Pennsylvania bittercress seeded in containers mulched with rice hulls had the fewest seedheads regardless of herbicide application on most evaluation dates (Fig. 4). In containers receiving no herbicide treatment (Fig. 4A), Pennsylvania bittercress in pine-bark-mulched containers had significantly fewer seedheads compared with no mulch treatments but were still higher than containers mulched with rice hulls. In isoxaben-treated containers (Fig. 4B), both mulch materials provided an added benefit in terms of seedhead reduction in relation to nonmulched containers, but the only difference in mulch materials was noted at 11 WAS when containers mulched with rice hulls had a lower seedhead count compared with containers mulched with pine bark. Similar to data observed with Pennsylvania bittercress coverage, in prodiamine-treated containers, pine bark provided no benefit compared with use of prodiamine alone (no mulch), but use of rice hulls significantly reduced seedhead counts compared with pine bark or no mulch (herbicide only).
The seedhead number of Pennsylvania bittercress grown in different mulch treatments (no mulch, pine bark, and rice hulls) applied with water [A (no herbicide)], isoxaben (B), and prodiamine (C) 9 to 11 weeks after sowing (WAS). Different letters above within the specific WAS means significant difference among mulch treatments according to least significant difference test.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
The seedhead number of Pennsylvania bittercress grown in different mulch treatments (no mulch, pine bark, and rice hulls) applied with water [A (no herbicide)], isoxaben (B), and prodiamine (C) 9 to 11 weeks after sowing (WAS). Different letters above within the specific WAS means significant difference among mulch treatments according to least significant difference test.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
The seedhead number of Pennsylvania bittercress grown in different mulch treatments (no mulch, pine bark, and rice hulls) applied with water [A (no herbicide)], isoxaben (B), and prodiamine (C) 9 to 11 weeks after sowing (WAS). Different letters above within the specific WAS means significant difference among mulch treatments according to least significant difference test.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
Biomass data generally followed the same trend as was observed with coverage and seedhead counts where both mulch materials resulted in significantly lower Pennsylvania bittercress biomass compared with the no mulch control or the use of isoxaben alone without mulch. Rice hulls reduced Pennsylvania bittercress biomass compared with the use of prodiamine alone, whereas pine bark mulch provided no benefit, and similar biomass was observed in containers treated with prodiamine without mulch and prodiamine with a 1-inch layer of pine bark (Fig. 5).
The biomass of Pennsylvania bittercress grown in different treatments (water, isoxaben, and prodiamine). ***Significantly different from the no mulch treatment within each group at P ≤ 0.001 according to least significant difference test; 1 g = 0.0353 oz.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
The biomass of Pennsylvania bittercress grown in different treatments (water, isoxaben, and prodiamine). ***Significantly different from the no mulch treatment within each group at P ≤ 0.001 according to least significant difference test; 1 g = 0.0353 oz.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
The biomass of Pennsylvania bittercress grown in different treatments (water, isoxaben, and prodiamine). ***Significantly different from the no mulch treatment within each group at P ≤ 0.001 according to least significant difference test; 1 g = 0.0353 oz.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
With regard to emergence, there was generally an increase–decrease–increase trend observed, most pronounced with the isoxaben- and prodiamine-treated containers. This is similar to previous studies with rice hulls (Altland et al., 2016), hand weeding trials in nurseries (Cross and Skroch, 1992), and previous evaluations with herbicides (Gallitano and Skroch, 1993; Ruter and Glaze, 1992) where weed emergence is initially low, then sharply increases when given ample time to germinate and then decreases as plants reach a larger size. Once weeds reach the reproductive stage, as was noted at 8 WAS in this study, emergence may then sharply increase when new seeds are introduced on the potting soil surface. The changing trend is most likely due to herbicide dissipation and Pennsylvania bittercress reproduction. In the southeastern United States, herbicides often dissipate rapidly immediately after application, after which weeds may begin to emerge, especially for Pennsylvania bittercress, which is capable of rapid seed production and has no dormancy requirement (Gallitano and Skroch, 1993; Judge et al., 2003). Judge et al. (2003) reported rapid trifluralin dissipation over time starting with day 1 when the highest concentration of trifluralin was observed (∼80 µg·g−1), but by 3 d after treatment, trifluralin concentration quickly began dissipating to a much lower level (∼28 µg·g−1) and leveled off ∼21 d after application. Although trifluralin was not evaluated in this study, both isoxaben (Drakeford et al., 2002) and prodiamine (Wehtje et al., 2010) have been shown to degrade more rapidly in nursery environments. On the basis of previous studies showing significant reduction in control from common preemergence herbicides at 10 weeks after application (Altland, 2019), it would be expected that by 11 WAS, reduced control would be observed from secondary seeding from mature plants. This is similar to results reported by Gallitano and Skroch (1993), where a marked increase in hairy bittercress was noted on later evaluation dates and it was assumed the increase was due to self-seeding in the container from mature plants. Our results were similar (Fig. 6), with a single treatment showing an increase of more than 100 seedlings over 3-d period. Although herbicide-only treatments showed this significant increase in emergence at later dates, emergence counts did not increase to the same degree in mulched treatments. This indicates that, similar to previous reports (Marble et al., 2019), mulch alone can outperform a single herbicide application and, when combined with preemergence herbicides, would give growers more flexibility with application timing while still achieving satisfactory weed control.
Emergence of Pennsylvania bittercress in pots treated with prodiamine after 11 weeks of sowing and herbicide application.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
Emergence of Pennsylvania bittercress in pots treated with prodiamine after 11 weeks of sowing and herbicide application.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
Emergence of Pennsylvania bittercress in pots treated with prodiamine after 11 weeks of sowing and herbicide application.
Citation: HortTechnology 32, 6; 10.21273/HORTTECH05102-22
Rice hulls provided better control of Pennsylvania bittercress compared with both pine bark and the use of either herbicide alone on almost all evaluation dates for all parameters collected. Mulch provides weed control through several mechanisms, including light exclusion, providing a physical barrier, and reducing available moisture, which all depend on mulch type, weight to area ratio, application depth, seed placement, and particle size (Chalker-Scott, 2007; Ngouajio and Ernest, 2004; Saha et al., 2019b; Teasdale and Mohler, 2000). As rice hulls are hydrophobic, the primary mechanism of control they provide is reducing available moisture for weed seed germination, explaining why emergence is typically lower when seeds are placed on top of rice hulls compared with when seeds are placed below the rice hulls layer (Altland and Krause, 2014; Altland et al., 2016). In contrast, pine bark has been shown to provide better weed control when seeds are placed beneath the mulch as it serves as a strong physical barrier to emergence (Saha et al., 2019a, 2019b). While in a landscape it would be more important for a mulch to suppress emergence of weed seeds below the mulch as there would likely be a seed bank in a field soil, in a nursery environment the potting soil would ideally be weed free, at least at potting; thus, it would be more important to have a mulch that is more efficacious on seeds introduced on top of the mulch layer over time, such as rice hulls.
For both nontreated (no herbicide) and isoxaben-treated containers, both pine bark and rice hulls provided a significant decrease in Pennsylvania bittercress emergence and growth parameters. The exception was in containers treated with prodiamine, in which coverage, seedhead counts, and biomass parameters where similar in nonmulched and pine-bark-mulched containers. This is possibly due to the higher soil adsorption coefficient (Koc) and lower solubility of prodiamine (Weber, 1990). Prodiamine has a relatively high Koc (13,000 mL·g−1), low solubility (<1.0 mg·L−1), and high hydrogen bonding potentials (due to its chemical structure contains nitrogen dioxide groups), and very low mobility, making it strongly sorbed to organic matter, such as pine bark (Briggs et al., 2011; Wehtje et al., 2010). Isoxaben, on the other hand, presents a lower Koc (190–1270 mL·g−1), higher solubility (1.04 mg·L−1) and higher mobility, making more prone to leaching than the prodiamine (Shaner et al., 2014; Weber, 1990). Pine bark was only applied at a 1-inch depth, and thus it is possible that Pennsylvania bittercress seeds were washed through the mulch layer, reached the soil surface, and were able to germinate. Less isoxaben may have adsorbed to the pine bark mulch due to its lower Koc, whereas prodiamine likely had a higher adsorption to the mulch and was unavailable on the soil surface to provide control of germinating seedlings. This has been reported previously, where prodiamine applied on top of a pine bark mulch provided only 60% control of large crabgrass compared with 100% control when applied to the soil surface (Saha et al., 2019b). This same effect was not observed with rice hulls, which tended to provide close to 100% Pennsylvania bittercress control regardless of herbicide application.
Isoxaben and prodiamine were not directly compared due to an interaction effect, and the objective was to evaluate individual herbicide performance in combination with mulch. However, when examining nonmulched containers, prodiamine surprisingly provided a seemingly higher level of Pennsylvania bittercress control compared with isoxaben, with a reduction in biomass of more than 90% compared with an approximately 60% reduction observed with isoxaben. This is in contrast to previous reports in which isoxaben is often recommended as a highly efficacious herbicide on Pennsylvania bittercress in general (Neal et al., 2017) and has even been shown to provide postemergence control of Pennsylvania bittercress before the reproductive stage of growth (Altland et al., 2000). Other authors have, however, reported a higher level of control with prodiamine compared with isoxaben (Gallitano and Skroch, 1993), but higher prodiamine rates were evaluated than were examined in the present study. Both herbicides were applied at the highest single application rate, but at this rate, no more prodiamine could be applied due to annual limitations, whereas two additional applications could be made with isoxaben. These and other factors should be considered by growers when making herbicide selections. Further work is needed to examine more closely herbicide the efficacy on different bittercress species in general to determine species differences in terms of response to different preemergence herbicides.
Overall, results from this experiment indicate that application of pine bark or rice hull mulch provide a definite weed control benefit, similar to previous work (Altland et al., 2016; Bartley et al., 2017; Burrows, 2017; Marble et al., 2017; Richardson et al., 2008). Further, this benefit was still observed when combined with two of the most commonly used preemergence herbicides in nurseries, prodiamine and isoxaben, where the mulch + herbicide combination outperformed the use of these herbicides applied alone. This indicates that mulch could be used as an effective weed control strategy, and improved control could be achieved when combining the use of mulch with these herbicides. For seeds introduced on top of the mulch surface as would be typical in a nursery environment, rice hulls outperformed pine bark when both were applied at a 1-inch depth. However, growers considering use of mulch should consider cost, stability in the container (breakdown over time and ability to stay in the container during severe weather events), and availability in addition to weed control benefits. As some negative effects were observed with the prodiamine and pine bark combination, additional research is warranted to determine optimal herbicide timing in relation to mulch application for growers wishing to combine both methods.
Units
References
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