Weed management in container plant production is challenging, primarily due to limited postemergence herbicide options and the consequential need for supplemental hand weeding (Case et al., 2005). Currently, weed control in container nurseries is primarily achieved using preemergence herbicides. Although effective, crop phytotoxicity can be a concern with many ornamental species that are sensitive to these herbicides. Further, limited options exist that are both safe on a wide variety of ornamentals and that also provide broad-spectrum weed control (Neal et al., 2017). Nontarget loss is also a concern in many instances, both in terms of cost and in terms of environmental stewardship. In smaller containers (e.g., 1 gal) spaced can tight (container to container with no spacing), losses can exceed 20%. At a pot spacing of 8 inches, 51% of the applied granular herbicide falls in-between containers, unavailable for weed control. At 12-inch spacing, up to 80% nontarget loss can be observed (Gilliam et al., 1992). Herbicides deposited in-between containers can increase the amount of herbicide detected in captured or reused irrigation water in containment ponds, and could lead to problems with phytotoxicity in some instances (Wilson et al., 1996). These nontarget losses are especially problematic for producers of large container ornamentals (7 gal or larger) that may be spaced several feet apart to facilitate pruning, fertilization, weeding, and other tasks along with allowing ample space for crop growth.
Many mulch products have been recommended as a means of addressing weed control challenges in ornamental plant production (Chalker-Scott, 2007). Richardson et al. (2008) reported over 5 months of weed control with pine bark nuggets in 3- to 7-gal nursery containers, and Cochran et al. (2009) reported that 1 inch of pine bark mulch reduced spotted spurge (Euphorbia maculata) and eclipta (Eclipta prostrata) growth by more than 80% compared with a nonmulched control. Similarly, Bartley et al. (2017) reported 90% to 100% control for more than 3 months with three different wood-derived mulches. Weed discs, plastic bags (Chong, 2003), rice (Oryza sativa) hulls (Altland et al., 2016), pelletized shredded newspaper (Smith et al., 1998), chopped paper waste (Pellett and Heleba, 1995), and many other materials have been evaluated with varying levels of success (Case et al., 2005; Mathers, 2003). Previous reviews of the literature show that mulch depth and physical characteristics, that is, their ability to block light, dry quickly, and decompose slowly, contribute more toward weed suppression than the type of mulch being used (Case et al., 2005; Chalker-Scott, 2007; Marble et al., 2015). In addition to weed control benefits, mulch also may increase growth of ornamental plants. Many studies have demonstrated that mulch aids in establishment and survival of landscape or field-grown plants (Chalker-Scott, 2007; Cregg and Schutzki, 2009; Smith et al., 2000). Fewer studies have focused on the effects of mulch in container plant production, but depths of ≈2 inches or less have been shown to either increase plant growth or have no effect (Amoroso et al., 2010; Billeaud and Zajicek, 1989).
Mulch applied at an adequate depth, typically 1 inch or greater, has been shown to provide greater weed control compared with one herbicide application, as evidenced by previous reports (Bartley et al., 2017; Marble et al., 2017; Saha et al., 2019); however, use of mulch for weed control has not been widely adopted in the nursery industry. The primary barrier to adoption is cost and ease of application. Costs will vary depending on nursery location, mulch availability, mulch type, and application efficiency, but even with a significant amount of preemergence herbicide landing off-target, one application of preemergence herbicide is considerably less expensive than a single mulch application. For a 7-gal container, a hardwood mulch may cost ≈$0.15 per pot retail, whereas a standard preemergence herbicide would cost ≈$0.01, not including labor. Application is also difficult and time-consuming, especially when pots are already spaced in the field and mulching is not completed at potting. In Florida, strong winds during frequent summer thunderstorms may also blow mulch out of pots or result in pot blow-over, which also dislodges or removes mulch. Lighter mulch materials, such as rice hulls, also are prone to blowing out of containers or may be removed by birds.
Two novel mulch types or application methods have been described previously that may have the ability to reduce weed control in container plant production. Cline et al. (2011) reported on the use of a spray-on mulch derived from recycled newsprint fiber. The spray-on paper mulch also was evaluated with and without use of a guar-based tackifier. Tackifiers or “soil glues” are composed of guar gum or other natural starches or synthetic materials, and are often used in erosion control hydro-mulching operations. These tackifiers have been evaluated in combination with shredded paper in container plant production (Pellett and Heleba, 1995). In studies by Cline et al. (2011), a spray-on mulch provided greater reduction in weed counts compared with a glyphosate check on several evaluation dates over 3 years and increased apple (Malus sp.) yields. Another novel mulch in container plant production is plastic. Use of a plastic film, similar to plastic used in vegetable production, was evaluated by Steed et al. (2018). The authors reported that the plastic mulch provided a high degree of weed control in nonspaced ligustrum in 1-gal containers, but did slow production time by 2 to 3 weeks due to reduced growth rates. Both of these methods (spray-on mulch or plastic) could potentially solve application issues in container plant production and provide growers with additional weed management tools. Similarly, use of tackifiers could potentially keep mulch more stable in containers and more resilient to wind storms or during normal nursery operations when pots are moved throughout the nursery. The objective of this trial was to evaluate the use of common mulch materials with and without the use of a tackifier to alternative mulch materials including a paper slurry and a plastic mulch for weed control in container nurseries in Florida.
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