Mulching is one of the most widely adopted and effective methods of weed control in landscape planting beds (Chalker-Scott, 2007; Marble et al., 2015a). Although mulch is an effective weed management tool, preemergence herbicides may be used in combination with mulch to reduce labor costs associated with hand weeding, repeated application of postemergence herbicides, or both (Marble, 2015; Marble et al., 2015a, 2015b; Wilen and Elmore, 2007). In addition to weed control benefits, use of common mulch, such as PB or PS, has been shown to reduce runoff and leaching of preemergence herbicides, including pendimethalin, metolachlor, and isoxaben, by 35% to 74% in the landscape compared with application of these herbicides to bare soil (Knight et al., 2001). Container nurseries also may use various mulch materials for weed control (Case et al., 2002). Addition of mulch as part of an overall integrated weed management program could reduce the number of herbicide applications needed, improve weed control and crop safety when using preemergence herbicides, and could potentially reduce runoff or leaching of preemergence herbicides (Bartley et al., 2017; Mathers and Ozkan, 2001; Wilson et al., 1996).
In general, wood products, PB, and other plant residues have a high sorption capacity for many herbicides (Brás et al., 1999; Huang et al., 2006). Herbicide sorption to organic mulch is dependent upon herbicide properties, namely solubility and soil adsorption coefficient [Koc (Aslam et al., 2013; Senseman, 2007; Weber, 1990)]. In addition to herbicide properties, herbicide adsorption will be influenced by the chemical and physical composition of mulch or plant residues, which are dependent on material age or stage of decomposition, the origin of the material, and the particle size or surface area (Ahmad et al., 2001; Aslam et al., 2013; Benoit et al., 2008; Dao, 1991).
There have been few studies focusing on sorption of preemergence herbicides used in ornamentals with common landscape or nursery mulch materials. In field crops, increasing levels of plant residue on the soil surface in conservation tillage systems have been reported to cause reductions in herbicide efficacy due to herbicide adsorption (Buhler, 1992). Mulch or residue materials usually decrease efficacy by binding the herbicide and inhibiting it from reaching the soil surface or by increasing microbial activity and thereby increasing the speed of herbicide degradation (Banks and Robinson, 1986; Chauhan and Abugho, 2012; Locke and Bryson, 1997). Banks and Robinson (1984) studied the effects of oryzalin applied to straw-covered and nonmulched soils. The amount of oryzalin reaching the soil surface was reduced in the presence of straw at the time of herbicide application, and the oryzalin concentration in the soil declined as the amount of straw increased. Similar results have been reported with pendimethalin and oxadiazon when applied to soils mulched with rice (Oryza sativa) residues (Chauhan and Abugho, 2012).
In contrast, Crutchfield et al. (1986) investigated the effects of metolachlor when applied to wheat (Triticum sp.) straw mulch and reported that although soil concentrations of metolachlor were significantly lower when applied in the presence mulch, weed control increased with increasing mulch level due to the weed suppression ability of the mulch. In ornamental production, synergistic herbicide and mulch interactions have been reported, as mulch may act as a slow-release herbicide carrier, extending the longevity of weed control, or provide control of weeds when mulch alone or mulch depth is insufficient (Case et al., 2002; Mathers and Case, 2010; Saha et al., 2019). However, observed synergism with herbicides and mulch typically results when mulch is applied at an adequate depth, usually more than 2 inches (Marble, 2015). Herbicide degradation will occur more rapidly than mulch degradation, and after several months, the type and depth of mulch will be the most important factors in terms of weed suppression if sequential herbicide applications are not made (Bartley et al., 2017).
It has been established that significant herbicide adsorption can occur when applied to mulched soils, but impacts on efficacy may or may not occur depending on mulch depth and the weed species evaluated (Banks and Robinson, 1984; Chauhan and Abugho, 2012; Crutchfield et al., 1986). Most of the research on herbicide adsorption to mulch has focused on no-till situations that are not applicable in most nursery or landscape environments for several reasons. First, in container ornamentals or landscapes, mulch is often not applied at recommended depths of 2 to 3 inches (Chalker-Scott, 2007). This commonly occurs in nurseries, as containers are completely filled with potting media, and depths of 2 to 3 inches may be cost prohibitive or difficult in large landscapes. As many common landscape and container weed species can emerge through mulch depths of 2 inches or more, herbicide adsorption could decrease control if depths of 2 inches or less are used (Richardson et al., 2008; Saha et al., 2019; Teasdale and Mohler, 2000). Second, mulch is often lost in container plants from blow over during weather events or moving containers during normal operations. Consequently, significant adsorption could lead to poor weed control if mulch were removed from containers. Most research on mulch and herbicide use in ornamentals has focused on long-term efficacy of various mulch-herbicide combinations (Bartley et al., 2017; Case et al., 2005; Mathers and Case, 2010; Saha et al., 2019). For landscapes and nursery scenarios, it is unknown how much herbicide is immediately adsorbed or intercepted by mulch or how efficacy would be affected if mulch levels were not sufficient for weed control or if mulch was lost soon after application. Therefore, the objective of this research was to assess adsorption of preemergence herbicides to mulches commonly used in nurseries and landscapes using short-term bioassay and chemical assay experiments conducted soon after herbicide application.
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