Mulch provides many benefits to landscape ornamentals, including soil temperature regulation (Fraedrich and Ham, 1982; Montague and Kjelgren, 2004), increased soil moisture (Fraedrich and Ham, 1982; Iles and Dosmann, 1999; Kraus, 1998; Litzow and Pellett, 1993; Watson, 1988; Watson and Kupkowski, 1991), and improved overall plant growth and survival (Green and Watson, 1989; Greenly and Rakow, 1995; Litzow and Pellett, 1993). Similarly, mulch increases the growth of container-grown ornamentals by providing the same benefits in a production (i.e., nursery) environment (Amoroso et al., 2010; Lohr, 2001).
Mulch is most often applied in landscape planting beds for aesthetic purposes and for weed management (Chalker-Scott, 2007). Mulch is less commonly used in container nursery production but may be used as a nonchemical weed management option for sensitive plant species (Case et al., 2005). Many different mulch materials have been evaluated for weed control in container plants. Richardson et al. (2008) reported up to 150 d of yellow woodsorrel (Oxalis stricta) and hairy bittercress (Cardamine hirsuta) control in large (3–7 gal) container-grown ornamentals with 3 inches of PB mini-nuggets. Similarly, Cochran et al. (2009) showed that 1 inch of PB mulch reduced garden spurge and eclipta fresh weights (FWs) and weed counts by more than 80% compared with a nonmulched control. Reviews of different mulch materials as a sole means for weed control have been summarized for landscape (Chalker-Scott, 2007) and nursery production (Case et al., 2005).
A review of earlier research focusing on the use of mulch in combination with, or in comparison with, PRE herbicides was recently published by Marble (2015). In many cases, research focused on evaluating different mulch or herbicide + mulch combinations to determine the most effective on target weed species, or evaluated the use of herbicide-treated mulches (Case and Mathers, 2006a, 2006b). For example, Bartley et al. (2017) evaluated three different mulch types applied at three depths (1, 2, and 4 inches) with and without addition of dimethenamid-P. The authors reported that 168 d after treatment (DAT), herbicide was no longer a significant factor as dimethenamid-P had lost all efficacy, and mulch depth was the only significant factor, with depths of 1–4 inches providing 90% to 100% spotted spurge (Euphorbia maculata) control for up to 90 d after seeding. All of these previous reports establish that different herbicide + mulch combinations can potentially provide a high level of control of many different weed species.
Previous studies have shown that mulch can provide substantial weed control when applied alone at adequate depths (Cochran et al., 2009; Richardson et al., 2008; Wilen et al., 1999); however, it is unclear whether it is mulch or herbicide that contributed most to the observed weed control. Furthermore, a decrease in herbicide efficacy with certain herbicides such as dinitroanilines, which bind tightly to mulch, can occur with increased levels of organic matter on the soil surface. Consequently, the herbicide becomes unavailable for weed control (Buhler, 1992). Mulch depth is an important factor to consider as research conducted by Banks and Robinson (1986) and Chauhan and Abugho (2012) showed that application of thin mulch layers reduces PRE efficacy in several agronomic studies because of rapid degradation caused by increased microbial activity (Locke and Bryson, 1997). Alternatively, herbicide placement (i.e., application of herbicides under mulch) or making the application before mulch addition improves weed control compared with PRE application on top of mulch (Chen et al., 2013). However, in most landscape situations, this application could only be made initially, and subsequent applications would have to be applied on top of existing mulch layers.
Increasing posttreatment irrigation levels to mulched areas could be a means of increasing herbicide concentrations in the soil as PRE herbicides must be incorporated into the soil by irrigation following application. In most cases, PRE herbicide labels recommend irrigation volumes of 0.2–0.5 inches soon after application. Banks and Robinson (1986) reported that reduced amounts of acetochlor, alachlor, or metolachlor were received on the soil surface as wheat (Triticum aestivum) straw mulch depth increased, resulting in the need for higher irrigation volumes for thicker mulch layers. However, wheat straw has very different physical properties compared with common landscape mulch materials, and the herbicides that were evaluated are not commonly applied in landscape situations. Additional research is required to determine the extent to which activation irrigation can improve the efficacy of different herbicide + mulch combinations commonly used in landscapes and container nurseries. This experiment was designed to accomplish three primary objectives. First, we wanted to determine the efficacy of multiple herbicide + mulch combinations and determine which factors significantly affected the control, specifically focusing on herbicide formulation and posttreatment irrigation volumes. Second, we wanted to determine the efficacy derived from mulch or herbicides used alone under the same conditions as the herbicide + mulch combinations. Third, our goal was to identify differences in the additive effects of the herbicide + mulch combinations compared with the use of only herbicides or only mulch.
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