Weeds are plants that compete with a crop for essential components for crop growth and development [i.e., light, water, space, or nutrients (Neal, 1999)]. These components are critical in container production because of the limited amount of space within a container. Although the competitive effect of a weed is highly variable depending on the species of both the weed and ornamental, many researchers have documented the detrimental effects of weeds on container-grown ornamentals (Berchielli-Robertson et al., 1990; Fretz, 1972; Walker and Williams, 1989).
Two common management practices for weed control in container plant production are hand weeding and herbicide applications. Hand weeding is an increasingly expensive option because of increasing labor cost (Darden and Neal, 1999; Gilliam et al., 1990; Mathers 2003). Through communications with Monrovia Nursery Co. (Azusa, CA), one of the largest wholesale nurseries in the United States, Mathers (2003) reported that nurseries spend an estimated $500–$4000/acre annually for hand weeding. This cost varies depending on the weed pressure and species. To reduce the need for hand weeding, nursery growers typically rely on multiple applications of preemergence herbicides to reduce weed pressure. Preemergence herbicides are commonly broadcast in a granular form using a cyclone or “belly-grinder”-type rotary spreader despite more recent research revealing the ineffectiveness of some granular applications when compared with liquid formulations and applications (Wehtje et al., 2015). There are numerous problems associated with current application methods of preemergence herbicides including improper handling of materials and application procedures, injury to nontarget plants, and nontarget loss (Wooten et al., 1999). Heavy reliance on preemergence herbicides has also led to herbicide resistance in some weed species.
Mulches have proven to be an effective nonchemical alternative for weed control in both the landscape and nursery container industries. Tree-derived mulches such as chipped eastern red cedar, pine bark mini-nuggets (Pinus sp.), and douglas fir (Pseudotsuga menziesii) have widespread availability, reasonable consistency, and are generally accepted by consumers (Llewellyn et al., 2003). In landscape studies conducted on tree-derived mulches, weed control was deemed acceptable and better when compared with nontreated control plots (Billeaud and Zajicek, 1989; Broschat, 2007; Greenly and Rakow, 1995). Tree-derived mulches have also been effective for weed control in nursery production, providing acceptable, long-term control of common weed species (Richardson et al., 2008; Wilen et al., 1999). In other container plant studies, combinations of herbicides and mulches were deemed most effective (Case and Mathers, 2003; Richardson et al., 2008). Case and Mathers (2003) reported acceptable long-term weed control in containers mulched with douglas fir and pine bark nuggets in combinations with either acetochlor (Harness®; Monsanto, St. Louis, MO), flumioxazin (SureGuard®; Nufarm, Alsip, IL) or oryzalin (Surflan®; United Phosphorus, King of Prussia, PA). Neither oryzalin nor flumioxazin provided long-term control when applied alone. Likewise, pine bark nuggets and douglas fir mulches provided some control but did not meet commercial standards (≥7 on a scale of 0 to 10, 10 being perfect control). However, these mulches were applied at a depth of one mulch particle layer, not allowing the particles to overlap. Mulches, when applied for weed control, are most effective when applied at greater depths of 2–3 inches (Greenly and Rakow, 1995; Richardson et al., 2008).
Other readily available tree-derived mulches could be used in container production in lieu of commercialized pine bark mini-nuggets. The objective of this study was to evaluate four different types of mulch derived from readily available tree species at multiple depths for long-term weed control. The four species tested were eastern red cedar, ground whole loblolly pine, chinese privet, and sweetgum. These species were selected because of their relative abundance and low value in many southeastern areas of the United States. To assess these mulches in combination with conventional methods of weed control, mulch treatments were evaluated with and without dimethenamid-p herbicide (Tower®; BASF Corp., Research Triangle Park, NC).
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