Recently transplanted Christmas trees are susceptible to weed competition and herbicide injury. Traditional herbicide programs for recent transplants and established trees have included the photosystem II (PSII) inhibitors atrazine and simazine [Lantagne and Koelling, 1997; Neal et al., 1999; U.S. Department of Agriculture (USDA), 2004; White and Newton, 1990]. Repeated use of PSII inhibitors and other long-residual herbicides has resulted in the development of herbicide resistance in several weed species and an increase in numbers of weeds that are naturally tolerant of these herbicides (Gower et al., 2004; Peachey et al., 2011; Richardson and Zandstra, 2006). Regardless of resistance problems, chemical weed control is essential for profitable and successful Christmas tree production (Ahrens and Newton, 2008; Brown et al., 1989).
New herbicides for long-term weed control have been developed by the chemical industry, and these herbicides need to be evaluated for their safety in Christmas tree production and weed control effectiveness. Westar 75 DG (DuPont Corp., Wilmington, DE) is a combination of 68.6% hexazinone, a photosynthesis inhibitor, and 6.5% sulfometuron-methyl, an acetolactate synthase inhibitor (DuPont Corp., 2009). It is an effective herbicide providing both contact and residual control of many annual and perennial weeds (Martin et al., 2004). The label allows hexazinone plus sulfometuron to be broadcast over dormant 4-year-old trees that have been established in the field for at least 1 year. Preliminary research in the northeastern United States indicated that there was potential for needle burn with hexazinone plus sulfometuron, especially at high rates (Kuhns and Harpster, 2005; Martin et al., 2004). Hexazinone (Velpar, DuPont Corp.) is registered for use in Christmas trees. Sulfometuron (Oust XP, DuPont Corp.) is not labeled for Christmas trees but is labeled for conifer release in forestry plantations. Both herbicides provide excellent broad-spectrum weed control, but have potential for conifer injury (Rose and Ketchum, 2004; Seifert and Woeste, 2002).
Many weeds emerge in Christmas tree plantations (Richardson and Zandstra, 2009). Perennial and biennial weeds, along with invasive and persistent annuals predominate. In these experiments, the most common weeds were common ragweed (Ambrosia artemisiifolia), horseweed (Conyza canadensis), hoary alyssum (Berteroa incana), rabbitfoot clover (Trifolium arvense), and quackgrass (Elymus repens). Common ragweed and horseweed are annuals in the Asteraceae family, which tolerate many preemergence herbicides and germinate during the entire season. Rabbitfoot clover is an annual that inhabits sandy and rocky soils. Hoary alyssum is a perennial in the mustard family that inhabits sandy and gravelly soil. Quackgrass is a perennial on many types of soils (Muenscher, 1935).
The effects of repeated applications of the combination of hexazinone plus sulfometuron on fraser fir have not been reported. Therefore, these experiments were conducted to determine tolerance of recently transplanted and 1-year-transplanted fraser fir to hexazinone plus sulfometuron and to compare it with other Christmas tree herbicides for weed control effectiveness and crop safety.
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