Ornamental and native grasses and grass-like species are becoming increasingly popular as landscape plants (Dana, 2002; Ruter and Carter, 2000; Thetford et al., 2009). Numerous species are now widely available and can be selected to fit almost any need (e.g., aesthetic value, drought tolerance, and pest resistance) and many thrive in low-input landscapes (Thetford et al., 2009; Wilson and Knox, 2006, 2009). In addition to their low-maintenance characteristics, recent trends toward a more “naturalist” approach and landscape designs that feature predominately native plants have further increased the use and demand of ornamental or native grass species (Brzuszek and Harkess, 2009; Ozguner and Kendle, 2006).
Golf courses typically have areas referred to as “natural areas,” which are areas on the course beyond the maintained rough (Dunning, 2014). These areas are often promoted for their ecological function (Nelson, 1997) but can present/pose design and maintenance issues for golf course superintendents (Dunning, 2014). These naturalized areas average 25.8 acres or about 17% of the total golf course area (Gelernter et al., 2017), creating a need for readily available, low-input, and aesthetically pleasing plant species in these spaces (Dunning, 2014; Florida Department of Environmental Protection, 2007). Ornamental and native grasses have been previously evaluated to determine their suitability and are now often used in these naturalized areas on golf courses (Dunning, 2014; Maddox et al., 2007; Voigt, 2002; Weston, 1990).
Although many of the most widely planted ornamental grass species are relatively resistant to disease, insect pests, or both (Thetford et al., 2009; Wilson and Knox, 2009), weed control continues to be a challenge for both landscape applicators and golf course superintendents managing large monocultures of ornamental grasses. Most of the research on ornamental grass tolerance to herbicide applications has focused on preemergence (PRE) herbicides labeled for use in container production. Research focusing on container-grown ornamental grasses has shown that many species are tolerant to over-the-top applications of common PRE herbicides including pendimethalin, prodiamine, isoxaben, and others (Cole and Cole, 2007; Glaze et al., 1980; Neal and Senesac, 1991). However, these herbicides would provide little benefit to areas already infested with weeds on golf courses or in landscapes.
A few studies have investigated the impacts of postemergence (POST) herbicides on ornamental grasses. Hubbard and Whitwell (1991) evaluated response of 12 ornamental grasses from nine genera to applications of three graminicides including fenoxaprop-ethyl, fluazifop-P-butyl, and sethoxydim. Significant injury, growth reduction, or both were noted among most species with all three herbicides with the exception of fenoxaprop-ethyl applications to Calamagrostis arundinacea ‘Karl Foerster’. The authors also reported a high degree of tolerance in two other Calamagrostis cultivars [C. arundinacea ssp. brachytricha (Steud.) Tzvel. and Calamagrostis × acutiflora (Schrad.) Reichb. ‘Stricta’] to fenoxaprop-ethyl applications. Gilliam et al. (1992) evaluated the same graminicides for use in Pennisetum alopecuroides ‘Hameln’, Cortaderia selloana ‘Rosea’, Miscanthus sinensis ‘Purpurescens’, and M. sinensis ‘Gracillimus’ and reported similar findings, at least some injury was noted on all four species with each herbicide. Similarly, Voigt and Reicher (2009) reported that Pennisetum alopecuriodes was sensitive to POST applications of fenoxaprop. Everman et al. (2011) evaluated 10 and 18 herbicide treatments for efficacy on Miscanthus ×giganteus (a biofuel crop) and M. sinensis (primarily planted as an ornamental), respectively, two species that have become naturalized and problematic weeds in Asia (Hakoyama et al., 1977; Sugiura et al., 1970). Results showed that imazethapyr, imazamox, and rimsulfuron were the most efficacious treatments on M. sinensis. Miscanthus ×giganteus was most responsive to treatments of glyphosate, foramsulfuron, nicosulfuron, and imazamox, which all reduced above and belowground biomass in comparison with the nontreated control group. Although these studies show that there may be certain POST herbicides that can be used in ornamental grass plantings, graminicides evaluated by Hubbard and Whitwell (1991) would not control broadleaf weed species. Herbicides evaluated by Everman et al. (2011) that did not provide control of M. sinensis would still have likely caused a high degree of injury to plants used for aesthetic purposes. To effectively control broadleaf and grassy weeds, additional POST options are needed for practitioners managing areas planted with ornamental grasses.
Topramezone is a new POST herbicide that controls susceptible species by inhibiting the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD) (Grossman and Ehrhardt, 2007) and was registered for use turfgrass in 2014. Similar to other HPPD inhibitors, susceptible weeds turn white in color because of chlorophyll loss and growth stops. Topramezone has become an important tool for turfgrass managers as it effectively controls crabgrass species (Digitaria spp.), goosegrass [Eleusine indica (L.) Gaertn.], and many different broadleaf weeds (Elmore et al., 2013; Soltani et al., 2012; Zhang et al., 2013). In addition to controlling notable annual broadleaf and grass species, previous research shows that topramezone could be potentially used to selectively control bermudagrass (Cynodon dactylon) in cool-season turfgrass (Brosnan et al., 2011; Brosnan and Breeden, 2013; Elmore et al., 2011). Mesotrione, another HPPD-inhibiting herbicide, has been shown to have little effect on growth of M. sinensis (Everman et al., 2011) or P. alopecuroides (Voigt and Richer, 2009). As topramezone has shown to be minimally injurious when applied to certain grass species (Johnston et al., 2016), it could be used as a selective POST herbicide for natural areas planted with ornamental grass species. The objective of this research was to evaluate the tolerance of ornamental grass species following over-the-top applications of topramezone.
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