Habitat installations rich in floral pollen, nectar, and nesting resources can support local pollinator populations on farms, thus providing a farmscaping tool to alleviate native and managed pollinator declines and the accompanying declines in pollination ecosystem services (Vaughan and Skinner, 2008, 2015). Pollinator habitat installation is supported by federal programs such as the Environmental Quality Incentives Program and the Conservation Reserve Program, through which technical and financial assistance is provided to participating farmers by, respectively, the U.S. Department of Agriculture’s Natural Resources Conservation Service (USDA NRCS) and the Farm Service Agency (Vaughan and Skinner, 2008, 2015). Pollinator refuge habitats (PRs) in these programs are designed to provide blooms throughout the entire growing season and may also contain grasses depending on regional recommendations (FSA, 2013). As interest in these programs develops, there have been calls for research into the factors affecting PR establishment and effectiveness. Specifically, how can wildflower establishment success be maximized to obtain the full diversity of the mix planted (Williams et al., 2015)? Furthermore, how does PR establishment success and species composition affect pollinator communities (Venturini et al., 2017)? We conducted a study to address these questions, investigating PR establishment and pollinator conservation value relative to seedbed preparation, seed mix composition, and herbicide treatments.
Seedbed preparation is one of the most important actions affecting PR establishment success because of its long-term impacts on weed management (Aldrich, 2002; Martin, 1986; Wilson, 1992). Previous studies indicate that weed competition can be a critical factor limiting PR success (Aldrich, 2002; Howell and Kline, 1992; Norcini and Aldrich, 2004; Perry, 2005). Although seedbed preparation recommendations for PR installations vary (NRCS, 2007, 2012; Vaughan et al., 2013), a review of the literature suggested repeated tilling, herbicide applications, or both are better methods for perennial weed management than other strategies such as grazing, solarization, or burning (Aldrich, 2002). Studies comparing the results of a no-till/preemergent herbicide method with repeated tillage have yielded conflicting results (Ahern et al., 1992; Corley, 1991; Corley et al., 1993; Skousen and Venable, 2008), but a recent study showed that in the first establishment year, a mixture of annual, biennial, and perennial wildflowers planted on cropland is more successful in a no-till than a tilled system (Angelella and O’Rourke, 2017).
After preparing a PR seedbed, a critical question is which species of wildflowers to plant. Pollinator habitat mix composition may affect both wildflower and weed establishment as well as pollinator conservation. Annual wildflowers can reduce weed competition during the critical initial phases of establishment (Aldrich, 2002) and can provide floral resources in the first year, but species should be selected so as to minimize the risk of becoming weeds themselves if seeds get dispersed outside PRs (Menz et al., 2011). The benefits of perennial wildflowers include that they compete well with annual weed species once they are established (Lulow, 2006) and that they do not require frequent overseeding (Aldrich, 2002). Moreover, some research suggests bumblebees and butterflies prefer perennials over annuals (Feber and Smith, 1995; Fussell and Corbet, 1992), although other studies report a lack of preference (Carreck et al., 1999), or that a combination of annuals and perennials can work complementarily to attract more pollinators (Williams et al., 2015). Although PRs will certainly be influenced by the species-specific preferences of growers, more can be done to develop general recommendations about the benefits and trade-offs of species mixes that consist of all perennials or a combination of short- and long-lived species.
Managing weeds after planting wildflowers is another major challenge that growers and advisors must address during the PR establishment process. Some studies indicate that herbicides improve PR establishment, whereas others indicate that it is an unnecessary expense. The selective herbicide imazapic, for example, can enhance the establishment of tolerant wildflower species (Beran et al., 1999), but effects can vary by site and species (Frances, 2008) as well as by weather conditions, soil quality, and seed source (Norcini and Aldrich, 2004). Similarly, the graminicide sethoxydim enhanced wildflower establishment and richness in two studies (Hitchmough et al., 2008; Trowbridge et al., 2017), but was found to be ineffective in another (Frances, 2008). Further investigation into the consistency of herbicide effectiveness applied during the early stages of PR growth would enhance our ability to make recommendations regarding weed management practices for successful PR establishment.
To address questions of how to best establish PRs on cropland and the subsequent impacts on pollinators, we examined the effects of seedbed preparation, seed mix composition, and herbicide use on PR establishment in a controlled field experiment over two years. We hypothesized that 1) wildflower establishment would be higher and weed establishment would be lower following a no-till than a tillage seedbed preparation; 2) herbicide applications following PR planting would suppress weeds and enhance wildflower growth; 3) mixes containing both annuals and perennials would have more wildflower growth, more bloom coverage, and less weed growth than a mix containing only perennials; and 4) pollinator abundance would correlate positively to bloom density.
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Species and seeding rates of pollinator refuge plantings.
Wildflower bloom counts by mix, seedbed preparation, and a mix-by-seedbed preparation interaction effect, for 2016 and 2017. Counts are the number of blooming plants per species in 2016, and the number of blooming stems per species in 2017, in which the number of blooming branches off of the main stem was counted individually and umbels and composites were considered a single flower.