Evaluation of Five Herbicide Treatments to Control Yellow Toadflax (Linaria vulgaris)

Yellow toadflax (Linaria vulgaris) is an invasive weed of rangelands, forests, and croplands in North America and a common problem for land managers (Wilson et al. 2005). Endemic to central Europe and the Mediterranean region, yellow toadflax was intentionally introduced to North America during the 1800s (Wilson et al. 2005) as an ornamental, a fabric dye, and for ethnobotanical use. The species escaped cultivation and has become an opportunistic invader throughout the continental United States, Canada, and parts of Mexico (Jacobs and Sing 2006). Yellow toadflax has been legally designated as a noxious weed in much of western North America (USDA 2013); it thrives in disturbed, open habitats and can easily dominate native or desirable plant communities (Wilson et al. 2005).

Yellow toadflax poses a serious threat to ecosystems across North America due to its high reproductive potential and ability to invade existing plant communities (Ward et al. 2008). The species reproduces sexually via seed and vegetatively via adventitious shoots from spreading roots (Ward et al. 2008). Dense patches of yellow toadflax also can lead to problems in agricultural fields (Wilson et al. 2005); studies in Montana have shown that infestations of 12 yellow toadflax stems/m² can reduce yields of canola by 20%; similar reductions have been reported in wheat fields with infestations of 74 stems/m² (Jacobs and Sing 2006). Presence of yellow toadflax can cause a shift in flower timing, as well as decreased abundance, species richness, species evenness, and flowering duration of common, flowering native species (Wilke and Irwin 2010). These effects have the potential to alter vital pollination networks within ecosystems (Wilke and Irwin 2010). Additionally, yellow toadflax is not preferred forage for livestock or wildlife (Sing and Peterson 2011); therefore, invasion in pasture and rangelands can lead to diminished forage abundance and habitat quality.

Currently, effective methods for yellow toadflax control are limited. Mowing and prescribed burning have no effect on killing roots or decreasing seed bank (Lym and Travnicek 2010, Erskine-Ogden and Renz 2011) and only select herbicidal treatments have had limited success (Lym 2002). Biological controls, such as the toadflax stem-mining weevil (Mecinus janthinus) and the toadflax flower-feeding beetle (Brachypterolus pulicarius), decrease overall fitness (e.g., delay seed production) but do not result in eradication (Jacobs and Sing 2006). Thus, land managers need accurate information on treatment methods to implement effective control measures for yellow toadflax.

We evaluated five herbicide treatments, along with differences in the timing of application (flowering and post-flowering), to determine their ability to decrease yellow toadflax stem densities. Four herbicide treatments (E-2, Perspective^®, Telar^®, Tordon^®) were applied alone and one treatment was a mixture of two herbicides (Tordon^®/Overdrive^®; Table 1). The study was conducted on two heavily invaded U.S. Fish and Wildlife Service Waterfowl Production Areas (WPAs) located in the Kulm Wetland Management District in North Dakota, USA. These WPAs provide critical nesting habitat for waterfowl and other species of ground-nesting birds. Our study sites (Todd WPA and Malm WPA) were situated adjacent to one another and therefore, shared geographical and biotic characteristics. Climate of the region is characterized by cold winters (January average -12° C) and hot summers (July average 21° C). Total annual precipitation averages 48 cm (National Weather Service 2013). Todd WPA (64.7 ha) and Malm WPA (129.5 ha) were considered non-native grassland and seeded with dense nesting cover (intermediate wheatgrass [Thinopyrum intermedium], tall wheatgrass [T. ponticum], and alfalfa [Medicago sativa]) in 1979 and 1983, respectively. However, at the time of our treatments, vegetation at both sites was dominated by smooth brome (Bromus inermis) and Kentucky bluegrass (Poa pratensis) with established populations of Canada thistle (Cirsium arvense), absinth wormwood (Artemisia absinthium), leafy spurge (Euphorbia esula), and yellow toadflax.

Table 1.

List of herbicides evaluated for controlling yellow toadflax (Linaria vulgaris) in Kulm Wetland Management District in North Dakota, USA. Year and location of treatments are listed...