Invasive species can reduce habitat quality, lower diversity of native species, and negatively impact populations of rare species. Non-native plants that invade habitat of threatened or endangered plants can be difficult to control because many methods, such as herbicide application, aimed at reducing problem plant abundance can also harm desired vegetation. Tygh Valley milkvetch (Astragalus tyghensis) is listed as threatened by the state of Oregon. It is a perennial of dry, rocky soils within a narrowly restricted geographic range. Roughly 20% of the known populations occur on roadside rights-of-way that are frequently invaded by aggressive weeds, including two exotic annual grasses, medusahead (Taeniatherum caput-medusae) and cheatgrass (Bromus tectorum). We conducted a two phase experiment to determine effective methods to control exotic annual grasses without harming Tygh Valley milkvetch. In Phase I, we evaluated the effects of a grass-specific herbicide (fluazifop) and two pre-emergent herbicides (imazapic and rimsulfuron) singly and in combinations on invasive and native plant species. Plots were specifically located to exclude Tygh Valley milkvetch plants. In Phase we evaluated the effects of imazapic and fluazifop, the two most promising treatments from Phase I, singly and in combination, on Tygh Valley milkvetch, exotic species and other native plants.
We found all Phase I treatments provided overall annual exotic grass control for one growing season after application. Fall application of imazapic and spring application of fluazifop, alone and in combination, reduced exotic annual grasses for two years after treatment. All herbicide treatments reduced cover of both medusahead and cheatgrass the first year after treatment. Imazapic alone or in combination with fluazifop was consistently able to control medusahead for at least two years. Cheatgrass was not reduced for more than one year by any treatment. Other than a one year flush of annual native forbs seen with fluazifop treatments, the remaining (albeit degraded) plant community showed no significant response to the herbicide treatments. In Phase II we have observed no significant treatment effects to Tygh Valley milkvetch seedlings or mature plants, even after two years, but fluazifop provided significant reductions in medusahead and bulbous bluegrass (Poa bulbosa) for two years. Imazapic reduced medusahead and bulbous blue grass for one year, and cheatgrass for two years. Imazapic also reduced native annual forb cover for 1 year post treatment. Our results suggest carefully-selected herbicides can be a viable tool to reduce exotic annual grasses without negative effects to Tygh Valley milkvetch.