Four rates of aminopyralid (30, 60, 90, and 120 g ae ha⁻¹ [0.4, 0.9, 1.3, and 1.8 oz ae acre⁻¹]) were compared for their ability to reduce abundance of nonnative dicot species and favor native species in an invaded Cascade Mountain meadow near Trout Lake, WA. Treatments were applied in two replicated studies (June 2009 and 2010), and foliar cover and species richness were monitored for two years. First-year control of nonnative dicots from application of 30 g ae ha⁻¹ of aminopyralid (69%) was greater than that of native dicots (29%); whereas, significant control of both species groups occurred at the higher rates. By the second year after treatment, absolute differences in cover between treated and non-treated plots averaged −17% and −21% for native and nonnative dicots, respectively, and 1% and 27% for native and nonnative monocots, respectively. First-year control of Canada thistle and oxeye daisy was greater after treatment in 2009 (88% and 90%, respectively) than after treatment in 2010 (56% and 55%, respectively), probably because lower spring temperatures in 2010 limited vegetation development and plant susceptibility to aminopyralid. Cover of Kentucky bluegrass and sheep fescue averaged 20% and 6% greater, respectively, in treated plots than in non-treated plots. Application of 30 g ae ha⁻¹ of aminopyralid had no detectable effect on second-year richness of native and nonnative species relative to non-treated plots; however, higher rates caused 24% to 43% reductions in richness of each species group. Research results suggest that application of aminopyralid at 30 g ae ha⁻¹ has the potential to reduce abundance of nonnative dicot species in similar meadow communities of the Pacific Northwest with little or no negative impacts to abundance and richness of native species. As a potential strategy to limit the subsequent spread of Kentucky bluegrass, a grass herbicide, such as fluazifop or sethoxydim, could be added to the treatment.

Nomenclature: Aminopyralid; Canada thistle, Cirsium arvense (L.) Scop.; Kentucky bluegrass, Poa pratensis L.; oxeye daisy, Leucanthemum vulgare Lam.; sheep fescue, Festuca ovina L.

Management Implications: Herbicides can be used selectively to shift species composition of plant communities to favor native species over nonnative species. Such selectivity can be achieved through species' differences in herbicide tolerance, stature, and timing of development. Aminopyralid is an herbicide that provides selective control of dicot species with little or no injury to many monocot species. To test the feasibility of using aminopyralid to reduce abundance of nonnative dicots in an invaded meadow community in the Washington Cascade Mountains, we compared species abundance and richness one and two years after application of four rates of aminopyralid (30, 60, 90, and 120 g ae ha⁻¹) in separate replicated studies treated in June 2009 and 2010. At the lowest aminopyralid rate (30 g ae ha⁻¹), control of nonnative dicots was considerably greater than control of native dicots, and there was no detectable negative effect on richness of either native or nonnative species. Reductions in dicot abundance from the aminopyralid treatments stimulated a 27% increase in cover of nonnative monocots, primarily Kentucky bluegrass and sheep fescue. First-year control of Canada thistle and oxeye daisy was lower after treatment in 2010 than in 2009 because initial vegetation development had been limited by the cooler spring temperatures of 2010. These research results suggest that a low rate of aminopyralid can be used in similar meadow communities to control nonnative dicots with minimal injury to native dicots. A grass herbicide, such as fluazifop or sethoxydim, could be added to the treatment to control Kentucky bluegrass. Timing of treatme