Native forbs are important for plant community function and diversity, and provide food and cover for wildlife. Field studies have been conducted throughout the western United States to determine the impact of herbicides applied to control invasive weeds on native forbs. However, locating adequate populations of native forbs for evaluation of tolerance to herbicides is often difficult. The susceptibility of nine native prairie forbs to aminopyralid applied at 30 to 120 g ha⁻¹ was evaluated in a greenhouse study and results compared to the same or similar species in field trials. Forb susceptibility to aminopyralid varied by species. Of the forbs evaluated, azure aster, purple coneflower, and closed bottle gentian were the most tolerant to aminopyralid while prairie coneflower, great blue lobelia, harebell, and white prairie clover were the most susceptible and likely would be killed in the field. Blanket flower and showy goldenrod were moderately tolerant to aminopyralid even when applied at 120 g ha⁻¹. The susceptibility of greenhouse-grown forbs to aminopyralid was comparable to results for the same or similar species in the field. Results from greenhouse trials could be used to predict native forb tolerance in the field.

Nomenclature: Aminopyralid; azure aster, Symphyotrichum oolentangiense (Riddell) G.L. Nesom; blanket flower, Gaillardia aristata Pursh; closed bottle gentian, Gentiana andrewsii Griseb.; great blue lobelia, Lobelia siphilitica L.; harebell, Campanula rotundifolia L.; prairie coneflower, Ratibida columnifera (Nutt.) Woot. & Standl.; purple coneflower, Echinacea angustifolia DC.; showy goldenrod, Solidago speciosa Nutt.; white prairie clover, Dalea candida Michx. ex Willd.

Management Implications: Invasion by noxious weeds has become a major threat to the conservation of biodiversity and ecosystem sustainability in remnant and restored prairies. Newly restored prairies are especially vulnerable to invasion by plants such as Canada thistle because there is not a well-established plant community that can compete for available resources. Chemical treatment remains the primary method of Canada thistle control used in rangeland and wildland sites because there are no cost-effective alternatives. Aminopyralid is often used to control Canada thistle and other invasive weeds on native and wildlands. Field studies have been conducted throughout the western United States to determine the impact of herbicides applied to control invasive weeds on native forbs. However, finding adequate populations of native forbs to evaluate tolerance to herbicides is often difficult. A greenhouse study evaluated tolerance of nine native prairie forbs to fall-applied aminopyralid. Of the forbs evaluated, azure aster, purple coneflower, and closed bottle gentian were the most tolerant to aminopyralid while prairie coneflower, great blue lobelia, harebell, and white prairie clover were the most susceptible and likely would be killed with fall applications in the field. Blanket flower and showy goldenrod were moderately tolerant to aminopyralid even when applied at 120 g ha⁻¹. The susceptibility of greenhouse-grown forbs to aminopyralid was comparable to results of the same or similar species in the field and suggests results could be used to predict native forb tolerance in the field. Forb species that were tolerant to aminopyralid in the greenhouse likely would be tolerant in the field as well because steps were taken to maximize potential injury. This information is valuable to land managers, who must balance the benefits of an aminopyralid application to control invasive weeds with the potential of unintentional injury to desirable plant species.