Prairie response to Canada thistle invasion was evaluated by comparison of forage yield between aminopyralid treated and non-treated infestations in North Dakota. In general, forage production was unaffected by Canada thistle in both grazed and wildland locations. The economic threshold was 37 stems m-2 for a treatment to be cost-effective. Canada thistle should still be controlled to improve overall flora quality. The susceptibility of 10 prairie forbs to aminocyclopyrachlor at 0, 35, 70, and 105 g ha-1 was evaluated in the greenhouse. Blue flag iris was tolerant and harebell was moderately tolerant to aminocyclopyrachlor. American licorice, prairie rose, purple prairie clover, and wild bergamot were moderately susceptible; however, plants may regrow in the field as some survived at 105 g ha-1. Azure aster, Canada goldenrod, great blue lobelia, and purple coneflower were very susceptible to aminocyclopyrachlor and likely would be eliminated in the field.