Long-Term Effects of Herbicide on Spotted Knapweed in Grand Sable Dunes

Abstract

Over 80 hectares of the Grand Sable Dunes in Pictured Rocks National Lakeshore has been heavily invaded by spotted knapweed (Centaurea stoebe). This species' propensity to dominate plant communities and stabilize active dunes creates the need for management in order to protect the globally rare perched dune ecosystem and its unique inhabitants. Milestone® and Transline® herbicides were spot sprayed on research treatments of knapweed for 5 consecutive years. Herbicide treatments were discontinued in some areas, and the recovering areas were compared to areas never invaded by knapweed. Herbicide treatments continued to be effective in controlling spotted knapweed populations; neither herbicide was significantly better at control of knapweed nor significantly more harmful to native species. Treatments recovering from herbicide generally were not significantly different from herbicide treatments, though some showed early anecdotal signs of recovery or reinvasion by spotted knapweed. Continued monitoring of this site is highly suggested for increased understanding of long-term effects of herbicide treatments.