Abstract

Extensive areas of degraded lands and remnant natural areas in the upper Midwest have been invaded by the non-native perennial, spotted knapweed (Centaurea maculosa Lam.). Reestablishment of native plant communities requires the application of effective control measures. The objective of our study was to examine the interactive effects of mowing and chemical site preparation treatments (herbicides) combined with hand pulling on spotted knapweed control and native plant establishment on a knapweed-infested site in western Michigan. Initial mowing and herbicide treatments were applied to forty-eight plots in the summer of 2008, and we seeded these plots with a mixture of native grasses and forbs in the spring of 2009. We hand pulled seed-producing knapweed from selected plots in mid-summer, 2009, and determined residual knapweed densities and native plant occurrence on all plots in late July, 2009. All site preparation treatments began to reduce the knapweed soil seedbank, while both glyphosate and clopyralid herbicides substantially reduced mature spotted knapweed densities. Hand pulling effectively reduced seed-producing knapweed densities to less than 0.5 plants m-2 on mowed and glyphosate-treated plots; hand pulling was unnecessary on clopyralid plots because mature knapweed were totally absent in 2009. Only clopyralid, however, reduced juvenile and seedling knapweed densities significantly. Planted native warm-season grasses were present on all treatment combinations, but full development of a diverse native plant community is expected to take several years. Some combination of herbicide treatment and hand pulling of knapweed is recommended to facilitate reestablishment of a native plant community on knapweed-infested sites.