Plant Sciences


Invasive species, such as spotted knapweed (Centaurea stoebe L.), impose a huge threat to the reestablishment of native plant communities on degraded sites across the Midwestern United States. Twelve treatment combinations and their effects on densities of all life stages of knapweed have been studied in the Bass River Recreation Area, Ottawa County, Michigan since 2008. Treatments included site preparation by either mowing or mowing with herbicide applications, followed by seeding of native grasses and forbs. Additional treatments included annual burning or hand-pulling of mature knapweed. Knapweed densities in the seedbank, seedling, juvenile and adult stages were sampled on the 48 plots in 2015. The effects of treatments varied among the life stages of knapweed, but all treatments have reduced knapweed densities in comparison to untreated plots. Burning and hand-pulling treatments both significantly decreased adult densities, but only hand-pulling had significant impacts on the other three life stages. Burning effects have been slow to develop, and it is likely that continued burning treatments will also decrease knapweed densities at all life stages, and encourage the growth of native vegetation.