Controlling meadow knapweed with manual removal, mulching, and seeding

Meadow knapweed (Centaurea pratensis) is an invasive forb that has made it’s way into many grassland, riparian, and disturbed habitats within Oregon, Washington, Idaho, and Montana. Meadow knapweed is a fertile hybrid between two European species that are also invasive in the United States: black knapweed (C. nigra) and brown knapweed (C. jaceae). This aggressive weed is capable of forming near-monocultures in invaded areas and can easily out-compete many native tree saplings, forbs, and grasses, reducing their cover and richness within their environment.

The purpose of this project was to examine the efficacy of combinations of mechanical removal methods and mulching and seeding to control and suppress meadow knapweed at two sites near Eugene, Oregon. Specifically, this project addressed the following questions:

• How effective are three non-chemical methods (mowing, grubbing, solarization) of removing meadow knapweed?
• Does mulching inhibit germination of meadow knapweed seeds after removal of plants?
• Does sowing of native species inhibit reinvasion by meadow knapweed after removal treatments?
• Is one year of treatment sufficient to control meadow knapweed?

Between 2007 and 2009, the cover of meadow knapweed in the control plots increased at both sites. Most of the treatments showed at least some suppression of meadow knapweed. The most successful treatments were solar/mulch/seed, mow/mulch/seed, and grub/mulch/seed. However, even after two years of treatment, meadow knapweed was not eradicated from these plots. Thus, treatment for longer than two years will be required in order to prevent reinvasion.