Controlling meadow knapweed with manual removal, mulching, and seeding
Meadow knapweed (Centaurea pratensis) is an invasive forb that is a fertile hybrid between two European species that are also invasive in the United States. Meadow knapweed is prevalent across Oregon,
Meadow knapweed (Centaurea pratensis) is an invasive forb that is a fertile hybrid between two European species that are also invasive in the United States. Meadow knapweed is prevalent across Oregon, Washington, Montana, and Idaho. Particularly in western Oregon, populations of meadow knapweed have been rapidly expanding and new infestations are frequently found. Although meadow knapweed appears to first colonize roadsides, river and stream banks, and disturbed pastures, it is also capable of invading native prairies and meadows and establishing monocultures within their new environment.
The purpose of this project was to examine the efficacy of combinations of mechanical removal methods (mowing, grubbing, and solarization) and mulching and seeding to control and suppress meadow knapweed. Specifically, this project addresses the following questions:
- How effective are several non-chemical methods 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?
The preliminary data collected in 2007 indicate effective methods for controlling meadow knapweed. The grub/mulch/seed and mow/solar treatments each significantly reduced the cover of knapweed and the total number of individuals present. The mow/solar treatment effectively reduced the respective piece of ground to a blank slate where bare ground cover predominated. In 2008 these plots were mulched and seeded. If the supplementary native seed can compete with the seeds of knapweed and other invasive plants both in the seed bank and raining down from adjacent areas, this method may have great potential. The grub/mulch/seed method proved effective in reducing knapweed but does not affect any of the other invasive plants. We will be able to assess the effectiveness of this method as a treatment for all invasive species after we collect data in 2009.
Despite the seeding that took place in 2007, blue wildrye was only present at one Fire Station plot (20% cover) and one Spur Road plot (0.1% cover) in July 2008; Columbia brome was only found in the same Spur Road plot (0.1% cover). Blue wildrye was germinating with varied success at the Spur Road site after the July 2008 seeding. Much of the success in controlling meadow knapweed will depend on the establishment of native graminoids and forbs that can effectively compete with knapweed.
Effective control of meadow knapweed will likely require several years of knapweed removal. Seeds of related species have been reported to be viable for up to eight years and mangers have reported that treated sites are frequently reinvaded by meadow knapweed the year following treatment. Thus, we are planning to evaluate all treatments in 2008 and either reapply or add treatments in future years.