What is the species?
- Centaurea diffusa and C. maculosum (diffuse and spotted knapweeds)
- ODA “B” rank
Although spotted and diffuse knapweeds prefer the drier conditions found in eastern Oregon, several populations exist in Portland’s transit corridors. Plants overwinter in rosette form, then grow a stem (“bolt”) in May to flower and seed in June.
Why are we concerned?
Knapweeds are known to form dense patches of deep-rooted perennial plants. Knapweed infestations can lead to:
- Reduced plant diversity as other species are choked out
- Increased risk to human health.
How does it spread?
Knapweed flowers produce large amounts of seed which easily contaminate vehicles, soil, machinery, and equipment. Stems can also break off and roll in the wind. The presence of plants in disturbed areas, like railroad rights-of-way, mean that trucks can catch pieces of knapweed and transport it long distances. Knapweed seeds are moved by:
- Tires and machinery
- Contaminated soil and feed
What does it look like?
Knapweeds have a low rosette form and a tall, flowering form. Diffuse knapweed is more easily identified by its gray-white flowers. Spotted knapweed has purple flowers.
How do we deal with it?
- Manual: Digging up knapweed is moderately effective, though remaining roots usually grow a new plant. Excavated plants should be bagged and put in the garbage. Knapweed also gives off fumes that some find unpleasant, so a mask may be desirable.
- Herbicide: Herbicides can be effective, but depend on conditions. On patches of more than a few plants, herbicides are used in combination with the manual methods described above.
How can folks help?
If you suspect you’ve found spotted or diffuse knapweed, call either the City of Portland or contact the Oregon Department of Agriculture.