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Spotted knapweed flowers. Photo Credit: Rob Routledge, Sault College, Bugwood.org -
Spotted knapweed flowers. Photo Credit: Rob Routledge, Sault College, Bugwood.org -
Spotted knapweed infestation. Photo Credit: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
Also known as: Panicles knapweed
Although spotted knapweed prefers drier conditions found east of the Cascade Mountains, it has been found in the Willamette Valley. Spotted knapweed is a prolific seed-producer. When a plant dies back, it breaks away from the root and becomes a tumbleweed, spreading seeds wherever it roams.
Description:
| Life Cycle: | Biennial (life cycle lasts two years) or short-lived perennial (life cycle lasts more than one year) |
| Early Detection and Rapid Response species: | Yes |
| Height: | Up to 3 feet (1 meter) tall |
| Leaf Description: | Rosette leaves are deeply lobed and hairy. Mature leaves are pale green and hairy and become smaller and less lobed further up the stem. |
| Fruit & Flower Description: | Pink to purple flowers have black tipped bractsBracts Leaf-like structures below a plant’s flower., giving them a spotted appearance. Flowers are solitary, meaning a single flower grows at the end of a single stem. |
| Bloom Time: | June to September |
Habitat:
- Knapweeds are pioneer species that thrive in open, sunny habitats.
- Spotted knapweed can grow in places with moist soil such as wet meadows and along riverbanks.
Impact:
- Spotted knapweed outcompetes and suppresses native vegetation, decreasing plant diversity and wildlife habitat.
- It increases the cost of agricultural production by competing with crops and reducing forage for livestock.
What we’re doing about it:
- Diffuse knapweed is a priority species for the Tualatin SWCD. As such, our Invasive Species Program actively monitors the watershed for new infestations.
- If identified within Washington County, a specially trained crew can come out and survey for it. If found, the crew will treat the infestation for free.
What you can do about it:
- If you think you’ve found diffuse knapweed in Washington County, please report it to the Oregon Invasive Species Hotline.
- Small patches can be dug up with a shovel or hand pulled. Knapweeds are easiest to remove in the spring when the soil is moist, and the plants are still rosettes. Make sure to remove as much of the root as possible to prevent resprouting.
- Dispose of all plant materials in a tied plastic bag and throw it away in the trash – not the yard debris.
- Once removed, make sure to reseed bare patches of pastures with competitive, perennial grasses to prevent knapweeds from regrowing.
Look-alikes:
There are many invasive knapweeds across the Pacific Northwest including diffuse knapweed (Centaurea diffuse) and meadow knapweed (Centaurea pratensis). The key to identifying different knapweed species is to closely examine each species’ flowers. Spotted knapweed’s flowers are pink to purple, and its bracts are marked with dark upside-down V’s.
Noxious Weed Listing:
| State of Oregon: | Class B & T |
| State of Washington: | Class B |
More Resources:
For more detailed information, download the Spotted Knapweed Best Management Practices factsheet
- Pacific Northwest Weed Management Handbook: Knapweeds
- University of California, Weed Research & Information Center: Spotted knapweed