Diffuse knapweed infestation. Photo Credit: Jennifer Andreas, Washington State University, Bugwood.org
Diffuse knapweed. Photo Credit: (c) Bruce Newhouse, courtesy of OregonFlora
Diffuse knapweed. Photo Credit: Dave Powell, USDA Forest Service (retired), Bugwood.org
Diffuse knapweed flowers. Photo Credit: USDA APHIS PPQ - Oxford, North Carolina, Bugwood.org
Also known as: White knapweed, tumble knapweed, spreading knapweed
Diffuse knapweed is a prolific seed-producer with a single flower stalk producing up to 1,200 seeds. When a plant dies back, it breaks away from the root and becomes a tumbleweed, spreading seeds wherever it roams.
|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. Mature leaves are pale green and hairy. They become smaller further up the stem.|
|Fruit & Flower Description:||White to purplish flowers have spiny, yellow bractsBracts Leaf-like structures below a plant’s flower.. Flowers are solitary, meaning a single flower grows at the end of a single stem.|
|Bloom Time:||June to September|
- Knapweeds are pioneer species that thrive in open, sunny habitats such as pastures, forest openings, and roadsides.
- Diffuse knapweed grows in places with dry soil and arid conditions.
- Diffuse 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.
There are many invasive knapweeds across the Pacific Northwest including meadow knapweed (Centaurea pratensis) and spotted knapweed (Centaurea stoebe). The key to identifying different knapweed species is to closely examine each species’ flowers. Diffuse knapweed is unique with its white flowers while other species have pink to purple flowers.
Noxious Weed Listing:
For more detailed information, download the Diffuse Knapweed Best Management Practices factsheet