Diffuse knapweed identification and control

Why it’s a noxious weed

Diffuse knapweed can rapidly spread in disturbed areas, wildlands, and pastures. In natural areas it can outcompete native plants and increase soil erosion and wildfire risk. It crowds out desirable forage plants, degrading pastures and reducing productive land.  

Plant description

Diffuse knapweed is adapted to a range of habitats and soil types but is especially suited to relatively dry or well‐drained sites. Depending on conditions, it can be an annual (one year life cycle) or short-lived perennial (2+ year life cycle) that will look different depending on its age.

 

Young plants may remain as a rosette for multiple years depending on conditions. Flowering occurs June-September, depending on climate conditions. Flowerheads have distinct bracts (small leaves or scales at the base of a flower). Bracts help identify different species of knapweed by noting bract size, shape and color. After flowering the plant dries and breaks off at the base to blow around as a tumbleweed, spreading seeds over large areas. Diffuse knapweed reproduces solely by seed and the seeds can remain viable in the soil for up to 8 years. Seeds are also spread by animals, mowing, and moving contaminated soil.

 

Identification traits include:

• In all stages plants have deeply lobed grayish-green leaves and a stout taproot.
• 1st year+ plants are rosettes (a circular arrangement of leaves growing close to the ground).
• 2nd year/mature plants are upright with a main stem and many branches, giving it a ball-like shape
• Up to 5ft tall
• Stems are covered in small white hairs.
• Flowers are small, white to light purple.
• Bracts have a large primary spine and several smaller comb-like spines 

Be aware of look-alike plants

Many species of knapweeds look alike and identification can be difficult. Traits that help tell them apart are bracts on the flowerheads, color of the flowers, and leaf shape. 

Other regulated noxious weed species in King County that may be confused with diffuse knapweed include spotted knapweed (Centaurea stoebe) meadow knapweed (Centaurea × gerstlaueri), bighead knapweed (Centaurea macrocephala), Russian knapweed (Rhaponticum repens), brown knapweed (Centaurea jacea), black knapweed (Centaurea nigra), and yellow starthistle (Centaurea solstitialis). Please contact us with photos if you need help identifying any of these. 

This knapweed ID brochure is a resource that compares these look-alike species and offers helpful identification tips. 

What to do if you find it

Property owners are required to control diffuse knapweed on lands that they manage. Please let us know if you see diffuse knapweed growing in King County. 

Our program staff can provide you with site-specific advice on how best to remove it. We map all known locations of regulated noxious weeds to help locate new infestations in time to control them. For information in other counties in Washington state, contact your local weed board.

Control methods

In areas with few weeds, it is important to act quickly before they become harder to control. Make a long-term plan as it often takes several years to get rid of most weeds. Start in the least infested areas first and then move into more heavily infested areas. To reduce costs and environmental impacts, please consider a combination of methods.

For more in-depth control information, read our Best Management Practices for diffuse knapweed.

Manual control

Small infestations of diffuse knapweed can be dug out with a hand tool or pulled by hand. Manual control is easiest when soils are wet and/or loose. When digging or pulling, remove as much of the long taproot as possible to prevent the plant growing back.  

Mechanical control

Mowing is not a viable control method for diffuse knapweed. Plants will re-sprout and flower in the same season when mowed and can flower below the level of the mower. Knapweed that is regularly mowed can persist longer than it otherwise would. Repeated rototilling or plowing will effectively control diffuse knapweed. Discing will control young plants and seedlings, but established plants can survive if the taproot remains. 

Cultural control 

In pastures, good grazing practices and management of grass and forage species will greatly improve control of diffuse knapweed. Seeding desirable species in any area will help prevent weed infestations. 

Chemical control 

Stay safe when using herbicide:

• Always read the label before use.
• Wear a long-sleeved shirt, long pants, shoes, and eye protection.
• Follow state and local regulations.

Herbicide can be an efficient method for larger infestations. The best time to apply to knapweed is in the spring while plants are growing taller and getting buds, but before flowering. In grassy areas, it’s best to use selective broadleaf herbicides such as triclopyr or 2,4-D. They won’t damage the grass, which helps suppress meadow knapweed. Using glyphosate in spring is also effective but it will kill any contacted grasses, so you’ll want to add new vegetation afterward. New knapweed plants can also be treated in the fall. 

For specific chemical recommendations, please refer to the PNW Weed Management Handbook.

Biological control 

Several biocontrol agents are used on diffuse knapweed. Some of the most common are the knapweed root weevil Cyphocleonus achates, the blunt knapweed flower weevil Larinus obtusus, and the lesser knapweed flower weevil Larinus minutus. These insects and a few others feed on roots and seeds and take many years to have a significant impact on an infestation. They can only reduce a population, not eradicate it. Only recommended for large infestations or where other methods are not feasible. For more information, check out the WSU Extension’s Integrated Weed Control Project.

For more information or a site-specific recommendation in King County, contact the noxious weed program. For information in other Washington locations, contact your local weed board, conservation district, or extension office. 

Disposal instructions

Place all flowering plants in garbage bags and dispose of in household garbage containers or take to a transfer station. Do not compost or put in yard waste. Non-flowering plants can be left to compost on site.  

Noxious Weed Disposal - Washington State Noxious Weed Control Board

Are your weeds piling up? - Noxious Weeds Blog