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Meadow knapweed identification and control
About this weed
Meadow knapweed is a regulated Class B noxious weed in King County. This means control is required in King County under the state noxious weed law. Meadow knapweed is also on the Washington quarantine list meaning it is illegal to buy, sell or offer it for sale in the state.
Meadow knapweed, known as Centaurea x gerstlaueri, is a hybrid of two species native to Europe, black knapweed (C. nigra) and brown knapweed (C. jacea). It is in the sunflower family. Other common names for this plant are hybrid knapweed and protean knapweed. Other listed Latin names include Centaurea jacea x nigra and Centaurea x moncktonii.
Why it's a problem
Meadow knapweed can rapidly spread in disturbed areas, wildlands, and pastures. In natural areas meadow knapweed can outcompete native plants and increase soil erosion. It has low nutritional value and crowds out desirable forage plants, degrading pastures and reducing productive land.
Plant description
Meadow knapweed is native to Europe and grows in a range of habitats. It is often found in disturbed sites with moist soil such as roadsides, riverbanks, and pastures. This plant is a short-lived perennial (2+ year life cycle) that will look different depending on its age. It is is a hybrid between black knapweed (C. nigra) and brown knapweed (C. jacea) and its characteristics may vary.
Flowering occurs May to August, depending on climate conditions. Flowerheads have distinct papery bracts (small leaves or scales at the base of a flower). These help identify different species of knapweed by noting bract size, shape and color. Meadow knapweed reproduces solely by seed and the seeds can remain viable in the soil for up to 8 years. Seeds are spread by wind, animals, mowing, and moving contaminated soil. Dead stems from the previous year remain standing after setting seed.
Identification traits include:
- In all stages plants will be covered in fine white hairs and have lance-shaped, slightly toothed or lobed leaves
- First year plants are rosettes (a circular arrangement of leaves growing close to the ground)
- Second year/mature plants are upright with branching stems covered in small white hairs
- Basal leaves are larger than leaves higher up on the stem
- Up to 5 feet tall
- Flowers are small, dark purple to white
- Bracts are papery, finely combed and appear gold
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 meadow knapweed include spotted knapweed (Centaurea stoebe), bighead knapweed (Centaurea macrocephala), Russian knapweed (Rhaponticum repens), diffuse knapweed (Centaurea diffusa), 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 meadow knapweed on lands that they manage. Please notify us if you see meadow knapweed growing in King County. Our program can provide the property owner or appropriate public agency with site-specific advice on how best to remove it. We map all known locations of regulated noxious weeds in order to help us and others locate new infestations in time to control them.
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. King County’s best management practices have more details for controlling meadow knapweed.
Manual Control
Small infestations of meadow 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 meadow knapweed. Plants will re-sprout and flower in the same season when mowed and can flower below the level of the mower. Repeated rototilling or plowing will effectively control 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 plants will greatly improve control of knapweed. Planting desirable species in any area will help prevent weed infestations.
Chemical Control
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. Always read the label before using herbicide and follow state and local regulations.
Biological Control
The blunt knapweed flower weevil Larinus obtusus feeds on the seeds of meadow knapweed. These insects take many years to have a significant impact and can only reduce a population, not eradicate it. Only recommended for large infestations or where other methods are not feasible. For more information, see the WSU Extension’s Integrated Weed Control Project.
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 after removal.
Washington State Noxious Weed Control Board pamphlet on noxious weed disposal