What Are They?
Invasive knapweeds, including bighead, black, brown, diffuse, meadow, Russian, spotted, and vochin knapweeds, are native to Europe. They all are listed as noxious weeds in Washington. Knapweeds grow back every year and are very aggressive. Knapweeds can infest an area quickly if left unchecked. They normally are found in disturbed areas, forests, and pastures.
Are They Here Yet?
Yes. Knapweeds are found in various parts of Washington.
Why Should I Care?
Knapweeds may damage wildlife habitat, decrease plant diversity, and increase soil erosion. They also may cause crop losses, reduce plants grazed by animals, decrease the appeal of recreational lands, and pose wildlife hazards. Diffuse knapweed has spines that can damage livestock’s mouths and digestive tracts, significantly reducing grazing opportunities. Russian knapweed is toxic to livestock.
How Can We Stop Them?
Prevention is the most efficient strategy, because knapweeds are difficult to control once present. If in an area with a known infestation, carefully clean all boots, clothing, and equipment before leaving. Controlling established invasive knapweeds can take several years. Small infestations may be removed by hand, ensuring the entire root system is removed. Immediately place in a sealed bag, because plants still can produce healthy seeds after they have been pulled. Dispose in the trash.
Mowing is not recommended, as plants will resprout from the roots. Plowing or tilling over the course of several years may be effective. In pastures, good management of grass and forage plants will help reduce knapweed populations. Contact the local noxious weed control board to ask about herbicide control.
Finally, this plant is on Washington’s Prohibited Plants and Seeds list, meaning it may not be transported, bought, sold, or distributed in the state. If found, please report to the Washington State Department of Agriculture.
To learn more, visit our “Don’t Pack a Pest” and “Play, Clean, Go” campaign pages.
What Are Their Characteristics?
Knapweeds in General
There are seven knapweed species that are noxious weeds in Washington. They all look very similar and are difficult to tell apart. Visit this handy brochure if interested in identifying the different types of knapweed.
- Knapweed grows up to 5 feet tall.
- Flowers are yellow, white, pink, or purple. Flowers resemble small thistles growing at the end of clustered branches.
- At the base of flowerheads are small leaf-like structures called bracts. All knapweeds mentioned here have them, but their size, shape, and color vary depending on the species.
Yellow starthistle
- Grows to be 18-36 inches tall, with a deep taproot.
- Has alternate leaves with bases extending downward, producing a winged effect.
- Stems are dull green and covered with woolly hairs.
- Yellow flowers are thistle like, with short yellowish spines in star-like arrangement at the base of the flower head.
Additional Photographs
