Russian knapweed (Rhaponticum repens)
A Class B Noxious Weed in Benton County
Russian knapweed is a perennial forb that spreads by both seeds and by roots fragments. The roots are dark-brown or black and have a scaly appearance. Russian knapweed can grow to be 3 feet tall. The leaves are alternate. The rosette leaves are lance-shaped and taper at both ends with the broadest part at the tip. As the plant grows, the lower leaves are larger with the upper leaves becoming progressively smaller. The flowers are urn-shaped and pink to purple in color. The flowers turn to a straw color at maturity. Outer bracts under the flower heads are greenish to straw colored and have broad papery tips. The stem dies back to the soil surface each year.
Russian knapweed is allelopathic, which means it contains a toxic substance that inhibits the growth of surrounding plants. It also is poisonous to horses. Russian Knapweed can form dense colonies:
How to control:
The keys to controlling Russian Knapweed is to stress the plant, so that it uses the nutrient reserves in its root system, and then control its vegetative spread.
- Tilling can create root fragments that will sprout. Deep tilling over 3 years can kill the root system.