Spotted knapweed, bushy knapweed
Spotted knapweed is a short-lived perennial or biennial plant that can grow up to four feet tall. This plant has a strong taproot as well as lateral roots. Rosettes of this plant are grayish-green in color with leaves up to six inches in length. Each rosette may produce many stems. This plant will adapt if it is mowed and will grow shorter and shorter to bloom in very low heights which can vary from two inches to four feet. The stems and leaves are a blue-green color but short with thin hairs on the leaves that can give a silver-gray cast. One flower that is pink to pinkish-purple grows on each stem. The flowers are surrounded by oval bracts with black tips, which gives the plant the name of ‘Spotted’ knapweed. One plant can produce up to 300 flower heads and produce up to 140,000 seeds. This plant blooms from June to September. Many mistake Spotted knapweed as Canada thistle. Reproduction is primarily by seeds.
Pink to purple bachelor-button looking flowers, bracts with black tips directly under the flower head and deeply lobed lower leaves are fine with cob-web type hairs.
Spotted knapweed is a highly adaptable plant and can be found in various elevations in moist or dry conditions, is shade tolerant but is commonly found in sunny areas and prefers well drained or gravel/sandy soils. This is the number one weed for Montana that has over 1 million acres of Spotted knapweed.
There are herbicides and other control methods that commonly control spotted knapweed. Hand pulling and digging can be very effective if done before the flower and seed establishment. Dig below the growing point or pull by the roots. Herbicides that are effective include Tordon, Milestone, Escort, 2,4-D, and Dicamba. For more information on these herbicides and other control methods contact the Weed and Pest office.
Spotted knapweed produces its own natural herbicide called “catechin” that eradicates other plants around it.