Squarrose Knapweed

Squarrose knapweed is a perennial forb in the Asteraceae (daisy) family. It grows from a deep taproot to form a basal rosette in its first year of growth; in subsequent years it produces a main stem erect from 1 to 3 feet tall. Leaves are arranged alternately on stems. Lower leaves are pinnately lobed and thin. Leaves decrease in size up the stem and become linear. The plant may be covered in small hairs giving it a grayish-green appearance. Squarrose knapweed flowers from early summer to fall. Flowers are similar to those of spotted knapweed, but they are frequently smaller with narrow bulbs under the florets. These bulbs are actually bracts which are usually green to pale yellow. Flowers are terminal and usually solitary but may occur in small clusters. Flowers are clusters of florets which are pink to light purple. Unlike other knapweed species, squarrose knapweed flowers only consist of four to 10 florets. Seeds are small (2.5 mm), oval, white to light brown, and have pappus at one end.

Squarrose knapweed is native to Asia. It is now known how this species was first introduced to North America. It spreads by seed. Pappus on seeds catch on clothing, fur, and equipment and are transported to new areas. It is primarily found in disturbed sites, landscapes, and roadsides.

Prevention and cultural control strategies should be utilized as much as possible.

In Teton County, squarrose knapweed infestations are not dense enough to sustain biological control agents; however, some biological control agents in use for spotted knapweed are also effective in reducing populations of squarrose knapweed. These agents include seedhead feeding beetles (Larinus minutus and Larinus obtusus).

Because squarrose knapweed has a taproot, it can be readily managed by hand pulling or digging out the root. Wear gloves and long sleeves when handling knapweed species as they can cause allergic reactions. Mowing will not eradicate stands of squarrose knapweed, but it can be beneficial in preventing flower production. Do not mow while seeds are present.

Larger infestations can be controlled with herbicides. Spring and fall treatments of rosettes are recommended, but treatment of bolting plants can be effective. Read the label before using any herbicide. Contact TCWP if you have any questions about application rates or how to use an herbicide.

• Weed Report: Squarrose Knapweed. Weed Control in Natural Areas in the Western United States. (2013) DiTomaso, Kyser et al. University of California.
• Squarrose knapweed (Centaurea virgata). Bureau of Indian Affairs - Navajo Region.
• Squarrose knapweed. (2015) Colorado Department of Agriculture.
• Exotic Species: Squarrose Knapweed. (2010) Reeves, K. National Park Service: Southern Colorado Plateau Network Inventory and Monitoring Program.
• Squarrose Knapweed (Centaurea virgata ssp. squarrosa). Idaho State Department of Agriculture.