New weeds to watch for in Douglas County: Spotted Knapweed

By Aaron Rosenblum
Foster Creek Conservation District

This is the second in a series of articles that introduces readers to new weed invaders in Douglas County. The weed species featured are known from only a few locations in the county, but have the potential to become wide-spread and problematic. The goal of these articles is to inform and educate Douglas County residents, so that you can help us with the early detection and rapid treatment of small weed populations before they become major issues throughout the county.

 

Black spotted bracts appear on the base of the vase-shaped spotted knapweed flower head. (Provided photo/Washington State Noxious Weed Control Board)

This week’s weed to watch for is spotted knapweed (Centaurea stoebe ssp. micranthos). Spotted knapweed is a highly aggressive plant listed as noxious in the state of Washington. It is generally a short-lived perennial, meaning it lives and produces flowers and seeds for multiple years, but can sometime be biennial, meaning it completes its lifecycle in two years. The plant forms a rosette in the first year, where leaves are formed close to the ground in a circular pattern, and there is no visible stem. The leaves are hairy, rough and are deeply lobed.

In the second year of growth, a flower stem 2-5 feet tall forms. The stem is branched with each branch producing a terminal, solitary, vase shaped flower head, with flowers ranging in color from pink to purple. The lower portion of the flower heads are composed of structures known as bracts, and they all have black tips. It is these black tips that create a spotted appearance and lead to the name of the plant.

A rosette of deeply-lobed spotted knapweed leaves. (Provided photo/Washington State Noxious Weed Control Board)

Spotted knapweed thrives in disturbed locations, preferring full sun and well-drained soil. It is commonly found on roadsides, but is also found on vacant lots, pastures, ditches and rangeland.

Spotted knapweed is highly competitive and will quickly outcompete native plants, reducing habitat and forage value. This is in part due to the fact that spotted knapweed produces a chemical that inhibits or kills other plants nearby. This is an ecological process known as allelopathy.

There are multiple species of knapweeds in Douglas County that can be difficult to tell apart. Spotted knapweed grows in the same habitat type as diffuse knapweed, but has pink or purple flowers and has black spots on the bracts at the base of the flower head, while diffuse knapweed has white flowers (occasionally light purple) and has bracts that are comb-like rather than spotted. Russian knapweed has pink or purple flowers but can be distinguished from spotted knapweed because it prefers to grow in moist to wet soils and has green to straw colored bracts under the flower head.

Foster Creek Conservation District is leading the development of the Douglas County Cooperative Weed Management Area and has funding to assist landowners with the control of spotted knapweed, as well as the other weeds featured in this series of articles. If you believe you have spotted knapweed on your land please contact me at 423-5990 or arosenblum@fostercreekcd.org; or contact Dale Whaley with the WSU Extension at 745-8531, ext. 6352.

 

Aaron Rosenblum is the natural resource specialist for the Foster Creek Conservation District.