Walla Walla County Extension

Weed of the Month

Diffuse & Spotted Knapweed

Diffuse knapweed (Centaurea diffusa Lam.) and spotted knapweed (Centaurea maculosa Lam.) are members of the sunflower family (Asteraceae). These aggressive, exotic invaders from Europe were accidentally introduced as crop seed contaminants. Knapweed thrives on any disturbed soil in gravel pits, roadside right-of-ways, vacant lots, trails, and heavily grazed pasture or rangeland. It outcompetes native plants species, decreases forage production for livestock and wildlife, and is a wildfire hazard.

Diffuse knapweed is an annual or short-lived perennial plant with a stout taproot and a single rough stem, 1 to 2 feet tall, branched toward the top. Grazed plants may produce multiple stems. Rosette and lower shoot leaves are finely divided. Spotted knapweed is a short-lived, perennial that reproduces primarily from seed and forms a new shoot each year from a taproot. The weed produces one or more shoots that are branched and 1 to 3 feet tall. Basal leaves can be 6 inches long and deeply lobed. Upper leaves are similar to diffuse knapweed.

Spotted knapweed flowers are usually pinkish-purple, sometimes white. Flower bracts have dark margins with short fringe. Diffuse knapweed flowers are often white but may be purplish. Bracts under the flowers have yellow spines with comb-like teeth on the spine margins. Sometimes the bracts are dark-tipped or spotted like spotted knapweed. The long terminal spine differentiates diffuse from spotted knapweed.

Knapweed germinates in spring or fall and flower continuously from early summer into the fall, as long as moisture and temperatures permit. Many narrow flowering heads occur on shoot tips singly or in clusters of two or three. They are about 1/8 inch in diameter and 1/2 to 2/3 inch long. A single flower stalk can produce 1,200 seeds and seeds may remain viable in the soil up to 5-8 years. The seeds are dispersed when the plant breaks off at the base becomes a tumbleweed, often spread by vehicles. The seeds are also transported by shoes and clothing, by feeding rodents and other wildlife, and in contaminated hay and crop seed.

The best knapweed control is prevention. Avoid travel through infested areas, clean clothing and shoes after walking in infested areas, don’t graze livestock when ripe seeds are present, and use weed-free hay. The plant is usually easy to control with herbicides but an area must be monitored for several years and treated as necessary for seedling control.

Biocontrol agents used in Walla Walla County include three seed eating beetles, Larinus minutus, Larinus obtusus, Eustenopus villosus, and a bud weevil, Bangasternus orientalis.

Existing plants may be killed with 2,4-D or glyphosate but yearly reapplication is needed to control new plants germinating from seed stored in the soil. Products that contain clopyralid such as Transline, Curtain (clopyralid plus 2,4-D), and ReDeem (clopyralid plus triclopyr) should be applied in fall or early spring at rosette stage. Follow-up treatment the following year may be necessary to control seedlings. Banvel (dicamba) and Tordon provide longer control, perhaps for two or more seasons varying with soil type and precipitation patterns. Seeding suitable perennial grasses helps prevent weed reinvasion.

Please refer to the annually updated Pacific Northwest Weed Management Handbook at http://weeds.ippc.orst.edu/pnw/weeds for specific control recommendations. ALWAYS FOLLOW PESTICIDE LABEL INSTRUCTIONS.