DIFFUSE KNAPWEED (CENTAUREA DIFFUSA LAM.)
Biennial or short-lived perennial in the Asteraceae (sunflower) family. The plant is 10-60cm tall with erect, branching stems with short, stiff hairs. Flowers appear from June - September, and are urn shaped with white, rose-purple, or lavender ray flowers surrounding tubular disk flowers. Bracts have terminal spines and may be spotted. Seeding occurs in August and September; seeds are brown or black. Leaves are deeply lobed, hairy and grayish-green, and form basal rosettes in their first year.
Habitat and Ecology
Diffuse knapweed prefers arid, low elevation habitats including grasslands, open dry forests, pastures, roadsides, and other disturbed areas. Cultivation and excessive moisture prevent it from growing in irrigated fields.
Diffuse knapweed replaces high-value forbs and grasses in rangelands, reducing productivity for both livestock and wildlife. When it invades disturbed sites, it reduces biodiversity, depletes soil and water resources, and can increase soil erosion.
Biocontrol has been successful in reducing the population of this plant to acceptable levels. For localized infestations, plants can be hand pulled or mowed before seed-set. Hand pulling may need to be repeated annually. Herbicide application following an autumn mowing is usually successful. Be sure to check with the BC Ministry of Agriculture, Food and Fisheries Crop Production Guides for herbicide recommendations. There is mixed information about the effectiveness of fire on reducing populations; some studies show that burning will reduce populations or improve efficacy of herbicide application. On the other hand, recently burned areas provide the ideal germination conditions for knapweed.
Province of British Columbia. 2002. Guide to Weeds in British Columbia.
USDA Fire Effects Information System
Photo reference: Carey Minteer, University of Arkansas, Bugwood.org