Diffuse Knapweed

Diffuse knapweed

Common Name: Diffuse Knapweed

Scientific Name: Centaurea diffusa

Habitat: roadsides, disturbed areas, farmland, grassland

Provincial Designation: Prohibited Noxious

Prohibited Noxious weeds are plant species designated in the Alberta Weed Control Act. Prohibited noxious weeds must be destroyed when found, meaning all growing parts need to be killed or the plant's reproductive mechanisms need to be rendered non-viable.

Origin

Diffuse knapweed is native to Eurasia and was introduced to North America as a contaminant in alfalfa and clover seed.

Legislated Because

Diffuse knapweed becomes tough and unpalatable late in the season which causes grazing animals to avoid using this plant as forage. It can quickly establish on disturbed sites and invade native plant communities. Once established, it can form large infestations in both rural and urban areas. Its roots exude a chemical that inhibits the root growth of other plants.

Description

The flowers of diffuse knapweed are white to pink in colour and are surrounded by triangular bracts, which are modified leaves underneath the flower petals, that narrow into short, stiff spines. Leaves are alternate, deeply lobed, and covered in fine hairs that give them a grey-green appearance.

Diffuse knapweed has a single upright stem that can reach heights up to 100 cm.

Biennial - it develops a low-lying rosette (circle of leaves right from the ground) in the first year of growth. In the second year, it sends up the blooming stalk. A single plant can produce 18,000 seeds. In winter, plant skeletons break off and tumble in the wind, spreading seed - a type of ‘tumbleweed’.

It commonly causes rashes when touched with bare skin

Can Be Confused With ...

Other Knapweeds - Centaurea species - but each type has a unique bract characteristics which will be the determining factor in identification. A number of knapweed species had been listed as prohibited noxious in Alberta.

Bachelor’s Buttons - Centaurea montana - and related garden cultivars which are ornamental garden plants and fine for you to grow! When in doubt, please contact invasiveweeds@edmonton.ca