- Centaurea biebersteinii & Centaurea diffusa
Spotted (Centaurea biebersteinii) and diffuse knapweed (Centaurea diffusa) are biennial to short-lived perennials, and considered regionally noxious under the BC Weed Control Act. Currently distributed throughout southern BC, both spotted and diffuse knapweed are a major concern in the Omineca, Peace River, Kootenay, Okanagan, Thompson, and Cariboo regions.
Diffuse knapweed has hairy, greyish-green, split leaves on many branches growing from a single upright stem. The flowers are white or sometimes purple, with small, sharp, rigid spines on the bracts. Spotted knapweed has hairy, deeply-cut leaves and purple flowers (occasionally white) on one or more upright stems. Flowerhead lower leaves have a black-tipped fringe that gives a spotted appearance. Diffuse and spotted knapweed both have a taproot and grow to around 1 metre in height.
Spotted knapweed is a prolific seed producer, with individual plants producing up to 140,000 seeds per square metre. Seeds and plant fragments make their way into hay and the undercarriages of vehicles, allowing for new infestations over great distances. Diffuse knapweed plants can produce up to 18,000 seeds that can remain dormant in the soil for years. Both knapweeds spread by wind, livestock and people, preferring open areas and well-drained soils where they establish in grasslands, open forests, and along roadsides. Spotted is more intolerant to dense shade and prefers moister habitats than diffuse; however, spotted is still a problem in forested areas disturbed by logging, fire, or other factors. Diffuse cannot tolerate cultivation or excessive moisture and is therefore uncommon on cultivated land. They choke out desirable forage for livestock and wildlife and increase soil erosion.
Warning: Contact with spotted knapweed can cause skin irritation.