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Knapweeds

Knapweeds

The knapweeds (spotted knapweed, Centaurea maculosa; diffuse knapweed, Centaurea diffusa; meadow knapweed, Centaurea pratensis) cover thousands of hectares of land in BC. An individual plant can produce up to 140,000 seeds per square meter, giving little opportunity for native plants to grow. Once introduced, this species is highly competitive and degrades natural plant communities.

Spotted knapweed impacts wildlife and sensitive ecosystems, forming monocultures that displace native plants and reduce biodiversity, further threatening already rare and endangered species. As well, spotted knapweed reduces the grazing capacity of land, impacting ranchers and the agricultural community.

 
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Spotted knapweed can be identified by its light pink solitary flowers growing out of stiff black-tipped (“spotted”) bracts. The plant grows from a basal rosette in the first year to almost 2 m tall in the second. The leaves are deeply lobed and the plant is based on a stout taproot.

Once established, spotted knapweed is very difficult to get rid of. Prevention is the best control. If already established, the most effective way of controlling the plant is by mowing to prevent further seed spread. Control efforts should be especially diligent along linear corridors, such as roadsides and driveways. Care should be taken not to spread seeds on the undercarriage of your vehicle, on shoes or other equipment. For herbicide options, see website below.

Active irrigation of affected sites is another fairly effective method of control. Through constant, regular watering knapweed can be choked out and grasses will emerge. Saturated knapweed can then be easily pulled out of moist soil.

Several biological agents have been released and successfully established on the knapweeds in the Coastal region, including: the gall fly, Urophora sp., larvae that feed on the stem of the plant, stunting plant growth by diverting nutrients; the moth, Agapeta zoegana, larvae that mines into the plant root, decreasing bolting stems, above-ground biomass and seed heads, and; the root boring weevil, Cyphocleonus achates, larvae that stunts plant growth and destroys the interior of the tap root by boring inside and exposing it to bacterial and fungal invasion.

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