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Diffuse knapweed (Centaurea diffusa) and Spotted knapweed (Centaurea stoebe)



Diffuse knapweed is an extremely aggressive weed that inhibits the plants growing nearby by secreting an allelopathic chemical. So far 7 different types of biological control insects have been released on it and we have seen reductions in density in many areas.  


Spotted knapweed is a highly aggressive species that invades moist, rich grasslands, disturbed areas and road edges. Preventing further expansion of this weed is critical to maintaining the forage base for wildlife.  Biological control insects have been introduced into many areas but this knapweed continues to expand.


Diffuse knapweed (Centaurea diffusa) has degraded grasslands throughout the Boundary. It is currently widespread in the Boundary and is believed to have reached it’s ecological limit in many areas. Now hybrids of diffuse and spotted knapweed are appearing which may fill the gap in distribution.  


Spotted knapweed (Centaurea stoebe) is a relatively recent invader to the Boundary area. The main infestation corridors occur in the Greenwood, Canyon Bridge, Carmi and Paulson areas. Isolated plants and patches are located along roadside corridors outside the main infestation. ​

Diffuse knapweed has hairy, grayish-green leaves on many branches growing from a single upright stem. The flowers are white with small sharp rigid spines. It can grow up to 1 meter. 


Spotted knapweed is a biennial to short-lived perennial growing from a stout taproot. It has bright purple flowers which are visible July through September. The flower bracts are black tipped giving it a spotted appearance. It grows to 1.5 meters tall with branched stems.​


Spotted Knapweed
Diffuse Knapweed

Containment work will continue along road systems this year to keep spotted knapweed from spreading to uninfested areas. So far bio-control insects have not produced significant drops in plant density or size. Spotted knapweed is just a lot harder to kill than diffuse knapweed.


The diffuse knapweed is being controlled by bio-control insects.  Both spotted and diffuse knapweed are provincially designated as noxious under the Weed Control Act.  Which means you must control it if it is on  your property.

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