By Selena Cole 

What is knapweed and why should we care? 

Here on Anarchist, you can find both species of knapweed – Spotted (Centaurea stoebe) and Diffuse (Centaurea diffusa)  – which are considered invasive species throughout BC’s southern interior.

Diffuse Knotweed (photo by R. Meuller)

Spotted Knotweed (photo by Rob Routledge)

Knapweed is a biennial (typically lives for two years) or short-lived perennial weed. The plants start growing as ground hugging florets (see photo below). Flower buds start to form in early June and flowering occurs from July through October. Each plant can produce hundreds to thousands of seeds and the seeds can remain viable in the soil for five to ten years, making an infestation challenging to control. In winter, plant skeletons of diffuse knapweed break off and tumble in the wind, spreading seed over large areas. Seeds are also spread by wind off the standing plant, via livestock and in unclean fill material and topsoil.

Knapweed is an extremely tough plant that can tolerate drought, trampling, and very rocky and poor quality soils. Its roots exude a chemical that inhibits the root growth of other plants. Severe infestations of both species of knapweed will outcompete native plant species, especially plants of value to wildlife. Also, dead knapweed can significantly increase fire risk in the forested and open areas surrounding our homes.

Got knapweed? Now what…

The most effective way to control both spotted and diffuse knapweed is by prevention.

• Proactively remove infestations of knapweed from your property to reduce spread and fire risk.
  The plants have a large taproot and are surprisingly easy to remove by hand-pulling.
• Effective control methods include:
  o hand-pulling (for single plants and small infestations)
  o mowing (for large infestations)
  o chemical application (NOTE: make sure the product is approved for use in Canada and
  consider the impact of using chemical control on other plants and wildlife)
  o biological control agents (moths, weevils, flies, beetles, or rusts that feed on knapweed
  seeds or roots; this method has been used on Anarchist Mountain in areas with large
  infestations with considerable success)
• Maintain grasslands in a healthy, vigorous condition to ensure a productive plant community.
  Competitive perennial grasses and forbs utilize water, nutrients and space that would otherwise
  be readily available to knapweed.
• Regularly patrol your property for knapweed plants and immediately control or remove
  infestations before seed set.
• Cooperate with adjacent landowners and encourage them to prevent knapweed spread.
• Immediately re-vegetate disturbed bare soils with a suitable seed mixture that provides dense,
  early colonization to prevent weed invasion.
• Clean your vehicles and machinery of plant material and soil before leaving a knapweed
  infestation.
• Monitor your property annually until it is knapweed free for several consecutive years.
• Do not move contaminated soil or fill to a new area.

For more information about knapweed and other invasive plants you might encounter, go to: Invasive Species Council of BC

Selena Cole is a retired environmental scientist and program lead for the AMFRS Wildlife Safety Program