Spotted and diffuse knapweed

Identification & Biology
Latin Name : Centaurea biebersteinii & Centaurea diffusa
Category : Terrestrial Plants
Description :
  • Tap-rooted biennial to shortlived perennial up to 1.5 m tall 
  • Leaves form a flat rosette in the first year  
  • Divided leaves are greyish green and hairy  
  • White, pink or purple flowers 


While diffuse and spotted knapweed are similar looking plants, spotted knapweed can be distinguished by its black-fringed bracts on the seed heads, giving the plant its ‘spotted’ appearance. Diffuse knapweed and spotted knapweed can vary their lifespan. Plants that complete their juvenile growth by the fall, and overwinter as rosettes, bolt in spring. Plants that have not finished the juvenile stage by the end of fall remain as rosettes through the second year and bolt during the third year. Flower buds are formed in early June and flowering occurs from July to October. Both diffuse and spotted knapweed are prolific seed producers, producing respectively up to 900 and 400 seeds per plant, under dry land conditions. Seeds may remain viable in the soil for over 8 years. Seed is commonly spread by wildlife, domestic animals and movement of infested hay and soil as well as plants caught in the undercarriage of vehicles and machinery. Diffuse knapweed plants are also spread by the wind, as mature plants blow around in a tumbleweed fashion. 


Both diffuse and spotted knapweed were introduced from Eurasia in the early 1900’s, likely as contaminants of alfalfa seed. They are widespread at low to mid-elevation grasslands and dry open forests. They are commonly found on roadsides, fields and disturbed areas. Both species are well adapted to well-drained, light to coarse textured soils but are not tolerant of dense shade. Spotted knapweed tends to inhabit moister habitats than diffuse knapweed, but it can survive in very dry climates. Both species now occupy over 100,00 acres of grassland, forest, and rangeland in B.C. 

Impact & Risks
  • Infestations of diffuse and spotted knapweed are causing severe environmental deterioration to B.C.’s Southern Interior grasslands, forests and rangelands.  
  • Infestations degrade livestock habitat and reduce forage production. 
  • Infestations can decrease recreational enjoyment. 
  • Knapweed can cause an undetermined loss of wildlife habitat. 
  • Knapweed is generally unpalatable to livestock, and spines on the flowers heads of diffuse knapweed may cause injury to the mouth and digestive tract of grazing animals.  
  • On agricultural land, knapweed can cause serious reductions in yields, crop value, and may even significantly devalue the land itself. 
  • Dead plant material can increase fire risk. 
  • Other losses include soil erosion and reductions in wildlife populations due to the decrease in available forage. 
Prevention & Mitigation

The most effective way to ensure that your lands do not become infested with spotted and diffuse knapweed is by prevention. Here are some recommendations to prevent knapweed from invading your property: 

  • 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 site annually until it is knapweed free for several consecutive years.  
  • Do not move contaminated soil to a new area.  
Treatment & Disposal
  • The key to controlling existing infestations is to eliminate new seed production and deplete the existing seed bank. 
  • Small infestations may be hand pulled to prevent seed production. Plants must be pulled rigorously. Plan to pull when soil moisture remains high enough to remove the entire taproot with ease.  
  • Mowing or cutting may also be used for controlling knapweed. Generally, for light infestations, hand cutting or mowing should be done in the early flower stage; plants are less likely to re-sprout if allowed to bolt before cutting. Cutting must be repeated over several years to deplete the seed bank.  
  • If seeds have developed, removed plants must be bagged and buried deeply at a landfill. 
  • Fire is not an effective control for knapweed.  
  • Continued monitoring and follow up treatments will be required as knapweed has an extensive, long lived seed bank. 
  • Chemical control is also an option. Before applying herbicides, read the label for full use and precautionary instructions.  
  • For further information on the selection and application of chemicals to protect your crop, contact AgriService BC at 1-888-221-7141 or email 
  • 12 knapweed biocontrol agents have been released in B.C. to control knapweed.  
Okanagan Distribution

Priority Level Definitions

Watch For - Poses a significant threat (very high risk) and does NOT presently occur in the region OR is relatively new to the region and is very limited in extent.
High - High risk/impact; limited population with significant potential to spread in the region.
Medium - Medium risk/impact; limited distribution – broader population distribution with potential to spread further in a region.
Low - Low risk/impact; may be widespread or not, may be of concern in specific situations with certain high values – e.g. specific agriculture crops. Some species may be treated primarily with biological control agents.