General: Biennial or short-lived perennial
Flowers: Small, white and sometimes rose-purple or lavender. Flowers form alone or in clusters at the ends of branches. They have small, sharp, rigid spines on the bracts.
Stems: Single, erect, growing to 0.1 – 0.6 m tall with multiple branches. Entire stem is covered in short, stiff, white hairs.
Leaves: Upper leaves are stalkless and become bract-like near the flowers. The rosette and lower leaves are 5 – 20 cm long, rough, hairy and divided.
Seeds: Light brown to black, around 3 mm long.
Roots: Deep taproot.
Spotted Knapweed (Centaurea bierberstienii) has hairy, deeply-cut leaves with the lower leaves having a black-tipped fringe, which gives it its spotted appearance. Flowers are purple (white on occasion) on a single or multiple upright stems that can grow to 1 m tall. A single plant can produce up to 140,000 seeds. Spotted Knapweed prefers moist habitats as well as disturbed areas.
Habitat and Origin
Diffuse Knapweed comes from the Eastern Mediterranean and was brought over as a contaminant in crop seeds.
Diffuse Knapweed grows in disturbed, dry areas such as sandy areas, grasslands, pastures, woodland clearings, open forests, roadsides, right-of-ways and clear-cuts. It is intolerant of moist soils, flooding and shade.
Propagation & Vectors of Spread
Propagation is done solely by seed, with a single plant producing up to 18,000 seeds a season. Diffuse Knapweed seeds can remain viable in soil for up to 15 years. Seeds germinate in the fall or spring and develop rosettes within the first year of growth.
Diffuse Knapweed spreads solely by seed, breaking off when mature and tumbling in the wind. Seeds can spread up to 100 m from the parent plant. Seeds can also be spread by wildlife, livestock and humans via hay and vehicle undercarriages.
Ecological and Economic Impacts
- Diffuse outcompetes and replaces traditional forage plants on range and pasturelands. While it is not poisonous, it can lead to reduced nutrition for livestock and wildlife.
- Moreover, mature knapweed plants are coarse and fibrous and the spines on the bracts can be very irritating, or may even cause injury to the mouths and digestive tracts of grazing animals.
- Can deplete soil and water resources.
- Reduces biodiversity
- Reduces land values and increases road maintenance costs.
What Can I Do?
Diffuse Knapweed is currently found throughout the Sea to Sky Corridor, so the best approach to controlling its spread is by PREVENTION.
Learn to identify Diffuse Knapweed: use the images presented in this profile page to learn how to identify Diffuse Knapweed.
What to do if you spot it: You can report any Diffuse Knapweed sightings by clicking here.
- Regularly monitor properties for weed infestations.
- Ensure soil and gravel are uncontaminated before transport.
- Check wildflower mixes to ensure that they do not contain Diffuse Knapweed.
- Ensure that plants are disposed of in a garbage bag if found in floral arrangements to prevent seeds from spreading.
- Unload, park or store equipment or vehicles in infested areas; remove plant material from any equipment, vehicles or clothing used in such areas and wash equipment and vehicles at designated cleaning sites before leaving infested areas.
- Diffuse Knapweed in a garden, no matter how well-contained its enclosure may seem.
- Move soil that has been contaminated with Diffuse Knapweed.
- Do not compost Diffuse Knapweed.
Repeated hand-pulling and digging before flowering can be effective on small infestations, but must be done over several years. Make sure to wear gloves and a long-sleeved shirt as Diffuse Knapweed is abrasive and can cause skin irritation. Mowing also prevents seed production but the remaining root will re-sprout.
Foliar application of Dicamba, 2, 4-D, clopyralid, aminopyralid or glysophate are effective forms of control. Picloram is also very effective, but not appropriate to use in moist, coastal soils.
We recommend that any herbicide application is carried out by a person holding a valid BC Pesticide Applicator Certificate. Before selecting and applying herbicides, you must review and follow herbicide labels and application rates; municipal, regional, provincial and federal laws and regulations; species-specific treatment recommendations, and site-specific goals and objectives.
The Agapeta zoegana moth targets the roots, the Cyphocleonus achates weevil targets the roots and foliage, the Larinus minutus weevil, L. obtusus weevil and the Urophora jaceana fly targets seeds.
Alberta Invasive Species Council, Knapweeds, Identification Guide for Alberta Invasive Plants (p.26), https://aaaf.ab.ca/uploads/pdf/Weed_ID_Guide_2017.pdf
eFlora, Diffuse Knapweed, http://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Centaurea%20diffusa
Invasive Species Council of British Columbia, Diffuse Knapweed, https://bcinvasives.ca/invasives/diffuse-knapweed/
Invasive Species Council of British Columbia, Knapweeds TIPS Factsheet, https://bcinvasives.ca/wp-content/uploads/2021/01/Knapweeds_Factsheet_20_02_2019.pdf
Invasive Species Council of Manitoba, Diffuse Knapweed, https://invasivespeciesmanitoba.com/site/index.php?page=diffuse-knapweed