General: Spotted Knapweed is a biennial or short-lived perennial. It starts out as a rosette and can remain that way for several years before bolting and dying in a subsequent season.
Flowers: The flowers are light purple, aromatic, and found individually or in clusters on the stems. The bracts have black tips, giving the plant its spotted appearance.
Stems: The stems are upright and grow to around 1 – 1.5 m tall. First-year rosettes are comprised of deeply-lobed leaves on short stems, while mature plants have slender stems with many branches.
Leaves: The leaves are deeply lobed, greyish-green, hairy, and covered with translucent dots. Leaf arrangement is alternate. Rosette leaves are up to 20 cm long, while second-year leaves become progressively smaller with fewer lobes towards the top of the plant.
Fruit: Spotted Knapweed seeds are bristled.
Roots: Spotted Knapweed has a taproot as well as lateral roots.
Diffuse Knapweed (Centaurea diffusa) has white flowers
Meadow Knapweed (Centaurea debeauxii) has undivided leaves and larger flowerhead bracts with comb-like fringes at the tips
Russian Knapweed (Centaurea repens or Acroptilon repens) has smaller flowers and no black marks on the bracts.
Black Knapweed (Centaurea nigra) is shorter (about 10 – 80 cm tall) with pink to purple flowers.
Brown Knapweed (Centaurea jacea) has pink flowers and lace-shaped or elliptic leaves.
Habitat and Origin
Spotted Knapweed originates from Eurasia. It was first introduced in North America in the late 1800s as a contaminant of alfalfa seed.
Spotted Knapweed likes open areas and well-drained soils. It is somewhat shade tolerant and prefers more moisture than other knapweeds. However, it is tolerant of a wide range of soil moisture levels. Spotted Knapweed is typically found in grasslands, open forests, along roadsides, on right-of-ways, and clear cuts.
Propagation & Vectors of Spread
Spotted Knapweed reproduces by seed. One plant can produce up to 40,000 seeds in a season, and one square meter can contain up to 140,000 seeds. Seeds can remain viable in the soil for upwards of 8 years, so Spotted Knapweed infestations rapidly form a significant seed bank.
Spotted Knapweed seeds are generally dispersed over short distances by the wind. Longer distance dispersal is often caused by seeds making their way into hay and the undercarriages of vehicles, thus allowing for transportation over great distances. Livestock and wildlife also aid in seed dispersal.
Ecological and Health Impacts
- Displaces native species, threatening biodiversity.
- Carries allelopathic properties: produce chemical compounds that prevent the growth of other plant species.
- Increases fire load.
- Chokes out desirable forage for livestock and wildlife.
- Increases runoff and erosion, leading to sedimentation of watercourses.
- Causes skin irritation.
What Can I Do?
Spotted Knapweed is currently found throughout the Sea to Sky Corridor, so the best approach to controlling its spread is by CONTAINMENT.
Learn to identify Spotted Knapweed: use the images presented in this profile page to learn how to identify Spotted Knapweed.
What to do if you spot it: You can report any Spotted Knapweed sighting 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 Spotted Knapweed.
- Minimize soil disturbances (e.g. use grazing plants that prevent soil exposure from overgrazing), and use seed misses with dense, early colonization (e.g. alfalfa or barely) to revegetate exposed soil and resist invasion.
- Ensure that plants are disposed of in a garbage bag if found in floral arrangements to prevent seeds from spreading.
- Maintain grasslands in a healthy, vigorous condition to ensure a productive, competitive plant community.
- 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.
- Plant Spotted Knapweed in a garden, no matter how well-contained its enclosure may seem.
- Move soil that has been contaminated with Spotted Knapweed.
Small infestations can be hand-pulled but will have to be repeated throughout the season and over many years. Every effort should be made to remove the taproot with little soil disturbance. If not possible, cut the taproot 2-4” below the soil surface. It is best to treat Spotted Knapweed before the plants set seed. Mowing doesn’t kill knapweeds; plants mowed at the rosette stage will quickly recover, and mowing after flowering will only further disperse the seeds.
Effective herbicides include picloram, dicamba, 2,4-D, clopyralid, aminopyralid, and glyphosate. Note that picloram, while effective, is not suitable for wet, 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.
Twelve biocontrol agents have been released in BC for knapweeds. They are most effective when used in combination. Several agents show promise for a variety of habitats.
Grazing is not an effective eradication method; however having cattle, sheep, and goats graze Spotted Knapweed in early spring may help suppress flower and seed production.
- Alberta Invasive Species Council, Brown Knapweed Factsheet, https://abinvasives.ca/wp-content/uploads/2017/11/FS-BrownKnapweed.pdf
- Capital Regional District, High Priority Species Capital Region Invasive Species Program, https://www.crd.bc.ca/docs/default-source/es-watersheds-pdf/invasive-species/edrr-factsheet.pdf?sfvrsn=488bb1ca_2
- Invasive Species Council of BC, Knapweeds TIPS Factsheet, https://bcinvasives.ca/resources/tips/knapweed
- Okanagan and Similkameen Invasive Species Society, Diffuse & Spotted Knapweed Factsheet, http://www.oasiss.ca/pdfs/FACTSHEET_KNAP.pdf
- Ontario Invasive Plant Council, Spotted Knapweed Best Management Plan, http://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/07/OIPC_BMP_SpottedKnapweed_FINAL_Mar292017_D4.pdf
- UC Davis, Spotted Knapweed: A Weed Report from the Weed Control in Natural Areas in the Western United States, https://wric.ucdavis.edu/information/natural%20areas/wr_C/Centaurea_stoebe.pdf