Diffuse Knapweed

Diffuse Knapweed

Centaurea diffusa 
Diffuse-Knapweed-Bryan-Kelly-McArthur-600x425
Photo credit: Bryan Kelly-McArthur

Status in Squamish:

CONTAIN

Status in Whistler:

CONTAIN

Status in Pemberton:

STRATEGIC-CONTROL
Vectors of spread:
Synonyms

Diffuse Knapweed is also known as:

  • White Knapweed
  • Tumble Knapweed
  • Spreading knapweed
  • Actosa diffusa
ID Characteristics

Flowers: Small, white, sometimes rose-purple or lavender. Flowers form alone or in clusters at the end of branches. They have small, sharp, rigid spines on the bracts.

Stems: Single, erect with multiple branches. Diffuse Knapweed grows up to 1 m tall; stems have 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, grey-green, hairy and divided. They grow in an alternate arrangement along the stem.

Seeds: Light brown to black, about 3 mm long.

Roots: Deep taproot.

Similar Species
Invasive:

  • 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.
Report

Please report any sighting of Diffuse Knapweed by clicking here.

Habitat and Origin

Biennial or short-lived perennial that is Eastern Mediterranean in origin. It was brought to North America 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

Reproduction is done solely by seed. Dormant seeds can remain viable in the soil for over 15 years. A single plant can produce up to 18,000 seeds, germinating in the fall or spring and developing rosettes within the first year of growth. Mature seeds emerge in July or August.

Diffuse Knapweed seeds break off and tumble in the wind; they can spread up to 100m away from the parent plant. Seeds can also be spread by wildlife, livestock and humans via hay and vehicle undercarriages.

Distribution

Ecological and Economic Impacts

Environmental:

  • 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, so the spines on the bracts can irritate or cause injury to the mouths and digestive tracts of grazing animals.
  • Can deplete soil and water resources.
  • Displays allelopathic properties: inhibits other plants from growing.
  • Reduces biodiversity.

Economic:

  • Reduces land values and increases road maintenance costs.
What Can I Do?

Diffuse Knapweed is abundant in certain portions of the Sea to Sky region, but it has not yet infested all potential habitats. The goal is to contain the spread of Diffuse Knapweed to Pemberton / ISMA 3.

 

Learn how to identify Spotted Knapweed: Use the images presented on this profile page.

What to do if you spot it: You can report any Spotted Knapweed sighting by clicking here.

 

DO:
  • Regularly monitor properties for weed infestations.
  • Remove plant material from any equipment, vehicles or clothing used in infested areas and wash equipment and vehicles at designated cleaning sites before leaving infested areas.
  • Ensure soil and gravel are uncontaminated before transport.
  • Minimize soil disturbances (e.g., use grazing plants that prevent soil exposure from overgrazing), and use seed mixes with dense, early colonization (e.g., alfalfa or barley) to re-vegetate exposed soil and resist invasion.
  • Ensure invasive plant (particularly flowering heads or root fragments) are bagged or covered to prevent spread during transport to designated disposal sites (e.g., landfill). Do NOT compost.
DO NOT:
  • Plant Diffuse Knapweed in a garden, no matter how well-contained its enclosure may seem.
  • Move soil that has been contaminated with Diffuse Knapweed.
  • 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.
  • Compost Diffuse Knapweed, especially the seeds!

 

Control

Mechanical Control

  • 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.

Chemical Control

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.

Biological Control

There are 12 biocontrol agents used in the Mainland and Coast regions of BC. Biocontrol helps decrease the size and vigour of the plants, as well as their seed production, but is not considered an eradication method.

References

Alberta Invasive Species Council, Diffuse Knapweed Factsheet, https://secureservercdn.net/198.71.233.231/yjc.cc8.myftpupload.com/wp-content/uploads/2020/07/PN_FS-DiffuseKnapweed.pdf?time=1635289333

Central Kootenay Invasive Species Society, Diffuse Knapweed, https://ckiss.ca/species/diffuse-knapweed/

Electronic Atlas of the Flora of British Columbia, Centaura diffusa, https://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Centaurea%20diffusa

Fraser Valley Invasive Species Society, Diffuse Knapweed, https://fviss.ca/invasive-plant/diffuse-knapweed

Gayton, Don & Miller, Val, Impact of Biological Control on Two Knapweed Species in British Columbia, Journal of Ecosystems and Management, vol. 13, no. 3 (2012), https://www2.gov.bc.ca/assets/gov/environment/plants-animals-and-ecosystems/invasive-species/biocontrol/impact_of_biological_control_on_two_knapweed_species_in_british_columbia.pdf

Government of BC, Field Guide to the Biological Control of Weeds in British Columbia, https://www2.gov.bc.ca/assets/gov/environment/plants-animals-and-ecosystems/invasive-species/publications/field_guide_to_the_biological_control_of_weeds_in_british_columbia.pdf

Invasive Species Compendium (CABI), Centaurea diffusa (diffuse knapweed), https://www.cabi.org/isc/datasheet/12039

Invasive Species Council of BC, Diffuse Knapweed, https://bcinvasives.ca/invasives/diffuse-knapweed/

Invasive Species Council of BC, How Biocontrol Took Down Diffuse Knapweed, https://bcinvasives.ca/news/how-biocontrol-took-down-diffuse-knapweed/

Invasive Species Council of BC, Knapweeds TIPS Factsheet, https://www.bcinvasives.ca/documents/Knapweeds_FINAL_20_02_2019.pdf

US Department of Agriculture National Invasive Species Information Center, Diffuse Knapweed, https://www.invasivespeciesinfo.gov/terrestrial/plants/diffuse-knapweed