Knapweed, Diffuse

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Pest Information
Common Name: Knapweed, Diffuse
Family Name: Asteraceae (Sunflower Family)
Latin Name: Centaurea diffusa Lam.
Other Names: White Knapweed, Spreading Knapweed, Tumble Knapweed.
Provincial Designation: Prohibited noxious
Seed Act Designation: Primary noxious
Life Cycle: Perennial, Biennial
Mode of Spread: Seed

Detailed Information
Provincial Situation: Some infestations and individual plants have been noted in southern and western Alberta.
Description: Diffuse knapweed generally grows to between 30 and 90 cm in height. It has a highly branched stem and a large taproot, as well as a basal rosette of leaves with smaller leaves alternating on the upright stems. Flowers are usually white or pink and grow out of urn-shaped heads carried at the tips of the many branches. Diffuse knapweed often assumes a short rosette form for one year, reaching maximum size, then rapidly growing and flowering during the second year.
Key Characteristics: Upright stems have numerous branches (tumble-weed)
Deeply divided leaves
Urn shaped flowers, usually white
Bract is fringed with spines and ends in a longer spreading spine at the tip
Similar Species: Knapweeds (Centaurea sp.) –Other knapweeds can be differentiated by involucre bracts. Spotted knapweed is the most likely to be found with Diffuse Knapweed and both have similar growth characteristics.
Where it Grows (Habitat & Ecology): Diffuse Knapweed will grow on most disturbed soils but thrives on well-drained light textured soils.  It will tolerate dense shade, flooding, or poorly drained soil and is commonly found in pastures, rangeland, roadsides, right of ways, gravel piles, and waste areas.  Diffuse Knapweed is believed to be “allelopathic”.
How it Spreads (Mode of Spread): Reproduces by seed alone.
Reproduction (Dispersal): Seed is spread by wind, livestock, contaminated hay, and by the tumbling motion of the plant after it matures and the stem breaks away from the crown at the ground.
Economic Importance (Beneficial Aspects): Not Known.
Toxicity and Other Concerns: Knapweed infestations increase production costs for ranchers and impair the quality of wildlife habitat, decrease plant diversity, increase soil erosion, and decrease the visual quality appeal of recreational land.  The risk of wildfires increases as well.
Origin: Introduced from Eurasia to North America in the late 1800’s.
Seed Production: One plant can produce up to 18 000 seeds, which can remain viable for 12 years.
Prevention: Use hay that is free of Diffuse Knapweed.  Control established Knapweed infestations to prevent maturity and therefore “tumbling”.  Use only construction materials and equipment that has been cleaned of all debris.
Cultural Control: Hand Picking can be effective on individual plants and small infestations if the entire root is removed.
Competition: Diffuse Knapweed will out compete native plants in most areas by the release of “allopaths” into the surrounding soil, which slows the growth of other vegetation.  Planting hardy grasses along with selective herbicide application will reduce Knapweed.  Very early irrigation often favors the germination and development of Diffuse Knapweed to the detriment of desirable plants, which grow at a slower rate.
Fertility: Nitrogen applications on rangeland may increase the severity of knapweed.
Cultivation: Works well in row crops and before seeding dry land crops.
Mowing: Can reduce seed production but may cause shoots to spring up from the crown.  Plants can flower at heights below the mower cut level.
Grazing: Livestock and wildlife do not graze Diffuse Knapweed.
Fire: Diffuse Knapweed will re-grow from the crown after burning.
Bio-Control: Seed reducing flies (Urophora sp)  can reduce seed  production b 95%.  A moth (Metzneria paucipunctella) and a root- feeding beetle (Sphenoptera jugoslauica) also reduce seed production.  Other insects that help control are a root feeding moth (Agapeta zoegana) and a root -feeding weevil (Cyphocleonus achates), which attack plants with large rosettes.
Life Cycle: It is a Biennial or a Short Lived Perennial.

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Last Modified: January 16, 2012

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