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Diffuse knapweed

Diffuse knapweed
(Centaurea diffusa)

Short-lived perennial, annual, or monocarpic perennial knapweed with a long fibrous taproot and single upright stem covered in fine hairs. Plants form dense colonies and readily reproduce by seeds, which can travel long distances.

Overview

Regulated areas of Diffuse knapweed
This species is Prohibited (Red counties)

Other names for this plant include:

  • Common names: tumble knapweed, spreading knapweed, white knapweed
  • Scientific names: Acosta diffusa

Ecological threat:

  • Invades open grasslands, prairies, open forests, orchards, and disturbed areas such as ditches, cultivated fields and field edges. Also commonly found near water sources such as river bottoms, and irrigated areas.
  • Aggressively outcompetes native vegetation, reducing overall biodiversity.
  • Seeds readily disperse across long distances, enhanced by human activity.
  • Reduces forage quality for wildlife and livestock due to its prickly spines, fibrous foliage, and bitter flavor.
  • Plants are allelopathic, producing toxic chemicals that inhibit the growth of desired plants.

Classification in Wisconsin: Prohibited

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for diffuse knapweed was based upon this literature review developed by the department.

Identification

Leaves & stems: Single upright stem with many branche, growing up to 40" tall. Plants have a gray appearance due to tiny fine hairs. Leaves are finely divided and alternate. Stems easily break at the ground level, resulting in increased seed dispersal.

Flowers: Flowers are usually white, sometimes pinkish to purple and form at the tip of branches. Bracts contain multiple lateral spines and one prominent terminal spine. They do not have a pappus (parachute-like structure that aids in seed dispersal).

Fruits & seeds: Seed heads are urn-shaped, with many seeds compact in the flower head. The nodding flower head gives the plant its mobility, turning into “tumbleweed” when flowering stalks break off facilitating seed transport over long distances. Plants produce on average over a thousand seeds per plant.

Roots: Long fibrous taproots.

Similar species: Diffuse knapweed can be distinguished from spotted knapweed by their bracts. Bracts on diffuse knapweed may have darler coloration on the tips, but lack fringe present on spotted knapweed. Furthermore, diffuse knapweed's bracts have the rigid terminal and lateral spines. Russian knapweed bracts have light thin hairs. Visit the Resources tab for more information.

Distribution

Centaurea_diffusa
Counties in WI where diffuse knapweed has been reported (as of April 2013). Both vouchered and unvouchered reports included.

Do you have diffuse knapweed in your county but it isn't shaded on the map? Send us a report.

Control

In many cases, a combination of treatment regimes is the most effective approach to control. Because diffuse knapweed contains allelopathic compounds, plant debris must be removed from the site to facilitate revegetation. Reseeding with desired vegetation is critical to increasing treatment efficacy.

Mechanical:

  • For smaller populations, persistent hand pulling of plants can be effective. Pull as much of the taproot as possible (easiest in moist soils). Flowering plants should be bagged and disposed of.
  • Mowing during flower stage is effective in preventing seed production and reducing overall density after multiple years.
  • Tilling at a depth of 7 inches followed by dense reseeding of desired species can help prevent reinvasion. *Be sure you have identified species correctly! Similar-looking Russian knapweed increases under cultivation practices since plants spread via rhizomes fragments.
  • Burning alone is not effective as it stimulates new growth; follow up with herbicide treatment.

Chemical:

  • Herbicide treatments are most effective when combined with other control techniques and followed by re-seeding. Timing of herbicide treatments is essential to the effectiveness.
  • Mow during flower stage followed by herbicide application of clopyralid + 2,4D at 1 quart/acre is recommended. Visit this Montana State Univeristy Publication [exit DNR] for specific herbicide applications.

Photos

View diffuse knapweed pictures in our photo gallery!

Resources

Sources for content:

  • Knapweeds (Spotted, Diffuse, & Russian) - Noxious Weed Integrated Vegetation Management Guide, IPM Practitioners Association.
  • Montana State University Extension: Biology, Ecology, and Management of Montana Knapweeds; Knapweed Identification Monthly Weed Report, 2011.
  • Center for Invasive Species and Ecosystem Health; Invasive.org

Links for More Information

Last revised: Friday May 31 2019