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Diffuse Knapweed

Diffuse KnapweedDiffuse KnapweedDiffuse KnapweedDiffuse Knapweed

(Centaurea diffusa)

Common Names

white knapweed, spreading knapweed, tumble knapweed

Description

One of three invasive, state-listed knapweeds in Montana, diffuse knapweed is an annual, biennial or short-lived perennial plant. It can grow up to two feet in height and has a single taproot. The rosette consists of greatly divided and feathery looking leaves that are covered with small hairs. On the stem, leaves become alternate, smaller and less divided. They are lance-shaped and grayish-green in color. Stems are multi-branched and hairy as well. Stems and leaves are covered with fine, cobweb-type hairs. Numerous flowers are produced singly or in clusters of two or three at the end of the stems. Flowers tend to be white, but due to hybridization with other knapweeds, flowers can also be pink to purple. Triangular bracts are pale yellow green in color and have spine tips. Seeds are oval, dark colored, do not have a pappus, and are contained in small fruits; this plant reproduces entirely by seed. Each plant can produce up to 18,000 seeds annually.

Key Features

Lower leaves with cobweb-type hairs, white flowers most of the time, spine-tipped bracts at the base of the flower.

Habitat

Diffuse knapweed is generally found on light, dry, porous soils. It prefers semi-arid to arid conditions and therefore, is not commonly found on irrigated lands.  This plant will grow in open sunshine or shady areas. Diffuse knapweed can be found in grasslands, forests, right-of-ways, riverbanks, rangelands, and disturbed lands in both rural and urban environments.

Currently found in the following counties:

Beaverhead, Broadwater, Carbon,Carter,  Cascade, Custer, Daniels, Dawson,  Fallon,  Flathead, Gallatin, Glacier, Jefferson,Lake, Lewis & Clark, Liberty, Madison, McCone, Meagher, Mineral, Missoula,  Petroleum,  Prairie, Richland, Stillwater, Teton,  Valley,  Yellowstone

IWM

Interesting Facts

Diffuse knapweed will not tolerate flooding or shady areas. When handling diffuse knapweed, be sure to wear gloves as in some folks, this plant can cause a rash to occur.

Commonly Confused Plants

  • Many native members of Asteraceae resemble knapweed in the rosette stage.

Visit our library for additional articles on diffuse knapweed.

Photo Credits: Krista Fechner 2007; Richard Old, XID Services, Inc., Bugwood.org; K. George Beck & James Sebastian, Colorado State University, Bugwood.org; Carey Minteer, University of Georgia, Bugwood.org

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