White knapweed, spreading knapweed, tumble knapweed
Diffuse knapweed is an annual biennial or short-lived perennial plant. This plant can grow up to 2 feet tall 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 and they are lance-shaped and grayish –green in color. The stems are multi-branched and are also hairy. Stems and leaves are covered with fine, cobweb-type hairs. Numerous flowers are produced singly or in clusters of 2 or 3 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. The triangular bracts are pale to yellow green in color and have spine tips. Seeds are oval, dark colored and do not have a pappus, and are contained in small fruits; this plant reproduces entirely by seed and each plant can produce up to 18,000 seeds annually.
Lower leaves with cobweb-type hairs, white flowers most of the time and spine-tipped bracts at the base of the flower.
Diffuse knapweed is generally found on light, dry and porous soils. It prefers a semi-arid to arid conditions and therefore, is not commonly found on irrigated lands. This plant will grow in open sunshine or shady areas and can be found in grasslands, forests, right-of-ways, riverbanks, rangelands and disturbed lands.
There are herbicides and other control methods that commonly control diffuse knapweed. Hand pulling and digging can be very effective if done before the flower and seed establishment. Dig below the growing point or pull by the roots. Herbicides that are effective include Tordon, Milestone, Escort, 2,4-D, and Dicamba. For more information on these herbicides and other control methods contact the Weed and Pest office.
Diffuse knapweed will not tolerate flooding or shady areas. If you pull this plant make sure to wear gloves because it may cause a rash with some people.