Diffuse Knapweed


Centaurea diffusa Lam. Asteraceae (sunflower family)

Colorado Dept. of Agriculture Class B

List B Eradication Required

Aliases: Spreading Knapweed, tumble knapweed, white knapweed
Annual: (1 yr life cycle). Most commonly a short-lived perennial (survives the winter but doesn’t do well the 2nd yr.); occasionally a biennial (needs 2 yrs to complete its life cycle).

Flower/Fruit:. Flowers are white to rose, sometimes purplish in color. Heads are many and narrow on the end of each branch. The edges of the specialized leaf (bract) below the flower are divided, like the teeth of a comb. The bracts are tipped with a definite slender spine (important for differentiation from Spotted Knapweed), which makes the flower feel prickly after it dries.
Bloom: Flowers occur from July – September.
Stem: Upright, highly branched, 4” to 3 ‘ tall.
Leaves: Leaves are covered with short dense hairs. Basal leaves (leaves at the bottom of the stem) are short stalked and often twice divided into narrow lobes, growing to 7 to 8 inches. Stem leaves become smaller in size further up the stem with top leaves being stalkless and less divided.
Root: Taproot (large main root).
Reproduce: Reproduces by seed but may regenerate from the crown if cut and root is not dug up. When the plant dries it can easily break off at the base and become a tumbleweed spreading its seeds. Wind is the primary means by which Diffuse Knapweed seeds spread. It can have up to 18,000 seeds/plant.
Seed Viable: Several years, possibly up to 8 yrs.
Mechanical Control: Cutting or mowing the above-ground portion of the plant, before seed set, may be an effective way to reduce seed production, but it will not eliminate the infestation. Mowing usually increases Diffuse Knapweed density, due to increased germination from the soil seed bank. Mowing should therefore be followed by a fall herbicide treatment (Sebastian and Beck 1999). When a Diffuse Knapweed plant has been cut, the rosette may live and rebolt. Additionally, Diffuse Knapweed seeds can remain dormant for several years, requiring any cutting program to be repeated several times annually (spring, summer, and fall) to be effective. Mowing or fire can be used as a way to remove standing dead material such that subsequently applied herbicide will be more effective. (Roche’ and Roche’ 1999). Pulling can be effective for knapweed control, but it must be repeated frequently. Youtie and Soil (1994) suggested hand-pulling knapweed three times annually until the plant disappears. The first pulling is in spring when the soil is moist, allowing enough of the plant to be pulled to kill it. The second pulling in June focuses on bolted plants, with the third pulling just before seed dispersal to kill any remaining plants.
Herbicide Control: Single treatments provide temporary but not long-lasting control.
From CSP – Colorado State Parks (cpw.state.co.us)
Chemical Control: Please go to the links at the bottom of this article for recommendations from the Colorado Dept. of Agriculture and from Jefferson County.
Interesting Facts: There are over 400 species of Knapweed. Diffuse Knapweed was brought in from Europe (Mediterranean region). It produces toxic chemicals that can inhibit the growth of desired plants and is a highly competitive plant. Severe infestation (which we have in our mountain communities) result in more than a 70% reduction in elk & deer forage. May cause a rash in some people.

Diffuse knapweed | Department of Agriculture – Conservation Services
Diffuse Knapweed, Noxious Weed – Jefferson County, Colorado