Description:

DIFFUSE KNAPWEED — Centaurea diffusa Lam.

Asteraceae — (Sunflower family)

Diffuse knapweed was introduced from the Mediterranean Region of Eurasia and is a biennial or short-lived perennial forb which reproduces only by seed. The plant usually produces a single main much-branched stem that is one to two feet tall and rough to the touch. Leaves are pinnately divided; the reduced leaves of the inflorescence are mostly entire.

A basal rosette of leaves is present in young plants with each leaf divided into narrow segments. Seedlings of diffuse knapweed have finely divided leaves covered with short hair. Stem leaves in the mature plant become much reduced as you ascend to the tip and alternate one per node. Flowering heads are numerous and narrow. Flowers are white to rose or sometimes purplish and are located on each branch tip; margins of involucral bracts are divided like the teeth of a comb.

The bracts surrounding each flower bear four to five pairs of lateral spines and one, long terminal spine. Diffuse knapweed can resemble spotted knapweed, which has black-tipped bracts. The difference is the sharp spine at the end of the bract that is characteristic of diffuse knapweed.

Centaurea is a large genus of over four hundred species, most originating in the Mediterranean region. A number of the knapweed species have been introduced from Eurasia and now represent a threat to pastures, riparian areas and rangelands. Diffuse knapweed infests roadsides, waste areas and dry rangelands, and as a highly competitive plant, threatens to exclude many desirable species. It is a tough competitor on dry sites and rapidly invades and dominates disturbed areas. Flowering occurs from July to September.

Growth Habit: Annual or biennial, bushy, up to 2` tall. Rosette formed first year, flowering stalk elongates second year.

Leaves: Grayish-green, alternate, basal leaves whorled, upper leaves much reduced. Covered with fine hair.

Stem: Hairy, erect, single main stem from a rootstock, branched near or above the base.

Flower: Solitary, usually white, sometimes pink, rose or lavender; seed head bracts end as sharp rigid spines.

Roots: Elongated taproot.

Seeds: Oblong, dark brown or grey with longitudinal lines.

Other: May seriously reduce productive potential of infested rangelands.

(Courtesy of Weeds of the West)

CONTROL STRATEGIES

Herbicides are most effective when applied at the bud to bloom stage. The knapweeds are very stiff plants with reduced leaf surfaces and are hard to kill. Seed viability has been shown to exceed 8 years and one study found viable seed that was 40 years old. Once it goes to seed it will take many years of perfect control to wear out the seed bank. This species is very hard to control because single plants blend into other vegetation and are very hard to see. Once the infestation is really obvious a seed bank has been established making control a long term commitment.

Herbicides must be applied at the late rosette to early bud stage to be effective in stopping seed production for that year. 2,4-D will provide a rapid burn down, but not actual destruction of perennial plants. Curtail, Transline, Stinger, and Redeem all contain the same active ingredient, clopyralid. They will give good control when applied early, but bolting plants in the late bud stage will still make seed. The best herbicide, where it can be used is Tordon at 1 to 1.5 pints per acre. The addition of a surfactant will help get the herbicide into the plant. These recommendations apply to non-crop and rangeland sites. Call the Weed and Pest or Extension Office if you have Diffuse knapweed around the house or on crop land.

To learn about biological control agents for Diffuse Knapweed visit this link.