Diffuse Knapweed

Centaurea diffusa Lam.
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Status:
County Declared
•
Priority 1
Synonyms:
white knapweed, tumble knapweed

Description

Diffuse knapweed is a biennial or short-lived perennial forb in the Asteraceae (daisy) family. It grows from a deep taproot to form a basal rosette in its first year of growth; in subsequent years it produces a main stem erect to 2 feet. This stem is usually branched near or above the base giving the plant a bushy appearance. Leaves are divided with long, thin lobes. Basal leaves have stalks, and leaves decrease in size up the stem. The entire plant is covered in small hairs giving it a grayish-green appearance. Diffuse knapweed flowers from early summer to fall. Flowers are small and terminal; they are usually solitary but may occur in small clusters. Florets are usually white (though sometimes pink to lavender) and combined in a dense head enclosed in bracts that have a terminal spine with four to five smaller spines on each side. Seeds are small (0.5 mm), oblong with dark with longitudinal lines.

Origin and Spread

Diffuse knapweed is native to Eurasia and the Mediterranean region and was introduced to North America around 1900 as a seed contaminant. Plants break off at the ground and drop seeds while being blown like tumbleweed. It is primarily found in disturbed sites and on rangeland.

Management Options

Prevention and cultural control strategies should be utilized as much as possible.

There are several biological control agents available for diffuse knapweed; however, infestations of diffuse knapweed in Teton County are fortunately not dense or extensive enough to support viable populations of these agents. Agents in Teton County include seedhead feeding flies (Urophora affinis, Urophora quadrifasciata, and Terellia virens) and beetles (Larinus minutus and Larinus obtusus) and root boring moths (Agapeta zoegana) and beetles (Cyphocleonus achates and Sphenoptera jugoslavica).

Because diffuse knapweed has a taproot, it can be readily managed by hand pulling or digging out the root. Wear gloves and long sleeves when handling knapweed species as they can cause allergic reactions. Mowing will not eradicate stands of diffuse knapweed, but it can be beneficial in preventing flower production. Do not mow while seeds are present.

Larger infestations can be controlled with herbicides. Spring and fall treatments of rosettes are recommended, but treatment of bolting plants can be effective. Read the label before using any herbicide. Contact TCWP if you have any questions about application rates or how to use an herbicide.

Treatment Area Recommended Herbicides
Range, Pasture, Natural Areas aminopyralid or clopyralid
Pasture where manure or hay will be used for compost 2,4-D or dicamba
Lawn 2,4-D or dicamba
Riparian glyphosate (aquatic label)

Additional Resources