Spotted Knapweed

Centaurea stoebe L.
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Status:
County Declared
•
Priority 3
Synonyms:
Centaurea stopbe ssp. micranthos, Centaurea beibersteinii, Centaurea maculosa

Description

Spotted 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 3 feet tall. This stem is usually branched near or above the base giving the plant a bushy appearance. Leaves are pinnately lobed and are arranged alternately on stems. 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. Spotted 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 pink or white and are combined in a dense head enclosed in bracts. Bracts are brown to black at the tips giving the buds a “spotted” appearance. Seeds are small (2.5 mm), oval, black or brown, and have pappus at one end.

Origin and Spread

Spotted knapweed is native to Eurasia and the Mediterranean region and was introduced to North America in the late 1800 as a seed contaminant and through contaminated soil in ship ballasts. It spreads solely by seed. Pappus on seeds catch on clothing, fur, and equipment and are transported to new areas. It is primarily found in disturbed sites, rangeland, sagebrush steppe and other dry areas.

Management Options

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

There are several biological control agents available for spotted knapweed. 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 spotted 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 spotted 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