Spotted Knapweed

Centaurea stoebe

Species distinguishing characteristics: 

  • Pinkish-purple flowers with grayish-green stem and leaves
  • Rigid bracts below flower heads have brown, triangular tips with comb-like fringe
  • Grayish-green basal and lower stem leaves have deeply lobed segments
  • Seed head from previous year persists

Family Characteristics: 

  • Multiple layers of bracts beneath the flowers
  • Flower heads composed of many smaller (often tiny) flowers, each of which produces an individual seed
  • Flowers may contain disk florets, such as those in the yellow center of a daisy, and/or petal-like ray florets
  • Undivided leaves
  • Includes the food plants lettuce, artichoke, and endive

Growth habitat: 

Erect, short-lived, perennial or biennial up to 1.5 meters in height.  Previously used Latin names include:  Centaurea maculosa (this is the name found in most weed references) and Centaurea stoebe ssp. micranthos.

Leaves and stems: 

A basal rosette, with leaves up to 6 inches long that radiate from a common center point, emerges during the first year of growth.  Flowering stalks grow from the rosette during the second year.  Leaves and stems have a coarse texture, and are covered with translucent resin dots and fine hairs that give them a grayish-green appearance.  Stems on mature plants have many branches.  Basal rosette and stem leaves have deeply lobed segments.  Uppermost stem leaves are short and narrow.  Leaves arrange in an alternate pattern along the stem.  Very bitter to taste.

Flowers: 

One pinkish-purple (or rarely white) flower head develops on each branch of the stem.  30 to 50 disk flowers cluster into an urn-shaped head that resembles a single flower.  A single plant can produce as many as 300 flower heads.  The rigid bracts beneath the flower head are approximately 6 mm in diameter and have brown triangular tips with short, comb-like fringe, which give the plant its spotted appearance (and its name).  Unlike yellow starthistle (Centaurea solstitialis) and diffuse knapweed (Centaurea diffusa), the bracts do not have well developed spines.  The bracts of meadow knapweed (Centaurea X moncktonii) are not rigid and have tan to brown fringe that is longer than, or as long as, the bract width.

Roots: 

Deep, stout taproot.  Taproot and ability of the plant to begin growth early in the spring help spotted knapweed compete for water and nutrients.  Root and crown fragments re-sprout when disturbed by heavy equipment or cultivation.

Seeds: 

At maturity, the bracts spread apart and release the brownish or black seeds.  Seeds are about 5 mm (1/8 inch) long, have a notch on one side, faint ridges on the surface, and a short tuft of bristles at the tip.  Seed heads from the previous year persist.  A single plant may produce up to 40,000 seeds.  Most seeds fall within a 4-foot radius of the parent plant and are not carried by the wind due to its short fluff.  Seeds may move longer distances by soil movement, water, animals, humans, machines, and vehicles, or by use of commercial seed or hay with spotted knapweed seeds.  Seeds that do not germinate the following year form a seedbank and may remain viable for 8 or more years in the soil.

Habitat preferences: 

Found in fields, roadsides, and other open areas, from the plains to the mountains.  Can rapidly form virtual monocultures in disturbed sites and adjacent native plant communities.  Prefers light textured, well-drained soils that receive summer rainfall and ample sunlight.  Does not tolerate dense shade. 

Interesting facts: 

Although wildlife and livestock rarely use spotted knapweed as forage, it can cause injuries or obstructions in the digestive tract.  The plant exudes a chemical called catechin into the soil and prevents the establishment or germination of neighboring plants.  Researchers are currently studying the potential of catechin as a natural weed killer for agricultural areas.

Hand pulling or digging the entire plant before it goes to seed can be a highly effective management method.  Remove as much of the taproot as possible to prevent regeneration from the crown or broken taproot.  Wear gloves when pulling to prevent skin irritation.  If any portion of the flower is beginning to emerge or if seed heads have formed, bag and remove.  Flowers can develop into seeds after being pulled.  Minimize soil disturbance caused by pulling and digging, which can aid in the germination of any seeds present in the seedbank.  Follow up treatments will be required each year until the seedbank has been exhausted.   

Mowing or cutting is ineffective as spotted knapweed can bloom at mowed height and poorly timed mowing can increase seed production or disperse seeds.  However, a single mowing at late bud stage growth can reduce the number of seeds produced.  Repeated cultivation can provide some control for spotted knapweed but creates soil disturbance that allows future infestation. 

Biological Classification: