Spotted Knapweed

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[Graphic Version]

Spotted Knapweed (Centaurea stoebe ssp. macranthos), courtesy of Great Smoky Mountains National Park Resource Management Archives, USDI National Park Service, bugwood.org

Common Name(s):

Spotted Knapweed

Scientific Name(s):

Centaurea stoebe L. ssp. micranthos (Gugler) Hayek

Scientific Name Synonyms:

Centaurea maculosa auct. non. Lam.
Centaurea biebersteinii DC

Symbol:

CESTM

Description:

Life Span: Biennial or Perennial

Origin: Introduced

Growth Characteristics: Spotted knapweed is a taprooted, perennial forb that can live at least 9 years. It reproduces almost entirely from seed. Plants are also able to extend lateral shoots below the soil surface that form rosettes adjacent to the parent plant, and multiple rosettes on a single spotted knapweed root crown are common.

Flowers: The branched stems bear terminal flowerheads that are 0.25 inch wide and 0.5 inch long. Flowerheads are solitary or borne in clusters of 2 or 3, with 25-30 flowers per head, and up to 60 flowering heads per plant.

Fruits/Seeds: Achenes are oval, 1/8 inch long, bearing a pappus of simple bristles which are less than the length of the seed and persistent.

Leaves: The leaves form a basal rosette the first year. When mature, the leaves are alternate, triangular in shape, petiolate, and dark green. The leaves are very large, with the lower ones frequently measuring 18 inches in length. The leaf margins are coarsely and shallowly toothed.

Stems: Beginning usually the second year, each spotted knapweed plant produces 1 to 6 flowering stems (can be 15 or more on older plants) that stand 8 inches to 4 feet tall. The stems branch in the upper half of the plant.

Roots: A deep taproot as well as lateral shoots.

Ecological Adaptations:

Under favorable conditions, plants appear to remain as taprooted rosettes for several years before developing flowering stems. Spotted knapweed gains dominance in part by its ability to out-compete native grasses for nutrients such as nitrogen and phosphorus. Seeds germinate whenever moisture and temperature are suitable, and both fall and spring seedling emergence is common.

Spotted knapweed establishes and dominates on dry, disturbed sites, especially along roads.

Soils: Found on soils with a wide range of chemical and physical properties, and often on poor soils. It does especially well in coarse-textured soils that are well drained with low water holding capacity. It is poorly adapted to irrigated pastures where saturated soil is common

Associated Species: Big Sagebrush, cheatgrass, pinyon pine, Utah juniper, creosote bush, bluebunch wheatgrass, Idaho fescue, blue grama, bitterbrush.

Uses and Management:

Spotted knapweed is a noxious weed in Utah.

It has good nutritional value and is eaten by both livestock and wildlife. Use is generally highest during spring and early summer when plants are green and actively growing in the rosette and bolt stages. Use declines as it matures and protein and digestibility decrease. Flowerbuds and seedheads may be grazed in late summer.

Biological controls do exist, eight of these insects attack the flower heads and five attack the roots. The idea being that together, these agents will be able to reduce viable seed production and stunt the overall growth and strength of the plants. In addition to insect control agents, several microorganisms have been considered for potential control of spotted knapweed, and are still under study and investigation.