Spotted knapweed

Centaurea stoebe

Images of this species:

Common look-alikes:

Identification

Appearance

Centaurea stoebe is a bushy, winter-hardy, upright perennial forb living 3 - 5 years or sometimes longer, with a deep taproot.

Foliage

Rosette leaves bluish green, hairy and covered with shiny specks interspersed with translucent dots, 4-8 inches long. The size of the leaves decrease in size above the middle of the stem. They are alternate, spiraling and jutting out and upward. Basal leaves deeply divided into elliptic or linear lobes. Lobes become more slender and fewer on upper leaves.

Flowers

Flowers from June-November. Each branch topped by an egg-shaped flower bud covered with overlapping rows of dry, fringed green bracts with black tips. Bracts eventually part at top to allow a constricted pinkish-to-lavender thistle plume to radiate out and up, 0.75-1 inch wide and overall about 1 inch long.

Fruit

Fruits appear from June-February. Tightly packed seed heads of oblong, brownish, hairy nutlets (achenes), 0.1 inch long, topped by short stubby bristles. A thousand seeds can be produced per plant.

 

Biology

ECOLOGICAL THREAT

Knapweeds have the potential to rapidly spread throughout introduced areas, displacing native vegetation and reducing the amount of available forage for wildlife and livestock. They may also degrade soil and water resources by increasing erosion and surface runoff. 

Centaurea stoebe rapidly colonizes roadsides and disturbed lands, especially dry sites. It can invade adjacent undisturbed prairies and open forests. A severe invasive species spreading into the South by seeds equipped for dispersal by wind, water, livestock, wildlife, and human activity, with viability in the soil for many years.

ORIGIN

These species are native to Europe and Russia. No one is sure when brown knapweed (C. jacea) was introduced to North America. Black knapweed (C. nigra) and spotted knapweed (C. stoebe) were introduced in the late 1800s.

HABITAT

Meadows, fields, disturbed areas

LIFE CYCLE

These herbaceous plants are either biennial or perennial with pink to purple flowers that are roughly half an inch across and resemble small pineapples. They flower from June through October. Plants grow one to four feet high. They typically form a basal rosette of leaves in the first year and flowers in subsequent years. Individual plants can live up to nine years.

Management Options

This is considered a watch list species

Prevention

Prevention The first line of control should be preventing the spread of these plants to non-infested areas. Prevention measures include cleaning clothing and equipment after entering infested areas, forbidding livestock to graze when seeds are mature and ready for dispersal, and using certified weed-free hay.

MECHANICAL CONTROL

Manual and Mechanical Plants may be manually removed from the ground, ideally when the ground is wet, so long as care is taken to remove the entire plant, including the deep taproot. Mowing will reduce the number of flowers and seeds but will not eliminate the plants.

CHEMICAL CONTROL

  • Be careful not to damage or kill nearby native plants when conducting management work.
  • Always read and follow pesticide label directions. Application of pesticides may require a certification from the Agency of Agriculture, Food and Markets. The Agency website provides information on what applicator certification is needed.

Chemical control for larger populations may be the most effective choice, although it can be costly and difficult to implement successfully. A three percent solution of triclopyr herbicide mixed with water can be applied to the leaves in early spring or fall. This should be repeated several times a year for two or more years.