Ohio Perennial and Biennial Weed Guide

Family:

Sunflower Family (Asteraceae)

Other Names:

Centaurea bierbersteinii, Centaurea stoebe, bachelor's button, blue bottle, born flower, hard head, star thistle.

Origin and Distribution:

Spotted knapweed is a native of Europe. After it was accidentally introduced into the U.S. during the late 1800's, spotted knapweed spread throughout the Northeastern and north central states as far west as Nebraska, the Pacific Northwest, and southern Canada. In Ohio, the plant can be found in a few counties on Lake Erie and scattered throughout the northeastern and southern parts of the state. Spotted knapweed usually inhabits sites that are infrequently-disturbed such as roadsides, pastures, and rangelands. The species prefers to grow in dry, sterile, gravelly or sandy soils and full sunlight.

Plant Description:

Spotted knapweed is a biennial or short-lived perennial. In its first year of growth, the plant produces a rosette of basal leaves that are deeply divided into numerous leaflets. Both upper and lower surfaces of leaflets are slightly wooly and covered with shiny specks and translucent dots. Several 3-foot-tall leafy stems emerge during the second year. At the ends of main stems and axillary branches are solitary rosy-purple flower heads surrounded by prickly bracts with a fringe of dark hairs at the tip. Spotted knapweed reproduces by seeds.

• Seedlings and Shoots:

  Both the seed leaves (cotyledons) and first true leaves have rounded tips. Subsequent leaves gradually become narrower and more lance-shaped. Young leaves grow in the form of a basal rosette.

• Stems:

  Stems are branched, wiry, softly hairy, 3 feet tall, and either erect or may appear fallen over. As many as 6 stems emerge from the same root crown during a single growing season. Sometimes, stems have green or purple stripes.

• Leaves:

  Rosette leaves are alternate (1 leaf per node), 4 to 8 inches long, gray-green, slightly wooly, compound with deeply divided leaflets, and attached to the stem by way of a long leaf stalk (petiole). Leaflets may lack lobes or be irregularly lobed. Upper and lower surfaces of leaflets are covered with shiny specks interspersed with translucent dots. Leaves located on the lower and middle portions of the stem are similar in appearance to rosette leaves. Upper stem leaves are smaller, linear, and have fewer divisions. Petioles generally become shorter as the leaf position moves up the stem.

• Flowers:

  Individual rosy-purple (rarely white) disk flowers are clustered onto flowers heads that are approximately 3/4 to 1 inch wide. Flower heads are surrounded by oval, dry, prickly bracts tipped with a fringe of dark hairs. Flower heads occur singly at the ends of main stems and axillary branches.

• Fruits and Seeds:

  Single-seeded fruits are oblong, brownish, 1/8 inch long, and have a short tuft of bristles at the tip.

Similar Species:

Cornflower (Centaurea cyanus) has a similar appearance except it is an annual with bright blue flowers. White-flowered knapweed (Centaurea diffusa) can be distinguished from spotted knapweed by its white flowers, but there is a pink-flowered form that is very difficult to separate. White-flowered and spotted knapweeds hybridize producing plants with characteristics intermediate between the parents.

Biology:

Plants flower from July through August. A single rosette produces 1 to 6 flowering stems during the second year of growth. Perennial plants can have a main rosette and minor rosettes that form at the ends of lateral shoots extending horizontally just below the soil surface. A single plant produces 16 flower heads on average, and each flower head produces over 25 seeds. Once seeds mature, they are shed and germinate, providing conditions are sufficiently moist, and develop into a rosette by fall. Grazing animals generally pass over spotted knapweed in favor of more palatable native grasses and herbs. Light infestations can be controlled by hand pulling or mowing early in the flower stage. Plants are less likely to regrow if allowed to form flower stalks before cutting. All control methods must be repeated for several years until seeds stored in the soil seed bank become depleted. Spotted knapweed is resistant to some herbicides, likely due to wooly hairs limiting retention or penetration of product.

Toxicity:

None known, but more mature plants have spines that can cause mechanical injury to the mouth and digestive tract of grazing animals.

Facts and Folklore:

• 'Knapweed' comes from the German word 'knobbe' meaning bump or button.

• In the past, young women wore spotted knapweed flowers to attract bachelors.

• Spotted knapweed is not known for its edible qualities, but North Africans claim to have fed the plant to their camels.