Bill Johnson

Spotted Knapweed

Centaurea stoebe ssp. micranthos (Gugler) Hayek
Aster family (Asteraceae)

Origin: Europe and western Asia

Background
Spotted knapweed was accidentally introduced into North America in the late 1800s in contaminated alfalfa and clover seed and in soil used for ship ballast. In North America, plants generally live 3 to 7 years but can live up to nine years or longer and regrow from buds on the root crown. Reproduction is by seed. Individual plants are capable of producing an estimated 500-4,000 seeds per square foot per year. Most of the seed is viable at the time of dispersal and can remain viable in the soil for 5-8 years. Most seed is dispersed near the parent plant but can be moved great distances by people, livestock, wildlife, and vehicles and in soil, crop seed, and contaminated hay.

Distribution and Habitat
Spotted knapweed is widely distributed in the U.S. and is reported to occur in every state in the Lower 48 except Texas, Oklahoma and Georgia. Over two dozen states and seventeen large national parks across the country recognize it as a significant invasive plant of natural areas. It invades open habitats, preferring full sun and can tolerate nutrient poor soils and harsh dry conditions.

Ecological Threat
Spotted knapweed invades a wide variety of habitats including open forests, shale, serpentine and other barrens, meadows, prairies, old fields, and disturbed areas. It forms deep taproots allowing it to capture moisture and nutrients and spreads rapidly, displacing native vegetation and reducing the forage potential for wildlife and livestock.

John Cardina, OH State Univ.

John M. Randall, TNC

Description and Biology

• Plant: herbaceous biennial or perennial plant.
• Leaves: a basal rosette of deeply lobed leaves is produced the first year; rosette leaves are deeply lobed, stalked and about 8 in. long; stem leaves are alternate and may be slightly lobed or linear; leaves become smaller and less lobed toward the apex.
• Flowers, fruits and seeds: flowering stems are 2/3- 4 ft. tall and branched; flowers are produced early summer after the first year and resemble tiny pineapples topped with a showy ring of pink to light purple, highly dissected petals; the bract tips of the base (“pineapple”) are dark, giving the spotted look for which the plant is named.
• Spreads: by wind-dispersed seed.
• Look-alikes: other knapweeds and thistles including Canada thistle (Cirsium arvense) which occurs in similar habitats in the mid-Atlantic region.

Prevention and Control
The most cost effective management strategy for spotted knapweed is to prevent its spread to non-infested areas. Spread by seed can be minimized by avoiding travel through infested areas by: 1) cleaning footwear, clothing, backpacks, and other items after hiking through infested areas; 2) not grazing livestock when ripe seeds are present in the flower heads; and 3) using certified weed-free hay. Individual plants can be pulled by hand when the soil is moist as long as the entire crown and taproot are removed, using a shovel or weed-popper type tool if necessary. Control of spotted knapweed infestations is very difficult and may require large investment of time, labor and materials to remove using manual and mechanical means or repeated applications of herbicides often at higher rates (see Control Options).