This adventive plant is a short-lived perennial about 2-3' tall. It branches occasionally to frequently, becoming broader toward the flowering stems. The stems are ribbed and pubescent with a stiff woody texture. The alternate leaves are up to 3½" long and 1½" across; they are sparsely distributed along the stems. They are pubescent and pinnately lobed; these lobes are narrow, and the terminal lobe of each leaf is usually the longest. If the basal leaves haven't withered away, they are somewhat larger than the cauline leaves and deeply pinnatifid. Both the stems and the foliage are whitish or greyish green, although the base of the central stem often turns brown with age. The upper stems terminate in flowerheads about ¾" across. Each flowerhead consists of numerous ray florets that are pink to purplish pink, overlapping floral bracts that are greyish green, and no disk florets. Each floral bract is ovate, tapering to a black tip with coarse short bristles that are dull white to brownish black; there are several dark green veins toward the base of each bract. These bracts are appressed together. The ray florets are slender, thread-like, and branching; the outer florets are the longest and sterile. The blooming period occurs from mid-summer to early fall and lasts about 1-2 months. Each floret is replaced by an oblong achene that has a crown of short bristles on top; these bristles may fall off. The root system consists of a taproot. It often forms colonies at favorable sites, and spreads by reseeding itself. The achenes are spread to a limited extent by the wind.
Spotted Knapweed is common in NE and east-central Illinois, and occasional to absent elsewhere within the state. Official records undoubtedly underestimate the distribution of this plant; it is probably still spreading. Habitats include gravelly areas along roadsides and railroads, dry waste areas and eroding slopes, pastures and abandoned fields, and vacant lots in cities. This plant occurs in disturbed areas primarily; sometimes it is found along the edges of prairie remnants near railroads. In some of the Western states, Spotted Knapweed has become a serious weed, but it is less of a problem in Illinois. This species is adventive from Eastern Europe and the Ukraine, and first appeared in the United States and Canada during the 1890's.
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.
The preference is full sun, mesic to dry conditions, and a poor soil that contains gravel or clay; alkaline soil is tolerated quite well. Both the foliage and roots of this species exude a powerful toxin that can destroy the root systems of other plants. This is one of the reasons it often forms colonies.
Spotted knapweed is a widely distributed species reported to occur throughout Canada and in every state in the U.S. except Alaska, Georgia, Mississippi, Oklahoma and Texas (see map). It has been designated as a noxious weed in Arizona, California, Colorado, Idaho, Minnesota, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oregon, South Dakota, Utah, Washington, and Wyoming.
It has been identified as invasive in natural areas by eighteen organizations in twenty-six states (Arizona, California, Colorado, Connecticut, Delaware, Kentucky, Idaho, Illinois, Massachusetts, Maryland, Michigan, Minnesota, Montana, North Carolina, New Jersey, New York, Oregon, Pennsylvania, South Dakota, Tennessee, Utah, Virginia, Washington, Wisconsin, West Virginia, and Wyoming). Fifteen national parks also identify spotted knapweed as an invasive plant and a threat to natural habitats.
Spotted knapweed infests a variety of natural and semi-natural habitats including barrens, fields, forests, prairies, meadows, pastures, and rangelands. It outcompetes native plant species, reduces native plant and animal biodiversity, and decreases forage production for livestock and wildlife. Spotted knapweed may degrade soil and water resources by increasing erosion, surface runoff, and stream sedimentation. It has increased at an estimated rate of 27% per year since 1920 and has the potential to invade about half of all the rangeland (35 million acres) in Montana alone.
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.