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Spotted Knapweed
(Centaurea maculosa)

by Virginia Chatfield

Family: Asteraceae (Aster Family)

Habitat and Distribution: Spotted knapweed is native to central Europe, where it is found in light, porous, fertile, well-drained, and often calcareous soils in warm areas. It lives in dry meadows, pastureland, stony hills, roadsides, and the sandy or gravelly floodplains of streams and rivers. In 1893, spotted knapweed was collected in Victoria, British Columbia. It is assumed that soil carried on ships as ballast and unloaded in the port transported knapweed to this site at that time. Other sources believe it was accidentally introduced to North America, most likely in the 1890s, in alfalfa seed from Asia Minor. It was first collected in Michigan in 1911 in an alfalfa field on the Oceana/Newaygo County line. In 1915 it was collected in Gladwin County; in 1917, in Oakland County. It was probably established across a wide area of Michigan well before 1920. There are several invasive species of knapweed, and most tolerate and even thrive in very dry conditions, but spotted knapweed survives in higher moisture areas as well. This species of knapweed is the most prolific in Michigan.

Description and Method of Spread: Spotted knapweed is a short-lived, noncreeping perennial that reproduces from seed, which is its primary means of spread. Enormous numbers of seeds are produced, 30% of which remain viable for at least 8 years! Each year the plant produces one to 20 tough, wiry shoots from a thick taproot. These shoots are branched and grow one to three feet tall. Rosette leaves at the base of the plant can be six inches long and deeply lobed, but toward the top of the shoot, leaves become much smaller and narrower, with smooth margins. Soft, thistle-like lavender to purple flowers are solitary on shoot tips. Spotted knapweed germinates in spring or fall. Perennial plants resume growth in early spring and bolt in May or June, continuing to flower through the summer and into fall.

The Trouble with Knapweed: This weed is not relished by wildlife and livestock and has severely diminished the quality of wildlife habitat and the returns from livestock production. Knapweed is highly adept at capturing nutrients and moisture, and it may spread quickly on disturbed ground, choking out other vegetation. Its thick taproot draws moisture out of the soil, impoverishing site conditions for other vegetation and sometimes creating erosion problems. It has now spread aggressively throughout every Michigan county, in old fields, pasturelands, roadsides, on disturbed ground, and waste places. Infestations have taken over many open sites, especially in the northern part of the state. Over 7.2 million acres in nine states and two Canadian provinces are infested.

Related Species: Two other members of this genus are on the invasive plant list. One is diffuse knapweed, Centaurea diffusa, which is an enormous problem in our western states, where the climate is much drier. The other is an escaped ornamental, Centaurea cyanus (Cornflower or Bachelor's Button), which is particularly invasive in rare native Northwest grasslands and prairies. This seed is often added to commercial wildflower mixes, which is one of the reasons these mixes are not usually recommended by ecologists.

Management: Anyone involved in prairie restoration or creation is acquainted with the problem of controlling knapweed. Unfortunately, most literature on controlling knapweed has focused on reestablishing valuable range, pasture, or cropland. None has looked at the problem from the point of view of restoration ecology, with the intent of restoring the native community. A few studies have been done with mowing just after most flowering has ended but before the seeds have matured. This was found to have some effect in reducing seed production in small infestations. According to David Mindell of Plantwise, an ecological consulting and design firm, "Fire is decent knapweed control if you start burning before there's too much present. The problem is that the plant holds a good deal of moisture in the leaves (which often stay green through the winter), and if there's much present, it's impossible to get fire to carry through the site. Other controls are hand-pulling in small areas and spot-spraying with herbicide in the winter (warmer days when there's photosynthesis going on)." Although herbicide control of spotted and diffuse knapweeds is effective, it can be cost-prohibitive and difficult to use without killing desirable vegetation. Reinvasion after using herbicide is also a problem. Irrigation (where it is possible) may help to stimulate grass competition, and seeding of desirable native grasses may help combat reestablishment. The important point to note is that catching an invasion of this prolific seeder early on can really make a difference.

Another approach has been initiated by APHIS (Animal and Plant Health Inspection Service of the USDA)—in conjunction with USDA's Agricultural Research Service, state agricultural experiment stations, and the International Institute of Biological Control—which has imported, propagated, and redistributed hundreds of thousands of biological control agents to combat the knapweeds. Scientists employed by these agencies have scoured the European landscape where knapweeds originated, looking for native species of beneficial insects that attack them and can be used as biocontrol agents in this country. Currently, there are four beneficial insect species cleared by USDA for release in the United States. The insects work together by feeding on the seeds and stems or in the roots of the weeds, reducing the knapweeds' reproductive capability by inhibiting seed production or plant vigor. (Of course, one wonders what the effect of these insects might have on the ecosystem!)

Article reprinted from the Fall 2000 issue of Wild Ideas,
the Flint Wild Ones Newsletter.
Copyright © 2000 Wild Ones–Natural Landscapers, Ltd.


For more information...

...and photos of Spotted Knapweed, see the following web sites:


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