Spotted Knapweed, Centaurea maculosa

A ccidentally introduced into America in the late 1800's, the spotted knapweed has established itself throughout the country. This noxious weed was most likely brought to the United States in a shipment of alfalfa seed from Asia Minor. Generally, the spotted knapweed will establish a colony in areas where there isn't a lot of competition from other plants. Once established though, spotted knapweed will spread into nearby areas crowded with native plants. Spotted knapweed is most frequently found in dry meadows, pastureland, stony hills, roadsides, and the sandy or gravelly flood plains of streams and rivers where soils are light textured and well drained and receive summer rain showers.

Spotted knapweed is a biennial; this means that it lives less than 2 years. It usually forms between 1 and 20 slender, upright stems 1 to 3.5 feet (30­100 cm) tall. Most of the stems branch on their upper half. The knapweed has pretty flowers that can be white, pink, purple, or a mix of all three colors.

Like their cousin yellow starthistle, spotted knapweed can produce a large number of seeds, which drop after the plant dries out in late summer. These seeds can be carried away by the wind, animals, and even people. And the seeds are long lived: they can lie dormant up to 8 years if weather conditions don't favor reproduction and still sprout and produce viable plants. As many as 146,000 seeds/yd2 have been reported in areas of heavy infestation.

Spotted knapweed hogs moisture and nutrients to the detriment of native plants nearby. This noxious weed quickly cuts off native vegetation from water in the soil and then moves into the natives' space after they die. Knapweed infestations do not spread quickly over large distances, but their slow, methodical advance can wipe out whole pastures. Knapweed can thrive in areas of heavy grazing. Native grasses stop regrowing after repeated grazing by livestock. Once the grasses die back, knapweed moves in and displaces them permanently. Spotted knapweed can cause chewing disease in horses, but animals tend to avoid eating it because it tastes bitter. Essentially, spotted knapweed is a troublesome noxious weed that chokes rangeland and doesn't provide nutritious food for grazers or wildlife either.