Look out for pretty, but nasty, yellow starthistle

Invasive Species Awareness Week is ending, but gardeners should continue to be weed warriors on the lookout for yellow or purple starthistle. These multi-thorned flowers look pretty until you touch them or walk through them. Not only do the spines hurt, if horses eat yellow starthistle, they can get "chewing disease." With this disease, the animal first shows signs of twitching lips, tongue flicking and involuntary chewing. Permanent brain damage can result and horses may starve to death.

Yellow starthistle, Centaurea solstitialis, is a gray-green annual plant with a long taproot. It has yellow dandelion-like flowers surrounded by 1-inch spines. The plants may grow 6 inches to 72 inches in height. Each plant can produce nearly 75,000 seeds, resulting in 50 million to 200 million seeds per acre for a heavily infested area. The seeds can survive in the soil for as long as 10 years before germinating. It grows along roadsides, streams, ditches, in waste areas and other disturbed sites.

Yellow starthistle has a long taproot and quickly crowds out native species. Its roots grow more quickly and more deeply than native plants. To control yellow starthistle, first identify it. Then, pull it out if you find a few plants. Notify your neighbors to be on the lookout for this weed. With larger infestations, grazing by sheep, goats or cattle works if they consume it before it goes to flower, although goats will eat yellow starthistle even when the spines appear.

Yellow starthistle can be mowed, tilled or pulled by hand. For mowing to work, 2 to 5 percent of the seed heads should be flowering. Tilling in early summer can be effective when followed by replanting desirable plants. Hand pulling removes individual plants and prevents seed production. Hand pulling is the method of choice for most home landscapes. Repeated chemical controls can actually cause an increase in other weeds and resistance in yellow starthistle.

Purple starthistle, Centaurea calcitrapa, is less common than yellow starthistle, but still is problematic. It is known to be in Pershing County while yellow starthistle already occurs in most northern Nevada counties. Purple starthistle has pink to purplish flowers and grows to 4 feet in height. Management is similar to yellow starthistle.

With both these colorful weeds and other noxious invaders, early detection and rapid response is the answer. For more information go to the University of Nevada Cooperative Extension Web site, www.unce.unr.edu, and look under publications.

JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at skellyj@unce.unr.edu or 887-2252.