Marianne Ophardt

Garden Tips: Be on the lookout for yellow starthistle, a nasty noxious weed

Yellow starthistle’s long spines can damage the eyes of grazing animals. The plant is also toxic to horses.
Yellow starthistle’s long spines can damage the eyes of grazing animals. The plant is also toxic to horses. Pixabay photo

Several weeks ago I talked about the basics of noxious weeds in Washington and pointed out that Ravenna grass is a noxious weed that we should all be taking steps now to eliminate before it becomes too late to eradicate.

A faithful reader pointed out to me that I had not mentioned yellow starthistle. This non-native plant comes to us from Eurasia. While a number of noxious weeds were brought into this country as ornamental plants, yellow starthistle is believed to have been introduced around 1849 via contaminated alfalfa hay that was imported from Chile.

Yellow starthistle is not typically a weed that home gardeners need to worry about. However, it has become a big problem in the Western U.S., including parts of Washington where it is primarily found on rangeland and poorly managed pastureland. It is also common along roadsides, railroad tracks, and in areas where the soil has been disturbed.

Yellow starthistle is a particularly nasty plant because of the stiff, sharp spines at the base of its yellow flower heads. Early in the season, before it develops these spiny flowers, it provides a fair amount of nutrition for grazing animals. However, after it flowers livestock generally forgo eating it or any desirable grasses growing in the same area where this thistle is also growing. If they do browse in the area, the starthistle’s long spines can damage their eyes. The plant is also toxic to horses.

Definitely an undesirable plant, it has also made it onto the noxious weed list because it is invasive, crowding out more desirable vegetation in areas where soil moisture is limited, especially on steep hillsides. Not surprisingly, it has become a serious problem in Washington counties with sizable acreages of grazing land, including Asotin, Benton, Columbia, Franklin, Garfield, Klickitat, Stevens, Walla Walla and Whitman counties.

The starthistle plant is a winter annual. This means its seeds germinate in the fall or early spring. As a successful weed, it quickly grows deep roots and flowers in early summer, giving it an advantage over native plants that flower later in the season. Another factor in its success is its prolific production of seeds, up to 150,000 per plant. Most of the copious seeds germinate within a year, but some remain viable in the soil as long as ten years.

While the plant itself is not difficult to control via traditional methods, including mechanical (hoeing, pulling and tilling), cultural (burning, grazing and re-vegetation), and chemical (post-emergent and pre-emergent herbicides), large scale infestations on hilly rangeland are particularly difficult to control.

The use of biological control agents is one approach being utilized in Washington. These bio-control agents are insects that attack the flower heads, thereby reducing seed production. One of these, a seed head weevil (Bangasterus orientalis), was found to reduce seed production by about 60 percent. This weevil has been released and become established in some areas with yellow starthistle infestations. Other bio-control agents have also proven effective in reducing thistle plant populations and allowing other more desirable plants to become better established.

Dr. Stephen Van Vleet, WSU Regional Extension Specialist in Whitman County, has spent the last 12 years helping educate growers on how to fight invasive weeds, including yellow starthistle, in Whitman County. As an entomologist, he has been on the front line of research into the use of bio-control agents to keep yellow starthistle and other invasive weeds in check in eastern Washington. To learn more about these invasives and their management, refer to “Invasive Weeds of Eastern Washington,” WSU Extension Manual EM005 authored by Van Vleet.

Marianne C. Ophardt is a retired horticulturist for Washington State University Benton County Extension.