Photo by Jerry Asher

Yellow starthistle (Sunflower Family) is a winter annual with a deep taproot. It is native to the Mediterranean region and was introduced to North America in contaminated seed. Flowers are bright yellow and located singly at the ends of branches. Flowers have sharp, straw-col­ored spines, up to 2 inches long, surrounding the base and radiating in a star shape. Stems are rigid and appear winged due to extending leaf bases. The stems of yellow starthistle are covered with cottony hairs giving a whitish appearance. Mature plants are usually 2 to 3 feet tall.

Habitat and impacts

Yellow starthistle favors disturbed sites like road­sides, ditches, waste areas, and overgrazed range­land. When site conditions are ideal, this plant can invade excellent condition rangelands. Starthistle requires at least 10 inches of annual precipitation that peaks in winter or spring. This plant will estab­lish on deep, well-drained soils and shallow, rocky soils. Yellow starthistle does not tolerate shade. It requires light on the soil surface for rosette and tap­root development.
This weed is a serious invader. It infests over 10 million acres in California and occupies large areas in Idaho, Oregon, and Washington. Starthistle appears to be moving north and eastward. Starthis­tle has been reported in 8 Montana counties since 1958. New invasions were detected early and im­mediately eradicated. Current trends indicate yel­low starthistle will continue to invade Montana. To stop spread, it is important to anticipate invasion and detect new plants early as they invade.

Biology and spread

Yellow starthistle is a rapid colonizer. It germinates quickly under most conditions. Seeds germinate in the fall and overwinter as seedlings. Rosettes form during early spring and bolt during late spring. The plants flower and produce seeds in June through August, and then lose their leaves and dry in early fall. With fall rains, seeds begin germination and the cycle is repeated. Seedbank development and a long seed life make this plant extremely difficult to control. It is important to locate new invasions prior to reproduction and seedbank development.

DOWNLOAD THE YELLOW STARTHISTLE FACT SHEET