Why Is It a Noxious Weed?
Italian thistle invades pastures and rangeland. It crowds out more desirable forage, as well as native plants, and excludes livestock grazing where infestations are dense.
How would I identify it?
Italian thistle is a spiny annual or sometimes biennial that can grow 6 to 8 feet in height.
Flowerheads narrow and in terminal clusters of 1 to 5. Bracts at the base of flowerheads are stiff and upright with forward pointing hairs. Flowers are purple.
Leaves green and nearly hairless above and have cobwebby hairs below. Leaves are pinnately lobed into spiny-lobed segments, with the terminal spine being most prominent.
Italian thistle has spiny, winged stems.
Fruit Seed Description
Seeds are light brown to brown with veins, 0.18 to 0.24 inches (4 to 6 mm long), with hairs attache at one end.
May Be Confused With
Slenderflower thistle, Carduus tenuiflorus, is similar but it generally has more flowerheads (5-20), than Italian thistle, Carduus pycnocephalus (1-5 flowerheads).
Where does it grow?
Italian thistle grows in dry, open area such as pastures, rangeland, right-of-ways, and waste areas. Please click here to see a county level distribution map of Italian thistle in Washington.
How Does it Reproduce?
Italian thistle reproduces by seed.
How Do I Control It?
Deferring autumn grazing of sheep has been effective in reducing stand density because the thistles grow etiolated and less spiny when competing with ungrazed grasses. Then sheep will graze the thistle along with the grass.
Two fungi have been evaluated as agents on Italian thistle. More information can be found by downloading our Written Findings.
For More Information
See our postcard for early detection information about Italian and slenderflower thistle.
See our Written Findings for more information about Italian thistle (Carduus pycnocephalus)
Clark County NWCB Fact Sheet on Italian Thistle