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U.S. Forest Service

Plant of the Week

Cirsium undulatum range map. Cirsium undulatum range map. USDA PLANTS Database.

Flowering head of wavyleaf thistle (Cirsium undulatum).A closer look at the attractive flowering head of the wavyleaf thistle (Cirsium undulatum). The flowering heads are distinctly larger than its noxious weed relative, Canada thistle (Cirsium arvense). Photo by Jill Larson Welborn.

Wavyleaf Thistle (Cirsium undulatum), shown in a coniferous forest opening.Habit of the wavyleaf thistle (Cirsium undulatum), shown here in a coniferous forest opening. Photo by Jill Larson Welborn.

Leaves of wavyleaf thistle (Cirsium undulatum).Leaves of the wavyleaf thistle (Cirsium undulatum). Note the gray or white color of the leaf owing to the tomentose hairs on the leaf surface. Photo by Jill Larson Welborn.

Wavyleaf Thistle (Cirsium undulatum (Nutt.) Spreng.)

By Jill Larson Welborn

Thistles (Cirsium ssp.) sometimes get a bad rap, since some members of this large and diverse genus in the Sunflower or Aster family (Asteraceae) are considered noxious weeds. Thistle species can also be difficult to distinguish, adding to the confusion. Introduced, noxious species of thistle include Canada thistle (Cirsium arvense) and bull thistle (Cirsium vulgare). But these often problematic species should not be mistaken for North America’s many native thistles. One of the more common of our native plants is wavyleaf thistle (Cirsium undulatum). Primarily a western species, it grows from the arid plateaus of the Pacific Northwest to the Great Plains, and south into northwestern Mexico. It is also found sporadically in many eastern US states, but some of these populations may be introduced.

Wavyleaf thistle is a perennial with simple or branched stems. Leaves are densely hairy, lending a white or gray cast to the leaf surface, and are often deeply lobed, the lobes spine tipped. The discoid flowering heads are solitary to several per stem, and broadly urn shaped. Involucres (bracts subtending the flowers) range from 2.5-4.5 cm high. The phyllaries or individual bracts have a prominent glutinous ridge down the center and the outer phyllaries are spine tipped. Corollas are lavender, pink, or occasionally white. The pappus bristles are white..

Wavyleaf thistle is found in a variety of prairie habitats, pinyon-juniper woodlands, montane coniferous forests, and disturbed areas. As with many thistles, they are frequently visited by bumblebees and a variety of other pollinating insects. The pappus bristles are used by birds, such as goldfinches, to line their nests. Native Americans have many uses for wavyleaf thistle, from food to medicine.

A close relative of the wavyleaf thistle is Tracy’s thistle (Cirsium tracyi (Rydb.) S.L. Welsh). This species is not as widespread as wavyleaf thistle and is restricted to the Colorado Plateau region in Colorado and Utah, extending south into the southern Rocky Mountains of northern New Mexico.

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