Plant of the Week
Cirsium discolor range map. USDA PLANTS Database.
Plant at edge of redcedar thicket. Photo by David D. Taylor.
Head of flowers showing white pollen at tips of anthers. Photo by David D. Taylor.
Immature head showing sharply bent spines on each scale. Photo by David D. Taylor.
Monarch butterfly on head. Photo by David D. Taylor.
Field Thistle (Cirsium discolor (Muhl. ex Willd.) Spreng.)
By David Taylor
Field thistle is a member of the Asteraceae, the Sunflower family. In older manuals and guides, this family is called the Compositae because the 'flowers' are a composite of many flowers, often of different types. The many species of plants in this family are grouped based on the arrangement and type of flowers. All members of the family produce one or more heads (capitulum) of flowers. Members of the family typically produce two types of flowers, ray flowers and disc flowers and in turn, these can have male and female parts, or either one or the other. The ray flowers look like petals, but each is actually an individual flower. The disk flowers are at the center of the head, inside the ring of ray flowers. Thistles have all disc flowers that are shaped into long tubes. About 200 species are known worldwide from North America, Europe and Asia. There are 58 species of Cirsium thistles native to North America.
This thistle is 80 to 200 centimeters (about, 31 to 79 inches) tall. The stem is upright and usually hairy. Stems have few to many branches. Leaves are alternate or opposite (alternate near the top), wider at the base than the top, and are 10 to 25 centimeters (4 to 10 inches), long, sometimes longer, by 1to 13 centimeters (0.4 to 5 inches) wide. Larger leaves are deeply lobed with each lobed toothed, entire and or armed with sharp spines up to 5 millimeters (0.2 inches) long. Leaves are green above and covered in dense white hairs below. One to many heads are borne at the end of branches. Each head is 2.5 to 3.5 centimeters (1 to 1.4 inches) high before flowering. The scale-like bracts (phyllaries) around the outside of the head are armed with a bent spine. The heads hold 100 plus disc flowers 17 to 26 millimeters (2/3 to 1 inches) long. Flowers are pink to purple, rarely white. The entire head is 3 to 4.5 centimeters (1.2 to 1.75 inches) wide.
Field thistle, as the name implies, is an open land species, found on moist to dry soils. It is a species of tallgrass prairies, pastures, old fields, roadsides, savannas and forest edge. It is found from Maine south to Georgia and west to the Dakotas and Texas (but not Oklahoma). It is also found from Quebec to Manitoba.
This species flowers in June to October depending on the part of the country in which it is found. Numerous bees, beetles and butterflies are attracted to the flowers which produce copious nectar. Finches, small mammals and insects eat the seeds. The plant is usually biennial and must seed from year to year to maintain a presence at as site. The plant is often mistakenly identified and killed as bull thistle (Cirsium vulgare), a weedy and aggressive European species. Bull thistle has leaves green on both sides. Field thistle has leaves white underneath. Field thistle is very similar to tall thistle (C. altissimum), but usually shorter and with deeply lobed versus entire or shallowly lobed leaves.