Field thistle

Description

Field thistle is a 36″ to 84″ tall, erect, biennial or short-lived perennial forb that rises on a single stem from a slightly thickened taproot and fibrous roots.

The stems are erect with few to many ascending branches. They are covered with long, white, spreading, unmatted, soft hairs when young, becoming almost hairless as they age. They are not winged and do not have spines.

In the first year the plant appears as a rosette of basal leaves. In the second year it sends up a flowering stem.

Basal leaves are firm, broadly lance-shaped to egg-shaped or elliptic in outline, 4″ to 20″ long, and 1½″ to 9″ wide, but usually no more than 10″ long and 5″ wide. They are bluntly angled or sometimes rounded at the tip and taper at the base to a winged leaf stalk. They are deeply lobed (pinnatifid), the lobes cut more than halfway to the midrib. The upper surface is green and hairless or moderately covered with stiff, straight hairs. The lower surface is white and densely covered with felty hairs. The margins are coarsely toothed, spiny, and rolled under. Basal leaves are usually absent at flowering time.

Stem leaves are similar, alternate, stalkless, 1½″ to 10″ long, and ⅜″ to 5″ wide, becoming smaller as they ascend the stem. The leaf bases are sometimes somewhat clasping but do not extend down along the stem. The leaves at the branch tips are smaller. The upper leaves are well developed.

The inflorescence is usually a single flower head, sometimes a few flower heads, at each branch tip. The flower heads are stalkless or on short stalks (peduncles). The peduncles are leafy, up to 2″ long, and are not overtopped by upper stem leaves. Each head is subtended by a ring of spiny, leaf-like bracts. These bracts arch upward and encage the developing flower head at first, spreading gradually as the head matures.

The whorl of bracts at the base of the flower head (involucre) is egg-shaped to broadly cylinder- or bell-shaped, ¾″ to 1⅜″ long, and ⅝″ to 1 3 ⁄16″ wide, as long or longer than wide. It usually has a few cobwebby hairs. The bracts of the involucre have a 1 ⁄16″ to 3 ⁄16″ long, straw-colored spine at the tip.

The flower heads are 1½″ to 2″ wide. There are numerous pale, pinkish-purple, tubular flowers.

The fruit is a tan to brownish, ⅛″ to 3 ⁄16″ long achene (cypsela). The cypsela has a straw-colored collar near the tip and a tuft of white hairs at the tip.

Height

36″ to 84″

Flower Color

Pale pinkish-purple

Similar Species

Bull thistle (Cirsium vulgare) stems have wings with spiny margins. The leaf bases extend down along the stem. The underside of the leaf is green.

Flodman’s thistle (Cirsium flodmanii) is a shorter plant, usually no more than 32″ in height. The stems remain densely white tomentose with age. The upper leaf surface is tomentose when young. The flower head is not subtended by a ring of spiny bracts. The flower head is smaller, no more than 1″ wide. The collar on the cypsela is yellow and conspicuous.

Tall thistle (Cirsium altissimum) leaves are shallowly lobed, the lobes cut less than ⅓ of the way to the midrib.

Habitat

Dry to moderate moisture. Prairies, fields, forest openings, river bottoms, roadsides, disturbed areas. Full sun.

Ecology

Flowering

July to October

Pests and Diseases

Use