swamp thistle

not listed

N5 - Secure

SNR - Unranked

not listed

FACW - Facultative wetland

OBL - Obligate wetland

OBL - Obligate wetland

Swamp thistle is a 12″ to 90″ tall, erect, biennial forb that rises from fibrous roots and a cluster of fleshy, stout, white to brown roots, rarely from a single taproot. The first year it appears as a rosette of basal leaves. The second year it sends up a single flowering stem.

Basal leaves are egg-shaped to broadly elliptic or inversely egg-shaped in outline. They can be 6″ to 21½″ long, and 1½″ to 8″ wide, though they are usually no more than 12″ long and 4″ wide. They are on spiny-winged leaf stalks. The leaf blade is tapered at the base and deeply lobed (pinnatifid), the lobes cut ⅞ of the way to the midrib. The lobes are linear to lance-shaped and are sometimes lobed again. They taper to a point at the tip with straight or concave sides along the tip. The upper surface is green and nearly hairless or sparsely covered with short, curled hairs. The lower surface is green and nearly hairless or sparsely covered with long, soft, cobwebby hairs. The margins have a few irregular teeth and many fine, soft spines, the main ones 1 ⁄16″ to ⅛″ long. Basal leaves are usually absent at flowering time.

The stem is erect, light green, ridged, and round in cross section. It has a few to many ascending branches, mostly above the middle. It may be hairless or sparsely covered with long, soft, spreading hairs, especially near the base. It is not winged and is not covered with a whitish, waxy substance (glaucous).

Stem leaves are alternate and 1¼″ to 6″ long. The leaf blade is pinnatifid, the lobes relatively narrow and cut more than ½ of the way to the midrib. It is otherwise similar to basal leaves. Lower stem leaves are stalked. Stem leaves become smaller and shorter stalked but no less divided as they ascend from the middle of the stem toward the inflorescence. Upper stem leaves are stalkless, well developed, and often slightly clasp the stem at the base.

The inflorescence is a single flower head or a more or less open, branched, cluster (corymb) of 2 to 5 flower heads at the end of each branch. The flower heads are stalkless or are borne singly on up to 6″ long stalks (peduncles). At the base of each peduncle there is a single 1″ to 3″ long, leaf-like bract. The corymbs are sometimes overtopped by the uppermost leaves. The flower heads are not subtended by a ring of spiny, leaf-like bracts.

The whorl of bracts at the base of the flower head (involucre) is broadly cylinder-shaped or bell-shaped, ⅝″ to 1¼″ long, and ⅜″ to 1¼″ wide. It is often somewhat cobwebby hairy. The bracts of the involucre (phyllaries) are in 8 to 12 series. They are green with a dark green central (dorsal) patch surrounding a white, gummy, sticky ridge. The outer (lower) and middle phyllaries are egg-shaped and taper to a appressed or ascending, sharply or bluntly pointed tip. The inner phyllaries are linear. The phyllaries are tipped with minute, insignificant spines. They appear spineless except under close examination.

The flower heads are to 1¼″ to 2″ in diameter and have numerous tubular disk flowers. The disk flowers are ⅝″ to 1¼″ long and reddish-purple or dark purple, rarely white

The fruit is a dark brown, ⅛″ to 3 ⁄16″ long seed capsule (cypsela). The cypsela has a conspicuous yellow collar near the tip and a tuft of white or slightly grayish-tinged hairs at the tip.

12″ to 90″

Purple to pink

The Native Thistle Test

Thistles native to Minnesota do not have spiny stems. Grasp the stem near the base of any native thistle loosely in your fist, then slide your fist upwards to just below the inflorescence. If the plant is a native thistle, you will not get a single prickle – it will be “ouchless”. If the thistle is thought to be an exotic (non-native) species, this test is not recommended. Tall thistle is native.

Wet. Meadows, prairies, marshes, swamps, bogs, woodland openings, streambanks. Full sun.

July to October

Sources

2, 3, 4, 5, 7, 28.

Native

Common

Cardueae is a synonym of the tribe name. Cynareae was published first and has precedence. Nevertheless, most sources use the name Cardueae for the tribe.

Plant

Inflorescence

Corolla

Involucre

Leaves