Weed List

Swamp Thistle
(Cirsium muticum

Thistles of ND

As its name implies, swamp thistle is found in moist low-lying woodland areas, thickets, and near rivers and especially wet meadows. It is found in eastern North Dakota and in the Bottineau area. Swamp thistle is an elegant long-stemmed flowering plant that usually grows 3 to 6 feet tall. Swamp thistle has very few and weak spines on the leaf margins and no spines around the flower head, which is an easy way to tell it apart from field thistle and tall thistle.

Swamp thistle is a biennial with a soft, hollow, and ribbed stem green in color with fine hairs. The leaves are light green, oblong with deep lobes to lanceolate, and the lobes usually are tipped with a small weak spine and almost hairless.

The leaves are up to 10 inches long and 4 inches wide at the base but become progressively smaller and more lanceolate towards the tip. The rosette leaves are generally oblong and can grow to 22 inches long and 8 inches wide.

Swamp thistle flowers somewhat earlier than field thistle, mid-July through early September. The heads are about 1.5 inches across and bell shaped, and the bracts are sticky and gray to white resembling cobwebs. The flowers are dark purple to lavender, generally the color of red wine (rarely white) and can be solitary or in clusters. The achenes are about 0.25 inches long, narrow, and dark brown to black in color with a yellow apical band.

The swamp metalmark butterfly will lay eggs only on swamp thistle which is the caterpillar's only food source.