Flower:

Few to many, short-stalked flower heads in tight clusters at the top of the stem and at the ends of long, mostly leafless ascending branches on the upper stem.

Flower heads are about ¾ inch across, lavender to pinkish-purple. The bracts (phyllaries) are in 5 to 7 layers, flat except for the tip which peels back away from the flower head; the tips are pointed but lack sharp spines. Bracts are green to purple-tinged, fringed with hairs around the edges, and covered with a thin layer of fine cobwebby hairs.

Leaves and stems:

Leaves are deeply lobed, toothless but some lobes further segmented, wavy around the edges and a sharp spine at the tip of each lobe. The upper leaf surface is variably hairy, the underside more densely hairy especially along the veins. Lower and basal leaves are up to 12 inches long and 4 inches wide, mostly crowded on the lower stem, becoming much smaller and widely spaced as they ascend the stem.

Leaf bases extend down the stem creating spine-tipped “wings” that are very sharp. Stems are erect, single, unbranched except in the flower clusters, ridged, densely to moderately hairy. Hairs are short and somewhat curly to long and silky.

Fruit:

Fruit is a light brown to straw-colored seed with a tuft of long white hairs to carry it off in the wind, spreading itself far and wide.

Notes:

Marsh Thistle is a new addition to Minnesota's growing non-native invasive species list. Reports from Michigan say it forms impenetrable spiny stands in wetland communities and is rapidly spreading across the upper Midwest. It is cold hardy and is found all the way to the arctic circle, and can invade boreal forest. It's been recognized as a prohibited weed in Wisconsin for some years but ignored in Minnesota, likely because it hadn't been seen in the state. Well, now it's here. While it was rumored to be here somewhere in Cook (and possibly Houston) County, we reported the first confirmed population in the state, found along the Gunflint Trail a few miles north of Grand Marais. We reported it to the Superior National Forest Gunflint Ranger Station within a few hours and their invasive species crew started taking steps to wipe it out within a week. This is what “early detection, rapid response” is all about!

Marsh Thistle is fairly easy to recognize, even passing by at 60 mph. It's generally tall (to 7+ feet) with a single stem, unbranched except near the top, with long, nearly leafless ascending branches and the short-stalked flower heads densely clustered at branch tips. The lower leaves are somewhat similar in shape to the native Swamp Thistle (Cirsium muticum) and the phyllaries are much like (the also invasive) Canada Thistle (Cirsium arvense), but Marsh Thistle has spiny winged stems, which the other two species lack. The spiny stems are characteristics of several other non-native purple thistles, which have spine-tipped phyllaries plus flower heads with longer stalks that aren't as densely clustered at branch tips. In any case, the spiny stem indicates a non-native thistle and it's best to eradicate it, no matter what species it is.