European Marsh Thistle (Cirsium palustre)

European marsh thistle (Cirsium palustre) is a high priority invasive species in the NCWMA region that can spread quickly by seed. Because of a persistent seed bank, infestations need to be treated annually for numerous years to eradicate even small sites. These thistles have been documented at only a handful of locations within our four county region in NW Wisconsin, making it important to treat all of the known sites to prevent it from becoming more widespread.

European marsh thistles invade wet ditches, wetlands, beaches, and dunes. Plants are biennial or monocarpic perennials. This means that they persist vegetatively with basal leaves for one or more years before the stems bolt and develop flowers. After the flowers develop seeds, the plants die.

If you see European marsh thistle this coming summer, be sure to report it to the NCWMA. This is a species that we track and treat. Most sites have been treated manually, by digging out plants before they develop seeds.

Identification

European marsh thistles have can grow over 7 feet tall and have spiny wings growing lengthwise along the stems. The plants have single stems that are unbranched except near the top. The branches arch upwards, have few leaves, and have tight clusters of flowerheads at their ends.

Inflorescences have densely clustered pinkish purple flower-heads with short stalks. Flowers are relatively small (3/4 inch across) compared to other thistles. Bracts (the base of the flower head) have cobwebby hairs. The tips of the bracts are pointed, but do not have spines. Plants flower in June to July.

Leaves are alternate along the stem and deeply divided into pinnate segments with spiny teeth along margins. Upper leaves are small, while lower leaves are up to 12 inches long.

Basal leaves are deeply pinnately lobed.

Flower heads are small (< 3/4 in) and have short pedicels. Numerous flower heads are bunched together.

Control

Small sites can be treated manually by cutting the tap root with a shovel 1-2 inches below the surface before it goes to seed. flowering plants should be bagged and disposed in the trash. Large sites may need to be treated with an herbicide. Herbicides with Aminopyralid as the active ingrediant (such as Milestone) are effective.

Similar Species

The native marsh thistle (Cirsium muticum) does not have spines on the stems.

Other invasive thistles:

Canada thistle (Cirsium arvense) has spiny leaves but non-spiny stems and flower heads. The upper leaves are not deeply lobed.

Bull thistle (Cirsium vulgare) has sharply spined leaves, stems and flower heads.

Plumeless thistle (Caduus acanthoides) has sharply spined leaves, stems and flower heads.

References

Minnesota Wildflowers. Cirsium palustre (Marsh Thistle). Accessed 1/16/2025. https://www.minnesotawildflowers.info/flower/marsh-thistle

Midwest Invasive Species Network (MISIN). European swamp thistle (Cirsium palustre). Accessed 1/16/2025. www.misin.msu.edu/facts/detail/?project=misin&id=39&cname=European%20swamp%20thistle

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