European swamp thistle (Cirsium palustre)

Synonyms: Carduus palustris L., Cnicus palustris (L.) Willd.

Common Names: European marsh thistle, marsh thistle, European swamp thistle

Description:

European swamp thistle (Cirsium palustre) is a tall, spiny wetland thistle from Europe that has spread across the upper Midwest since the early 1900s [1][3]. It forms dense, impenetrable spiny stands in wet meadows, fens, and ditches, crowding out native wetland plants [3]. MISIN tracks it because it invades high-quality, undisturbed wetlands ? habitats few other thistles reach.

Habit:

European swamp thistle is a biennial (or short-lived) thistle that grows a low cluster of spiny leaves in its first year, then shoots up a single tall flowering stalk ? often 5 to 7 feet, sometimes taller ? in its second year [1][2]. The most reliable feature is the stem itself: it is lined top to bottom with continuous spiny "wings," so the whole stalk looks prickly, not just the leaves [2][4]. The flower heads are small for a thistle ? only about half an inch across ? and packed in tight clusters of a dozen or more dark purple (occasionally white) heads near the top [1][2].

It is easiest to spot in June through August, when the tall purple-topped stalks stand above surrounding wetland plants. First-year plants are easy to miss ? a flat rosette of spiny, deeply lobed leaves hugging the ground.

At-a-glance cues:

1. Stem lined with continuous spiny wings from base to tip ? the whole stalk feels prickly [2][4]
2. Tall, mostly single, unbranched stalk reaching 5?7+ feet in bloom [1][2]
3. Many small purple flower heads (~0.5 in across) packed in tight clusters at the top [1][2]
4. Grows in wet ground ? fens, wet meadows, ditches, and forested wetlands [3]

Leaves:

1. Arrangement: alternate; first year as a low basal rosette, second year along the stem.
2. Shape: narrow and lance-shaped, deeply pinnately lobed (cut into segments).
3. Margin: spiny-toothed, each lobe tipped with a sharp spine.
4. Size: basal and lower stem leaves 6?12 in (15?30 cm) long, becoming shorter up the stem.
5. Surface: dark green and often somewhat hairy, especially on the underside; leaf bases run down the stem to form its spiny wings.

Stems:

1. Habit: erect herbaceous stalk, usually single and unbranched except near the top, reaching 5?7 ft (1.5?2 m) or more.
2. Wings: continuous spiny wings run the full length of the stem (formed by the down-running leaf bases) ? the key separating feature.
3. Surface: often covered with fine sticky or cottony hairs; frequently flushed purplish.
4. Cross-section: round, hollow-stiff, rigid enough to stand well above surrounding vegetation.

Flowers:

1. Color: dark reddish-purple, occasionally white.
2. Heads: small for a thistle, about 0.4?0.6 in (10?15 mm) across, made entirely of disk florets.
3. Arrangement: many heads (often a dozen or more) packed in tight clusters near the top of the stem and branch tips.
4. Bracts: spine-tipped but only weakly so compared with larger thistles.
5. Bloom time: roughly June through August.

Fruit and seeds:

1. Fruit: a small dry seed (achene), about 2.5?3.5 mm long.
2. Pappus: topped with a tuft of feathery white bristles for wind dispersal.
3. Output: a single plant can produce up to ~2,000 seeds [1][3].
4. Dispersal: mostly wind; most seed falls within ~33 ft (10 m) of the parent, but strong winds can carry it much farther [1]. Seed also moves on equipment, mowers, and in mud.

Habitat:

European swamp thistle grows in moist to wet ground: fens, wet meadows, sedge meadows, bogs, forested and partly forested wetlands, and the wet edges of ditches and roadsides [1][3]. It favors acidic soils, tolerates partial shade (though it flowers more heavily in the open), and readily colonizes both disturbed sites and high-quality, undisturbed wetlands ? a trait that makes it especially troublesome [1][3].

It is native to Europe and was introduced to North America in the early 1900s (recorded in New England by the early 1900s and in Michigan by 1934) [1]. It is now established across the northeastern U.S., eastern Canada, and the Great Lakes region, where populations are still expanding south and west [1][3].

Reproduction:

European swamp thistle spreads only by seed ? it has no creeping roots. The feathery seeds blow in on the wind from nearby stands, and also hitch a ride on mowers, equipment, vehicles, and in mud moved between wet sites. Most seed lands close to the parent plant, so infestations tend to thicken and creep outward, but the occasional long-distance gust starts new patches far away [1].

It is a biennial or short-lived monocarpic plant: it spends its first year (sometimes longer) as a ground-hugging rosette, then bolts into a tall flowering stalk, sets seed, and dies [1]. A single plant can release up to about 2,000 seeds [1][3]. For control timing, the key point is to stop seed from forming ? remove or cut plants in the rosette stage or while bolting, before the purple heads open and go to fluff in summer.

Impact and Damage:

For people on the land, dense European swamp thistle is a physical nuisance: its tall, spiny, winged stems form prickly, nearly impenetrable stands that make wet pastures, trails, and ditch banks hard to walk or work through [3]. It offers little forage value and can dominate wet field edges.

Ecologically, it is a wetland specialist that invades fens, sedge meadows, and bogs ? high-conservation habitats that resist most other invaders. It forms dense colonies and carpets of rosettes that shade and crowd out shorter native wetland plants, including the native swamp thistle, reducing plant diversity and degrading habitat structure [3]. Because it establishes in undisturbed natural wetlands, not just roadsides, it is rated a medium-to-high threat in the Great Lakes states [3].

Similar species:

Several thistles share the spiny, purple-flowered look; check the stem and flower-head size to separate them, and be careful not to destroy native thistles.

Swamp thistle (Cirsium muticum) is the native wetland thistle most easily confused with it, sharing wet habitat, but its stem lacks the continuous spiny wings and its flower heads are larger with soft, spineless, cobwebby bracts.

Bull thistle (Cirsium vulgare) also has spiny-winged stems, but it is shorter and bushier with large, solitary flower heads (1.5?2 in across) and stiff spine-tipped bracts, versus the many small clustered heads of European swamp thistle.

Canada thistle (Cirsium arvense) spreads by creeping roots into large patches, has smooth (unwinged) stems, smaller pale lilac heads, and is dioecious ? quite different from the tall, single, spiny-stemmed European swamp thistle.

Tall thistle (Cirsium altissimum) is a native thistle of drier ground with white-woolly leaf undersides and unwinged stems, distinguishing it from the spiny-winged, wetland-dwelling European swamp thistle.

Monitoring and rapid response:

Signs to watch for. Tall, prickly, purple-topped stalks standing above wet meadow, fen, or ditch vegetation in June through August, and flat rosettes of spiny, lobed leaves on wet ground in spring and fall. The continuous spiny wings running the full length of the stem confirm it ? but check that you are not looking at the native swamp thistle, which has smooth, unwinged stems.

What you can do.

1. Confirm identity first ? the native swamp thistle is a look-alike worth protecting. When unsure, get the plant verified before pulling.
2. For a few plants, dig or pull in the rosette or early bolting stage, removing as much of the root crown as possible; wear thick gloves for the spines.
3. Cut or remove flowering stalks before the heads open and set seed; bag and dispose of any heads that have started to flower rather than leaving them on site, since cut heads can still ripen seed.
4. Because it spreads only by seed, stopping seed set for a few years can exhaust a local patch ? return each season to catch new rosettes.
5. Clean boots, tools, mowers, and vehicles after working in an infested wetland to avoid carrying seed to new sites.

When to call a professional. Large or well-established stands, infestations in standing water or sensitive fens and bogs, and sites where you cannot reliably separate it from native thistles are best handled with professional guidance and coordinated, multi-year, multi-visit removal ? prioritize small outlier patches first to keep it from reaching intact wetlands.

1. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Fryer, Janet L. 2010. Cirsium palustre. In: Fire Effects Information System (FEIS). U.S. Department of Agriculture, Forest Service. research.fs.usda.gov
2. Wisconsin Department of Natural Resources. European marsh thistle (Cirsium palustre). dnr.wisconsin.gov
3. U.S. Geological Survey, Nonindigenous Aquatic Species. Cirsium palustre species profile. nas.er.usgs.gov
4. Washington State Noxious Weed Control Board. Marsh thistle (Cirsium palustre). nwcb.wa.gov/weeds/marsh-thistle

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