Invasive Species Factsheet: Marsh Thistle

Marsh thistle is a tall plant, capable of growing 4-7' in height. Flowering typically occurs from June through August when marsh thistle produces clusters of 12 or more purple flowers (up to 0.75" wide) at the tips of leafless branches in the upper stem. There are no stem branches below the flowers, and the unbranched stems have spiny leaf bases called wings.

The leaves of marsh thistle have spines at the tip of each lobe and are usually densely hairy on the underside. Note: Be careful when handling this plant as its spines are capable of piercing human skin.

The current known distribution of marsh thistle in North America includes a handful of northern U.S. states and several Canadian provinces. Pennsylvania does not have any documented occurrences of this species, making it an early detection species in the Commonwealth. Any suspected findings should be reported immediately.

Marsh thistle prefers to grow in moist areas such as wetlands and in places containing acidic ground including marshes, bogs, fens, wet fields, stream sides, and forest edges. This species can also grow in almost any disturbed setting such as roadsides and wet ditches as long as adequate moisture is present.

It can tolerate growing in somewhat shaded conditions, and therefore can be found in moist woodlands as well. Marsh thistle is not able to compete well in upland sites where moist soils are limited or not present.

Marsh thistle can produce up to 2,000 seeds per plant. These seeds can be transported parachute-style on windy days, but generally can only travel short distances. Methods of longer-range dispersal include birds, water, vehicles, and equipment.

To help prevent the spread of this and other invasive species, clean your shoes after hiking using a boot brush station and leave on site any seeds or plant fragments you find.

Several species may look similar in appearance to marsh thistle. Some of these species include:

•  Canada thistle  (Cirsium arvense)
•  Bull thistle  (Cirsium vulgare)
•  Plumeless thistle  (Carduus acanthoides)
•  Swamp thistle  (Cirsium muticum)

Marsh thistle can be distinguished from most other species of thistle by its single un-branched stem with spiny wings. Additional information on the growth form of the above-named look-alike species is provided below:

Canada thistle: Has a different growth form; a shorter plant with flowering heads in loose clusters on leafy stems, growing rhizomatously. No spiny wings on stem.

Bull thistle: Flowers are at the tops of branching and leafy stems. This species also has spiny wings, but is more branchy.

Plumeless thistle: Flowers are at the tops of branching and leafy stems.

Swamp thistle: No spiny wings on stems.

Marsh thistle is capable of forming dense thickets that can crowd out shorter native plants and is considered a prolific seed producer. Floodplain forests, herbaceous wetlands, and other wet areas with canopy openings are subject to invasion. Resulting impacts to native habitats include a decline in biodiversity and compromised ecological integrity.

In areas where rare native species exist, marsh thistle poses a significant ecological threat. Attempts should be made to recognize this species in the wild and remove it where possible.