Marsh Thistle
Cirsium palustre
Washington State Classification
Class A — Eradication Required Statewide
Marsh thistle is quickly expanding in North America, and has a high potential to infest many parts of Washington State.
Quick Reference
Identification
Growth Habit
Herbaceous biennial or monocarpic perennial thistle. This means it only blooms once, but may live for multiple years until it does so. For the first year, or until it blooms, it lives as a rosette of spiny leaves, before bolting into a tall flowering stalk.
Leaves
The hightly dissected and spiny leaves of the basal rosette can be up to 1 foot long. The leaves on the flowering stalk are similarly dissected and lobed, but grow smaller the higher on the stem they are. In the shade, leaves will be wider and less spines, than those growing in sun.
Flowers
Clusters of pink to pink-purple thistle heads at the top and tips of flowering stems. The petals are made of disk flowers, and look very frilly. Each flowering head has a bulb of green and green-purple bracts, which often have spines on the tips.
Fruit & Seeds
Small, brown to black seeds attached to a feathery pappus (like a dandelion seed)
Impact
Marsh thistle is quickly expanding in North America, and has a high potential to infest many parts of Washington State. There are several infestations in British Columbia, which are close to the Washington border.
Ecology & Spread
Habitat
Moist, wet, and damp sites, in full shade to full sun, in both Eastern and Western Washington. Click here to see an informal map of known marsh thistle locations in Washington as of January 2025.
Spread Mechanisms
Reproduction
Seed only, however when hybridized with creeping thistle (Cirsium arvense), it is likely to be able to reproduce by creeping underground rhyzomes.
"