Tom's Blog

Saturday, September 19, 2015

Swamp thistle: an attractive wetland plant

We have always had swamp thistle (Cirsium muticum) in our wetland, but this year it is particularly prolific in the part of the wetland we burned this April.

The C value of swamp thistle is a relatively high number of 8 (in Michigan it is 10!).

Occasionally we have a few swamp thistle plants in our wet mesic prairies (Crane and Valley), but it is most prolific in the real marsh areas where the water table is close to the surface.

Swamp thistle is native to Pleasant Valley Conservancy and thrives without any help from us. However, we do collect seeds for planting in other parts of the wetland, and are happy if it spreads into areas where we have not seen it before.

In an earlier blog post this year I discussed the use of the hemiparasite lousewort to promote wetland plants. Swamp thistle was one of those that benefited, but my impression is that burning is even more important for this species. 

Like our most common native thistle (old field thistle; C. discolor), swamp thistle is a biennial. It grows from seed its first year as a rosette. Like other biennials in northern climates, it probably requires a cold period to induce flowering. 

This is a beautiful plant with rich purple-colored flowers. It is not surprising that the insects love it!


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