The marsh thistle (Cirsium palustre) is another of the common members of the thistle genus found in Dorset. It mainly grows, as its common name implies, in marshes, damp meadows and grassland, by rivers and ponds, in damp woodland; indeed anywhere the soil is damp (for some of the time at least). It can be quite widespread and can even form quite dense patches in suitable habitat.
This plant has a tall central stem (which is very prickly) from which other stems occasionally shoot out to the sides and at the end of each stem you get a tight cluster of small dark purple flowers, just a few in flower at any one time. The leaves are thin and pointed with sharp spines on the edges. It is not a particularly attractive plant perhaps but it can certainly be impressive and I have seen them growing to nearly eight feet tall.
It flowers from July to September and is really quite distinctive once you recognise its main features.