The Marsh Thistle is quite tolerant of shade and is often found in woods along the paths and fire breaks. Also as its name suggests it prefers damp soil. You will find it growing outside woodlands as well, sometimes in wet meadows, moorlands and roadsides. It can grow very tall, over 2 meters high is quite possible. The flowers though are quite small as thistle flowers go and a fairly deep purple colour.
It is also the species of thistle which more often than any other produces white flowers, sometimes it will even produce pink flowers. I had one a couple of years ago growing in Ninewells Wood in Monmouthshire which had pink flowers but despite allowing its seeds to disperse and the growth of subsequent generations I have not seen any more pink ones. The flowers (purple or white) are produced between June and October.
It is a fairly statuesque plant, growing tall but with few side branches, and even these are quite short. The stem is adorned with spiny wings and these and the leaves can develop a purple tinge.
The flowers are much visited by insects as they are rich in nectar, below is a slide show of the flowers with the Five Spot Burnett moth and a Red Admiral butterfly visiting.