True thistles such as Cirsium are long-lived perennials and can be divided in spring or autumn; ... The equally harmless melancholy thistle C. heterophyllum has similar purple flowers on tall stems.

Another species, the melancholy thistle (C. Heterophyllum) can be found even by roadsides at this time of year and I have seen its purple heads bristling along grass verges in Upper Wharfedale.

Alistair S. Jump, F. Ian Woodward, Seed Production and Population Density Decline Approaching the Range-Edge of Cirsium Species, The New Phytologist, Vol. 160, No. 2 (Nov., 2003), pp. 349-358. ...