I probably should have done this after the Cinnabar Moth post or after the Marsh Ragwort post: what was I thinking? I just don't know what got into me!
So, somewhat later than perhaps expected, here is Common Ragwort, Senecio jacobaea.
Besides being the food plant of the Cinnabar moth it is used by over seventy invertebrates, with thirty being exclusively dependent on it. Of the thirty, ten are rare or threatened.
Unfortunately the plant contains nasty alkaloids which cause liver cirrhosis in livestock. Although the living plant is bitter and rarely browsed, when dried it becomes palatable. This makes hay a particular hazard should it contain any. It is listed as an 'injurious weed' under the 1959 Weeds Act and some by-laws make it illegal to allow it to grow on your land.
This reputation as meant a variety of common names reflecting its negative attributes: Mare's Fart is a particular favourite. Others include Stinking Willie and Staggerwort.
Perhaps surprisingly, given the lack of practical uses, it is the national plant of the Isle of Man where it is known as Cushag.