Just to clarify ... Ragwort is poisonous to horses and cattle if they eat it. It's even more poisonous dead than alive, so needs to be removed from grazing pasture and hay meadows etc. However, it is very good for pollinating insects and is the main food for the larvae of the Cinnabar moth and other insects.
Posts
https://static1.squarespace.com/static/595ca91bebbd1a1d0aaab285/t/5a316245085229bad0f89849/1513185863463/Ragwort.pdf
https://butterfly-conservation.org/sites/default/files/ragwort-leaflet-june-20082.pdf
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.