Now is when they are easiest to identify and kill before they can set seed in November. We use a combination of herbicidal spot spraying and, an old favourite, the mattock. We always carry a mattock on the quad bike when we inspect the sheep and a couple of big hits is all it takes to physically remove the plant.
Mostly we are winning but we also have all four thistle species, some blackberry (under control) and plaenty of horehound. The latter used to used in brewing a non alcoholic and bitter 'root' beer. Don't care fort the taste myself but some of the beer labels were fabulous.