Does the mullien moth decimate other plants - or just stay on the Verbascum?
Yes , they're partial to the Figworts as well (Scophularia sp..) ; although these are commonly encountered around lake and pond margins .Popular with wasps and not often grown in gardens . Maybe a plant for an aquatic or wildlife garden .
There are loads of verbascums, small ones, tall ones, pink ones, yellow ones..and they are in same family as foxgloves so quite a bit of similarity. But your triffid is a verbascum to me...
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.