I have heavy clay soil too. My chickens free range a lot in the garden. They have not touched the clumps of geraniums - "proper" geraniums which are perennial, not pelargoniums which we usually refer to as geraniums (you know - the ones that stink IMHO!)
Weigela viburnum euonymous lavatera and rhododendrons have also been ignored. Mimulas and verbascum have been demolished.