Answer: In regions where they are marginally hardy, butterfly bushes (Buddleia) can be winterkilled quite badly, some years even to the ground, so it's a good idea to take some precautions. Wait until spring to cut them back -- leaving the top growth seems to help protect the roots during bad winters. Also, once the ground is frozen, mulch around the shrubs fairly heavily. In the spring, you can prune back the tops to remove any dead wood, and then trim what's left for shape.
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