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If you're mulching in order to protect tender plants from harsh weather, rather than dig them up and store them elsewhere, then yes, the crowns need to be well covered with a dry mulch to protect them from the frost. It's usual to cut them back first.
Depending on what sort of mulch you use, most of it will have started to rot down by the spring, so when all chances of frost have gone you can clear the mulch away from the crowns of the plants and incorporate it with the rest of the garden soil.
There's some good advice here http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=337
When you say tender plants- which ones?
As far as I'm aware all of those are hardy in the UK and do not need mulching for protection. In fact it might do them harm as it might attract slugs! They'll be fine - they'll die down in the winter and in the spring they'l reappear. The paeony takes it's time and will be a little later appearing than most other herbaceous perennials, but appear it will.