Chris J Dixon writes
K wrote:

Alternative is to grow clematis in large pots away from slugs (I grow in
greenhouse) until they have about 4ft of woody stem and then plant them
out. And don't cut them back lower than this. Although they still get
slug/snail damage, it's a lot less at that height, and they soon get to
a level of vigour where they are growing faster than the slugs can eat.

I note that it is generally advised to prune clematis fairly hard
after flowering, yet some of the most impressive displays I have
seen have been those where it has just been left to itself on a
wall or trellis.

Pruning depends on the clematis species (early, mid season or late
flowering) and is partly to bring the flowers into vision rather than
somewhere in the tree tops.
--
Kay