I cut mine half back in the autumn, to avoid wind-rock, then very hard back in early spring. I have Lavatera 'Cynthia Nixon', and she's grown about 10' from the base this year!

Yes agree with spritzhenry, lost mine last winter to the severe cold weather.

Thanks Spritz and Julien , I'll try the prune , I always do so with much tepidation ,I must say !

You don't need to - they'd even grow back from a little stump! I look for the first growth on each branch and saw it off to that.