I cut mine back and it sprouted again. I don't think I did it at the right time but it didn't seem to matter. To be on the safe side I didn't do it all at once, waiting until the cut parts began to grow again before cutting the other parts.

I agree with Steragram..I have pruned mine several times ,without any problems.. after flowering.. around late spring/early summer would be a good time..I have actually cut them back hard a couple of times,over the years...a bit drastic,but they grew back nice and bushy..

I had to cut mine back after a late hard frost killed the new growth in April last year. Not a good time to have to prune, but it is thriving and in better shape than it has been for years.