Cut it back by about 1/3 to 1/2. Be careful about cutting back more than this - I did that one year and the plants didn't rebound quickly, they sulked for quite a while. I also find it looks better if I vary the cuts - some branches I cut back 1/3, other branches 2/3. just cut mine back and they've already starting growing back and throwing blooms, they look magnificent! -Birdy