They are so slow to bloom I don't cut back at all....just deah head the spent blooms. Some are ment for a hanging basket...so they really don't get "bushy".
Linda C

Agree - don't cut down. Just deadhead.

Depends on what variety you get. I have had a few cutting grown ones who were vigorous and rampant in their growth habit. Others, not.....especially the seed started miniature marine.