In the south, I would just cut off everything thinner than a pencil, and simplify the branch structure a little, in early spring. That will encourage more vigorous flowering twigs, and fewer little sprangly "whiskers". In the north, where it is more of a big perennial, I would chop it all off to about 15 cm tall in late fall, and maybe cover it with mulch. Some hybrids with L. fauriei might be hardy enough to become shrubs in those places, and will respond to "pencil pruning".