Usual procedure is to shear over after a main flush of flowers ends, taking about an inch of foliage at the same time, shearing to shape, about twice a year, usually in about late June and again late August to mid September. You must never cut into old wood.

does this apply to french lavender as well Bamboo.

Yes, Helen, although if you have young plants (any variety of Lavender) and they're straggly in spring, they can be cut back to make denser growth.

thanks bamboo