Remove any spent flower stalks and about 1 inch of leaf growth. Do not prune into the woody part of the plant. Then again in spring, prune them by removing another inch or two of the leafy stems, but do not cut down to the woody base.
What happens if you don't prune lavender? Their lifespan will be shortened, and without a trim they will grow a large, lanky, woody base that can split open and looks bad.
For a more information on pruning lavender, click here.
Photo credit: Garden Gate Magazine