Experts and BHG readers answer.
The very best time to cut back or prune lavender is right after it finishes flowering. This will help the plant stay compact and maintain an attractive shape. Be sure to cut it back after every flush of flowers (those purple smelly parts). Just trim the flower stems off and trim the remaining foliage to your liking. be sure not to cut back to the old wood as this will likely kill the plant.If you're not sure, try pruning in increments. Cut it back by 1/3 in spring to stimulate new growth; then cut it back again by another 1/3 after the new foliage has grown in. Never prune the old wood unless it is completely dead.