Wait for the flower heads to change from gray to purple during the growing season. Once the flowers bloom, select a dry, cool morning for harvesting the French lavender. If there is dew on the plant, wait for it to dry.
Fill a bucket a quarter of the way with cool water. Take the bucket into the garden with you when you harvest French lavender.
Cut the lavender with a pair of sharp scissors, making the cuts below the blossoms on the stem at your desired length. Cut taller stems first in order to keep the plant compact.
Drop the harvested stems into the bucket of water, cut-end first. This will keep the lavender cool and prevent the flowers from releasing its fragrant oils before you use them.