When a young lavender plant is first planted, it should be pruned back by one-third. Simply take a sharp pair of pruning shears and take off one-third of the growth. This is true whether it is planted in the fall or the spring. However, early spring is the best time to plant a lavender plant. This is because the newly planted lavender can grow quickly as the soil warms and not sit in damp soil without a good root system all winter. A healthy lavender plant grows rapidly after planted.
The lavender plant will probably bloom the first summer after planting. This is the right time to begin a pruning program. After the first bloom, you need to remove any old blooms left on the plant so the plant does not produce seed. During the seed production process, the lavender is using energy that could be used to produce more growth. Remove old blooms by cutting the stem where the foliage begins at the base of the flower stem. You can cut slightly more, but only to shape the plant. Prune off the old blooms in this manner after every bloom period.
This is an important pruning step that is often overlooked. Every spring, when the lavender plant begins to awaken and put on new growth, shear off 4 to 6 inches of the plant and form a nice mound. Never cut in to the woody part of the plant. The plant cannot recover and will always look damaged if you cut into the woody section. If there was not enough growth over the first year to cut off 4 to 6 inches before cutting into the woody stem section, cut off half that amount. This will allow the plant to produce a nice crop of blooms as well as keep it in shape for the next season.