Left to their own devices, lavender can become woody and ungainly, so to keep plants compact and attractive, it's best to trim them annually in late summer, just after flowering has finished. Remove any spent flower stalks and about 2.5cm (1in) of leaf growth. Pruning lavender each year provides several benefits including: Refining the shape of the plants; Encouraging bushier growth; Producing more flowers; Preventing woody and leggy stems with sparse growth.
Prune lavender plants at least once a year, although some gardeners prune them two or three times in a single growing season. The most important time to prune lavender is immediately after the plant flowers in summer to early fall, although you may want to give your plants a second pruning later in the season if they produce an additional flush of flowers. Because pruned lavender can be more susceptible to winter damage, stop pruning your lavender plants six weeks before your area's first frost date.
You can also prune lavender in the spring, but this can be tricky, especially with deciduous varieties. Lavender plants often won't grow when they are cut back into their woody sections. For this reason, if you prune lavender in spring, always wait for new leaves to sprout to avoid cutting into the woody section of the plant.
First-year lavender plants benefit from pruning, but it’s even more important to prune mature lavender. If you begin pruning lavender in its first year, you can shape the plant more easily, and pruning will encourage more vigorous growth and bushier stems.
The best time to prune lavender is usually immediately after the plant stops flowering, usually in late summer to early fall. Don’t prune lavender too late in the season, as plants pruned right before frost may not handle the cold well. It’s best to prune plants early in the morning on dry days because pruning cuts form a callus quicker under these conditions, and plant diseases are less likely to gain entry. Before pruning lavender, always disinfect your tools with rubbing alcohol to prevent the spread of plant diseases. You may also want to sharpen the blades before you work to ensure that the cuts are nice and clean.
Gather up a section of lavender stems in one hand and cut them off with pruners at least 2–3 inches above where the woody section of the plant begins. Continue this process, gathering up and pruning away lavender stems until you’ve cut the plant down by about one-third. If you’re working with only a few lavender plants, this is easy enough to do with pruners, but you may want to use shears for a large shrub or hedge.
Make pruning cuts only in the green sections of lavender plants. If you cut into the woody sections, there’s a good chance the plant won’t regrow. Only remove woody sections of the plant if they’re damaged or spoil the overall form of the lavender.
As you work, try to maintain the rounded look of the lavender plant by trimming stems slightly shorter toward the perimeter of the plant and leaving them a bit longer at the plant’s center. This creates a refined, mounded plant silhouette, but don’t worry too much about making your cuts perfect.
While it’s not strictly necessary if you prune your lavender in autumn, you may want to lightly trim it again in spring to clean up the lines of the plant and remove any winter-damaged stems. Just keep in mind that spring pruning may diminish the number of flowers your plant produces, so it’s best to prune early before flower buds appear.