By the time a plant is three years old, it is full grown & will definitely need trimming. Younger plants may be trimmed too if they need shaping or grew quickly, but by year three, plants should be trimmed at least once a year in early spring (after the last frost) or, in the fall (prior to the first frost).
This trimming is not only harvesting the blooms or flower stems, this is trimming to shape the main leaves & sometimes actual woody stems. Each cultivar will have a different size and shape and you will need to adjust how deep and the amount you trim accordingly. English varieties tend to be smaller and bushy while French are larger and have more uniform leaves & stems.
If a plant is never trimmed, it will become overgrown and may split in the middle, opening it up to disease and looking scraggily. It may also grow too many little bloom stems instead of a healthy amount it can support.
We prefer to hand trim with pruning shears. It's easier to create a uniform shape and the cuts are smooth. Plants look better later when the leaves grow in. But we always have to also use our electric trimmer because of the amount of plants we have. It would be impossible to get them all trimmed without it. We aim for trimming half of our field in the fall and half in the early spring.
Trimming English lavender is the same as trimming most bushes. Choose the height you want your plant to be. Cut down the top to that height, then shape the sides to blend smoothly to make a uniform ball shape. Sometimes we do trim more then most folks think looks safe & may even look skimpy. Some plants need a good hard trim due to overgrowth. But as long as the plant has time to seal the cuts prior to the frosts (2 weeks prior in the fall) it should not go into shock. Likewise, trimming during a heat spell is not recommended and may be too harsh for the plant to survive.
Here are some pics and video of Super Bleu (An English cultivar) being trimmed. Many of the plants did need a deep trim but will come back nice & bushy in the spring. We will post pics in the spring for comparison.
Finished plants:
*Warning- Trimming too harshly or during harsh weather conditions may cause shock. There is always the chance trimming may lead to shock. Please consult your local garden/landscaping specialist for additional advice if needed. This content is for informational use only and Wildwood Lavender Farm LLC is not responsible for damage caused when trimming is done improperly. (This is probably why it is hard to find advice on trimming overgrown plants :-(