I worked one summer at a lavender farm where almost every day we were asked, “when should I prune my lavender?” The answer is usually mid- to late-summer… when the flowers have turned brown and the bees are no longer interested.
Why prune lavender?
Cutting back lavender between mid-August and mid-September gives the plant time to put on some fresh growth before winter, leaving it looking neat and healthy right through until spring. If a plant is left unpruned for years it will keep growing bigger, but most of the growth will be at the outer ends of the branches. Eventually the weight of this growth will cause it to flop outwards leaving the woody centre exposed and maybe some broken branches too.
An annual prune keeps the plant smaller by encouraging growth from closer to the base.
How much should I cut back?
After “when should I prune lavender?”, the next question was always “how much should I cut back?” The key thing is to be bold but make sure there are still plenty of small green shoots on the plant. If the plant has been pruned every year, you can cut back about 1/3 of the growth to keep it in shape and prevent it becoming too big. If needed you can cut back further, but either way you must leave some green shoots or the plant won’t grow back. You can use shears for speed or secateurs for a more precise approach. The fragrant clippings can be put on the compost heap.
What if I forget to prune a lavender plant?
Although lavender should be pruned every year, it doesn’t have to be in summer. In fact, in a garden designed to attract wildlife there’s a good case for leaving some plants unpruned through the winter. Birds like goldfinches love the seeds and will feast on the spent flowerheads. For this reason, I prune half my lavender plants in late summer and leave the rest until spring. I’ve been doing this for years and really don’t notice any difference in the vigour or flowering time of the plants. I try to alternate so that a plant that was pruned in summer one year is left overwinter and pruned in spring the next… I don’t know if this makes a difference, but it feels fair!