There are different schools of thought on how to trim lavender plants.
You can trim once per year, as long as it's a hard trim (see below for details)
We think our advice to trim twice a year is the best way to keep English Lavender (L. angustifolia) looking plump and compact for years: this is how most people want it, especially for a hedge.
The aim is to make the Lavender stems produce only a couple of centimetres or so of mature, woody growth each year.
When Lavender is allowed to grow too much, it tends to become leggy, make fewer flowers and need replacing.
Restricing its growth with hard trimming effectively keeps it young for longer.
Trim New Lavender Plants:
Trim newly planted lavender hard after flowering, in August/September. From then on:
1st trim of the Year:
In late February (or early March if the weather has been cloudy), trim your plants lightly.
This will encourage them to flower hard and keep them looking tidy.
2nd trim of the Year:
Right after flowering (or by end Sept), give your plants a very hard trim.
Cut all the new growth back down to 1-2cms above the older, woody part of the stem - this should leave between one to three leaf buds. This little bit is the hardiest part of the new growth and will survive the winter well. By trimming early, the wound will have time to heal and any re-growth will have a chance to harden up before the frosts.
They will look a bit sad for a short time, but they bounce back a bit and look very neat all winter.
This is optional but you get more flowers if you do...
Right after the first flush of flowers, just cut off the spent flower stalks. This will encourage a stronger second flush of flowers. We think that the seed heads still look quite nice, so we usually don't bother deadheading the Lavender in our garden after the end of June and they still produce some new flowers in late summer.
Cutting back mature Lavender Plants.
The consensus about clipping lavender plants is that the leafy, silver-green stems should be cut down to two or three buds above where it becomes hard and woody (i.e. leaving about 2cms of the year's soft growth).
There is a bit of a phobia about cutting into the woody part of the plant. In our experience, it is best not to cut lavender back hard. Pruning lavender gently each and every year, as described above, will keep it compact and stop it getting leggy, so you will never need to be fierce.
To be honest, once a lavender plant is too leggy and woody, we think the best thing to do is to dig them out and replace with new young plants.
You can order Lavender at any time, for delivery from late April/earlyMay onwards which is the best time to plant.