How do I prune lavender, rosemary and other woody herbs?

The one rule that governs every Mediterranean subshrub — no dormant buds in bare old wood — how to prune within it every year, and what to do with a plant already past the point of rescue.

Difficulty: beginner
Time: 40 min
Read: 5 min

Short answer

Cut back by about a third every year, always leaving several centimetres of leafy green growth below every cut. Never cut into the bare grey wood at the base: these plants carry no dormant buds there and a stem cut into old wood simply dies. Do it once growth is established, and stop in good time before winter.

Lavender, rosemary, sage, thyme, santolina, cistus, helianthemum and several other silver-leaved plants share one piece of anatomy that governs everything about pruning them. Unlike a rose or a buddleia, they hold no latent buds in mature bare wood. A cut into that wood does not stimulate new growth — it removes the stem's only source of it. Learn that once, and every one of these plants becomes simple.

What you'll need

Sharp secateurs or single-handed shears
Gloves
Kneeler (optional)

The line you must not cut below on lavender, rosemary and sage

Step by step

Look at the plant and find where the green stops.Run a hand down a stem from the tip. There is a zone of soft leafy growth, then a point where the stem is bare, grey-brown and woody. Every cut you make must be above that point, with green growth left below it.
Take about a third off, no more.Shape the plant into a neat dome or mound, removing the spent flower stems and roughly a third of the season's growth. Aim for a tidy shape you would be happy to look at all winter, because that is what you will be looking at.
Do the main cut after flowering, in late summer.For lavender, around August, once the flowers have gone over. For rosemary and sage, after flowering in early summer. The plant then has time to make a little new growth and harden before the cold.
Stop cutting by early September.Growth stimulated later than that will not harden before frost, and soft new growth on a Mediterranean subshrub going into a wet British winter is exactly what rots.
Give a light tidy in spring if needed.In April, once you can see what has come through the winter, remove any frost-damaged tips and even up the shape. Keep it light — the main cut is the one after flowering.
Start in the plant's first year and never miss one.This is the whole game. A plant cut lightly every year from the day it is planted stays compact and inside the safe zone indefinitely. A plant left for three years is usually already past saving.
Prune to a shape, not to a line.Rounded domes shed rain and snow and stay furnished. Flat-topped subshrubs collect water and split open.
Handle rosemary and bay slightly differently.Rosemary can be trimmed after flowering and again lightly in late summer, and it dislikes hard cutting as much as lavender does. Bay is a true shrub and does regenerate from old wood, so it can be cut back much harder.
Accept when a plant is finished.A split, sprawling subshrub with bare grey wood in the middle and green only at the tips has no material to cut back to. Take cuttings — they root easily — and replace the plant. This is normal: lavender is generally replaced after eight to ten years even when well pruned.
Attempt a staged renovation only where there is low green growth.If you can see leafy shoots emerging from low down on the woody stems, you can cut back to those over two or three seasons, doing part of the plant each year. Without them, there is nothing to work with.

Tips

Drainage matters as much as pruning for these plants. Most are killed by winter wet, not winter cold, and a plant in heavy wet soil will fail however well you prune it.
Cuttings from lavender, rosemary, sage and thyme root very readily in summer. Taking a few every year means you always have a replacement ready before the old plant collapses.
Do not mulch these plants with rich organic matter against the stems. Gravel or grit around the crown suits them far better and keeps the base dry.
Sage and thyme both become woody and open with age exactly like lavender, and both are best replaced every few years rather than renovated.

Common mistakes

Cutting back into the bare wood to make the plant smaller — There are no dormant buds there, so the cut stems simply die back. The plant looks tidy for a fortnight and then is dead.
Skipping the annual cut because the plant looks fine — One or two missed years is how a compact plant becomes a woody sprawl. The cut is preventive, not corrective.
Pruning hard in autumn or late winter — It provokes soft growth going into cold wet weather, which is exactly what rots and dies. The main cut belongs after flowering in late summer.
Growing them in rich moist soil and feeding them — These are plants of poor, dry, free-draining ground. Rich damp conditions produce soft growth, few flowers, less scent and a short life.

If it doesn't work

Cut stems have died back completely

Cause: Cut into bare old wood — Fix: Nothing can be done for those stems. Take cuttings from any remaining green growth and replace the plant.

Plant split open with a bare hole in the middle

Cause: Never pruned, or pruned too lightly — Fix: Usually terminal. Take cuttings and replace, then prune the new plant annually from year one.

Leggy soft growth with few flowers

Cause: Soil too rich, too wet, or too shaded — Fix: Move it to a sunnier, drier, poorer position. Do not feed.

Died over winter with blackened stems at the base

Cause: Winter wet at the crown rather than cold — Fix: Improve drainage, plant on a slight mound or in gravel, and keep organic mulch away from the stems.

Questions people ask

Can I cut lavender back hard to rejuvenate it?

No. Lavender does not reshoot from bare old wood, so a hard cut into the woody base kills the stems. Prune a third off each year to stop it reaching that state, and replace the plant when it does.

Which plants does this rule apply to?

Lavender, rosemary, sage, thyme, santolina, cistus, helianthemum, hyssop, curry plant and most silver-leaved Mediterranean subshrubs. Bay and box are true shrubs and are exceptions — both reshoot from old wood.

When should I prune lavender?

The main cut is after flowering, around August, taking about a third off and leaving green growth below every cut. A light tidy in April deals with winter damage.

My rosemary is enormous and woody. Can I reduce it?

Only by cutting into growth that still carries leaves. If the lower part is bare, there is no way back. Take cuttings and start again — rosemary roots very easily.