Thuja: when and how to prune

Clip a thuja hedge in May–June and again in late August, by mid-September at the latest, and always only into the green: it will not regrow from bare brown wood.

Thuja is probably the most widely planted hedge in Belgium and the Netherlands: it grows fast, stays green and forms a dense screen. But like most conifers, yew excepted, it does not regrow from bare brown wood. Cut too deep and you will be looking at a brown hole for years. So the whole art is to clip often and lightly.

Updated on 10 October 2026

Key facts
Pruning months	May–June and August–September
Nesting season (15 March–15 August)	Take care in May–June and August
Good for bees, butterflies and birds	Yes, for birds
Native to Belgium	No, planted
Poisonous to people and pets	Yes, take care with children and pets

Thuja: when to prune it?

The first clip comes in May–June, once the spring growth is complete. Check carefully for nests first: a thuja hedge is a favourite nesting site.

The second clip is in late August, when most young birds have fledged. Don’t clip after mid-September: the new growth still needs to harden off before winter.

Don’t clip in bright sun, heat or drought: the cut tips scorch and the hedge turns brown. An overcast day after a little rain is ideal.

What else to prune in those months? May · June · August · September

Thuja: how to prune it?

Wear gloves and long sleeves: the sap can irritate the skin, and the foliage contains poisonous thujone.
With a hedge trimmer, cut back only the new, light green growth, to a few centimetres above the previous cut. Always stay in the green.
Cut the sides at a slight slant, with the top narrower than the base: the bottom then gets light and the hedge stays green down to the ground.
Stretch a line for a straight top. Cut the top as soon as the hedge reaches the height you want: if you let it shoot up first, it is hard to bring it down later.
An overgrown hedge can be reduced in height once by a good amount. The top stays bare for a while, until the surrounding side shoots grow upwards and close the gap. This doesn’t work on the sides: a bare side stays bare.
Rake up the clippings under and around the hedge thoroughly. Never put them in a field, and don’t give them to horses, cattle, sheep, rabbits or guinea pigs.

How much to cut, and how often?

Two clips a year keep a hedge neat and dense. A single clip in late August is also fine, but the hedge then looks a little shaggier in summer.
Only ever remove the current year’s growth. A hedge you let get a little wider every year is too wide after ten years, and there is no going back.
Brown scales on the inside in autumn are normal. Brown patches on the outside are usually caused by drought, cutting too deep, dog urine or the thuja leaf miner: cut out the brown twigs and water in dry spells.

What does it mean for wildlife?

A dense thuja hedge is one of the favourite nesting sites in the garden: blackbirds, song thrushes, dunnocks, greenfinches and woodpigeons like to breed in it. The nesting season runs from 15 March to 15 August, and disturbing an active nest is against the law: check thoroughly before the May–June clip and leave a wide area around any nest.
If you only clip once a year, do it in late August: the breeding season is then almost over.
In winter the evergreen hedge is a sheltered roost for sparrows, blackbirds and other small birds, and hedgehogs look for a spot at the bottom. Leave the fallen leaves under the hedge.
Thuja has little to offer insects: it is wind-pollinated and few caterpillars eat its foliage. Plant a strip of flowering and berrying shrubs in front of it, such as hawthorn, wild privet or holly, or choose a mixed native hedge when planting a new one.

Common mistakes

Cutting too deep, into the brown wood: bare patches never fill in again.
Letting the hedge grow for years and then trying to make it narrower in one go.
Clipping in heat or drought, so the cut tips scorch and the hedge turns brown.
Letting the top get wider than the base, so the hedge goes bare at the bottom.
Clipping in May without checking for nests first.

Caution: Don’t clip in bright sun or drought, and check first that no birds are nesting in the hedge. Thuja is poisonous: wear gloves and keep clippings away from children, pets and livestock.

Nearby: related pruning guides

Plant profile: Thuja · Pruning calendar and basic rules