Can I reduce the height of a leylandii hedge?

The one-way rule that governs leylandii — you can take the top off, but you cannot narrow it into brown wood — and how to plan a reduction that does not leave a permanent scar.

Difficulty: intermediate
Time: 4 hr
Read: 5 min
Safety: warning

Short answer

Yes on height, with care. The top can be cut down into bare wood and, although the cut ends will not reshoot, the side growth grows up and covers the flat top within a couple of seasons. The sides are different: cut into brown wood on the flanks and it stays brown for good.

Leylandii and the other common conifer hedges — Lawson cypress, Thuja, Chamaecyparis — carry green growth only on the outer skin. Behind it the wood is bare and buddless, and it has no capacity to produce new shoots. That single fact governs everything about managing them, and it is why a hedge that has been allowed to widen for fifteen years can never be brought back to a narrow one.

Safety

Check for active birds' nests before cutting; leylandii hedges are heavily used for nesting and damaging a nest is an offence. Reducing a tall hedge means working at height with a powered tool, which is the highest-risk garden job there is — use a platform tower or a long-reach trimmer from the ground, never a hedge trimmer from a ladder, and never a chainsaw at height without training and certification. Check for overhead power lines before raising any pole tool.

What you'll need

Long-reach hedge trimmer or pole saw
Platform tower (optional)
Eye and ear protection, gloves, hard hat
String line and canes

Step by step

Check for nests, and decide whether it is your hedge to cut.A thorough check of the hedge first. Then be clear on ownership: if the hedge is the neighbour's, you may only cut growth on your side back to the boundary, and you cannot reduce its height at all without agreement.
Decide the final height before you start.You get one chance. Set canes and a string line at the target height along the whole run, standing back to check it works with the ground level, which is rarely flat.
Cut the top 150mm or so lower than the final height.The side growth at the top will grow up and over the cut, closing the flat surface. Cutting exactly to the final height means the finished hedge ends up taller than you wanted.
Accept that the cut top will show at first.The sawn stems on top will not produce new shoots. For a season or two the top is visibly a cut surface, and then the side growth arches over and hides it. This is normal and it does resolve.
Never cut the sides into brown wood.The width is the part you cannot fix. Trim the flanks only into the green outer growth. If the hedge has grown too wide, the honest choice is between living with the width and taking the hedge out.
Cut a slight batter into the green growth as you go.Even within the green skin, taking a little more off the top than the bottom lets light down the face and keeps the lower growth alive for longer.
Do it in late spring or late summer.Late May to June and again in late summer are the good windows. Avoid hard frost, avoid drought, and stop by late September so any regrowth hardens off.
Work from a platform, never a ladder.A hedge trimmer needs two hands and a stable stance. Using one on a ladder is how people end up in hospital, and it is the single most common serious injury in garden work. Hire a tower, or hire a contractor.
Consider the honest alternative.A leylandii hedge that has become too tall, too wide and bare at the base is often better removed and replaced with something manageable — yew, hornbeam, beech or a mixed native hedge. Removal is a big job with substantial root systems, but it solves the problem permanently rather than deferring it two years at a time.
Set up a twice-yearly routine afterwards.Leylandii kept to size with two light cuts a year is a perfectly good hedge. Almost every leylandii problem in Britain is a maintenance gap rather than a plant fault.

Tips

Cut the top before the sides. It changes the shape enough that judging the flanks afterwards is easier, and everything you drop lands on uncut growth rather than a finished face.
Leylandii clippings are bulky and slow to compost. Chip them, or plan several trips to the green waste facility.
If your hedge is over 2m and the neighbours have complained about it, read up on the high hedges complaint route before you get into a dispute about it.

Common mistakes

Cutting the sides back into brown wood to narrow the hedge — There are no dormant buds in that wood. The brown face is permanent, and the only remedy is removal.
Using a hedge trimmer from a ladder — It is a two-handed tool used at arm's length, which is incompatible with holding a ladder. Serious injuries from this are common and entirely avoidable with a tower or a long-reach tool.
Reducing height in one hit from very tall — Taking several metres off leaves a large cut surface, a stressed plant and, on old hedges, sometimes wind instability. Take large reductions over two seasons.
Letting it go for another five years instead — Every year of neglect widens the hedge, and the width is the part that cannot be undone.

If it doesn't work

Brown scar along the side after cutting

Cause: Cut into wood with no green growth — Fix: It will not recover. Either accept it, screen it with planting in front, or remove that section.

Top looks flat and dead after reduction

Cause: Normal — cut stems do not reshoot — Fix: Wait. The side growth closes over the top within one to two growing seasons.

Hedge bare at the base

Cause: Light excluded by an over-wide, over-tall hedge for years — Fix: Reduce the height so light reaches the base, but on conifers the lower growth will not regenerate. Plant in front of it, or replace.

Neighbour complaining about the height

Cause: A hedge over 2m blocking light — Fix: Talk first, reduce voluntarily if you can, and understand the complaint route before it becomes formal.

Questions people ask

Will the top grow back after I cut it?

The cut stems will not reshoot, but the green growth on the sides at the top grows upwards and covers the cut surface within one or two seasons. The height is genuinely reducible; the width is not.

How wide can I cut the sides?

Only within the green outer layer, which on a neglected hedge may be as little as 150 to 300mm deep. Anything beyond it leaves a permanent bare face.

Should I just take it out?

If it is bare at the base, too wide to narrow, and causing a dispute, then yes, that is often the sensible answer. Yew, beech, hornbeam and mixed native hedges all give privacy without the same problems.

How tall is a leylandii allowed to be?

There is no general height limit on planting a hedge, but a complaint can be made to the council about an evergreen or semi-evergreen hedge over 2m that unreasonably affects a neighbour's enjoyment of their property. Councils charge a fee and only act after the parties have genuinely tried to settle it themselves.