Why has my conifer hedge gone brown in patches?

Six causes of brown patches in a leylandii or cypress hedge, how to tell them apart, and the uncomfortable fact that most of them cannot be reversed.

Difficulty: intermediate
Time: 40 min
Read: 4 min

Short answer

Work out whether the brown is on the surface or deep inside, and whether it spreads. Interior browning is normal shading. A sharply cut brown face is old pruning damage. Spreading brown patches with resin on the stems suggest cypress aphid or a canker; whole dead plants suggest root death from drought or waterlogging.

The critical background fact is that conifers other than yew have no dormant buds in brown wood. Whatever the cause, once a patch has gone brown it will not green up again, so diagnosis is about stopping the spread and deciding what to do with the scar rather than about curing it.

What you'll need

A pair of secateurs
A hand lens (optional)

Step by step

Look at where the brown is.Part the foliage. Brown, dead needles deep inside the hedge with a healthy green surface is completely normal — the interior is shaded and sheds. That needs no action at all.
Check for a flat, sharp-edged brown face.A brown surface with a straight edge that matches a cutting line means somebody cut into old wood. It is permanent. Nothing will bring it back and no treatment is relevant.
Look for cypress aphid damage.This is the commonest cause of unexplained spreading brown patches in leylandii. Damage typically starts low and inside, spreads outwards, and the browning often appears from late spring. Look for sticky honeydew, black sooty mould on the foliage below, and grey-green aphids up to about 4mm on the stems in spring.
Check the stems for resin and sunken bark.Bleeding resin, sunken dead patches on the bark and dieback beyond them point to a canker. Prune out affected branches well below the damage where the hedge allows, and disinfect tools between cuts.
Consider root problems if whole plants are dying.A single plant going brown from the top down or all over, while its neighbours are fine, usually means the roots. Waterlogging, drought, honey fungus and severed roots from trenching are the candidates. Dig gently and look at the root zone and the soil.
Rule out drought stress on a new or reduced hedge.Conifers hate dry roots and show it months later. A hedge planted within the last three years, or one under a wall or eaves where rain never reaches, is the classic case.
Check for scorch and salt.Cold drying winds, de-icing salt splash near a drive or road, and dog urine at the base all brown conifers in characteristic patterns — wind scorch on the exposed side, salt and urine at the bottom of the plants nearest the source.
Decide what to do with the scar.Since the brown will not regrow, the options are to live with it, to plant something in front of it, to remove and replace the affected plants, or to take the hedge out and start again with a species that regenerates.

Tips

Yew is the exception that proves the rule: it will reshoot from old wood, so a brown yew hedge often can be recovered. Nothing else in a common UK conifer hedge will.
Cypress aphid damage often appears after the aphids have gone, which is why people look for pests and find none. Look in spring, before the browning shows.
A hedge that has browned at the base after years of no light is not diseased. It is shaded, and the fix is height and width reduction, done early enough.

Common mistakes

Cutting brown areas out to encourage regrowth — There are no buds in that wood. Cutting only makes the hole larger and more visible.
Feeding a hedge that is dying from root damage — Fertiliser on damaged or waterlogged roots increases stress. Fix the drainage or the water supply first.
Assuming any brown means disease — Interior shading is normal, and old cutting scars are permanent damage rather than an active problem. Treating either achieves nothing.

If it doesn't work

Brown inside, green outside

Cause: Normal shading of interior foliage — Fix: Nothing needed.

Straight-edged brown face

Cause: Cut into old wood in the past — Fix: Permanent. Screen it, replace the section, or accept it.

Browning spreading from low and inside, with sticky foliage

Cause: Cypress aphid — Fix: Encourage predators, tolerate light infestations, and monitor in spring when the aphids are actually present. Damage already done will not green up.

One plant dead, neighbours healthy

Cause: Root death — waterlogging, drought, honey fungus or severed roots — Fix: Investigate the roots and the soil. Remove and replace the plant once the cause is identified and corrected.

Brown only on the windward side or beside a road

Cause: Wind scorch or salt spray — Fix: Provide a windbreak or accept it. Salt damage near a road is hard to prevent and argues for a more tolerant species.

Questions people ask

Will the brown grow back green?

On leylandii, Lawson cypress and Thuja, no. Those species have no dormant buds in brown wood. Yew is the exception and generally will regenerate.

Is it a disease I can treat?

Usually not. Cypress aphid and cankers are the two live problems, and neither has a good domestic chemical answer. Managing vigour, water and light does more than any product.

Should I replace the whole hedge?

If more than a third of it is dead or permanently brown, replacing is usually less work and better looking than patching. A species that regenerates from old wood — yew, beech, hornbeam — avoids repeating the problem.