← All guides · Diagnosis · Conifer & leylandii dieback · Updated September 2026

Leylandii or Conifer Hedge Going Brown in Patches? Diagnosing the Cause

A brown patch on a leylandii or conifer hedge is different from a brown patch on almost any other hedging species: with most conifers, the brown is permanent. They do not carry dormant buds in old bare wood the way deciduous species do, so once a section turns and stays brown, it will not push new green growth out of that dead material. Getting the diagnosis right before you act is not a nicety, it is the whole job.

Why brown is usually permanent on conifers

Deciduous hedging species such as beech, hornbeam and hawthorn carry dormant buds along their stems and branches. Cut them back into old wood and they will still push new growth from those buds. Most conifers do not work this way. Leylandii (x Cuprocyparis leylandii), Lawson cypress (Chamaecyparis lawsoniana) and western red cedar all carry their active growth buds only on live, green foliage. Old bare brown wood has no buds to activate, so a section that turns brown and dead will stay that way. The main exception is yew (Taxus baccata): despite often being grouped with conifers, yew does carry dormant buds in old bare wood and can regrow from a hard cut. If your dark-green, flat-needled hedge is actually a yew rather than a leylandii or cypress, see our separate guide on yew hedge browning and dying, which covers Phytophthora root rot, urine damage and over-cutting specific to Taxus.

This is why the cause matters so much before any action is taken. A cypress aphid problem caught while the affected section still has some green foliage can still be managed, and the remaining live growth will bulk out over subsequent seasons. A section that has been entirely brown for two growing seasons is not going to recover, and any treatment applied to it is wasted effort. Those are two completely different situations, and the diagnosis is what separates them.

Cypress aphid: the most common cause

The cypress aphid (Cinara cupressivora) is a large, grey-green aphid specific to cypress and conifer species. It feeds on sap in stems and shoots, and a significant infestation kills foliage patches by depriving them of nutrition. Research conducted jointly by the Royal Horticultural Society and East Malling Research found cypress aphid was associated with approximately half of all investigated brown-patch cases on conifer hedges in the UK, making it the most common identifiable cause by a clear margin.

The aphid typically causes browning in late spring and early summer, when populations are highest. It affects older inner foliage first, leaving the outer shoot tips green initially. Look for:

• Small grey-green insects on stems near the browning, 3-4mm long, clustering on the undersides of shoots and at branch junctions.
• Sticky honeydew secretion on leaves and hard surfaces beneath the hedge, sometimes with a thin film of sooty black mould developing on top.
• Browning that follows the interior structure of the hedge rather than a directional wind or sun pattern. If the damage tracks the inner frame of the hedge rather than the windward face, aphid is the more likely cause over scorch.

For a limited infestation where most of the section's outer green growth is still intact, an autumn or early spring spray with an aphicide labelled for garden use can reduce the population before it builds again. For a heavily infested hedge where large sections are already brown, the honest assessment is that treating the aphids protects the remaining green but does not regrow the dead patches. If the dead sections are visible and cosmetically unacceptable and the hedge is in a coastal exposed position, that may be the prompt for a wider conversation about species suitability.

Timing any treatment work matters for a different reason too. Under section 1 of the Wildlife and Countryside Act 1981, it is an offence to damage or destroy an active bird nest. Conifer hedges are a regular nesting site, particularly for goldcrest, wren and dunnock in Whitstable gardens. We do not carry out significant hedge work on a conifer between 1 March and 31 August without a qualified check for active nests first. Outside that window, late winter is the better time for managed reduction work on a conifer hedge, and autumn is preferable for aphid treatments.

Drought and wind scorch: the coastal factor

Leylandii and Lawson cypress are described as moderately salt-tolerant, which sounds more reassuring than it proves in practice on a seafront Whitstable plot. Gardens along the Tankerton seafront and behind Island Wall face north across the Thames Estuary with very little shelter. Persistent onshore wind from the north and north-east carries salt spray, and a dry, windy winter draws moisture out of conifer foliage faster than shallow roots can replace it. The result is wind scorch that browns the exposed face of the hedge and works inward.

Drought stress produces similar-looking damage but in a slightly different pattern: if the roots of a young or recently planted hedge cannot access enough moisture during a dry period, browning starts in the most exposed parts of the canopy and progresses inward. Free-draining sandy soils, which is generally beneficial for most hedging species but means young leylandii hedges in Chestfield, Yorkletts and Swalecliffe gardens need more reliable watering through the first two or three dry summers than they would on heavier clay ground.

The diagnostic distinction from cypress aphid: drought and wind scorch browning follows the geometry of the exposure (wind or sun direction), while aphid browning follows the interior structure of the hedge regardless of direction. Salt scorch specifically produces a clean face-specific pattern on sea-facing hedges. See our salt scorch guide for the full directional-browning differential.

For a seafront leylandii that has shown repeated browning on the windward face through successive winters, the pragmatic answer is usually that the species is in the wrong position. The alternatives we would specify for a Tankerton or Island Wall boundary are escallonia, Griselinia littoralis, tamarisk and sea buckthorn: genuinely salt-tolerant species that hold coastal exposed positions reliably without the annual dieback cycle.

Phytophthora root rot

Phytophthora root rot (principally P. cinnamomi and P. lateralis on conifers) kills hedging plants in irregular patches that do not respect exposure or aspect. The disease lives in waterlogged or persistently wet soil, and it kills the fine feeding roots before moving to the crown. An affected plant typically:

• Shows yellowing followed by browning across the whole plant rather than a single face or section of inner foliage.
• Fails to recover during wet weather. Unlike drought stress, which can partially stabilise after good rain, Phytophthora-affected plants often look worse in a wet spell.
• Has dark, discoloured or partially rotten roots at the base when you scrape the soil away just below the stem collar.

Kent's free-draining sandy soils along the coastal strip mean Phytophthora is less common here than on heavy clay in wetter parts of the country. It is not absent, though: low-lying spots near Whitstable harbour where the water table sits high, and hollows in Chestfield or Seasalter gardens where rainwater collects, can provide the waterlogged conditions the pathogen needs. There is no effective chemical treatment for Phytophthora in an established hedge. Affected plants need to be removed, the drainage improved, and replaced with a species more tolerant of wet soil if the underlying problem cannot be fully resolved.

Over-cutting into bare wood

This is entirely preventable, which makes it the most straightforward to diagnose after the fact. A conifer hedge trimmed back beyond the outer green growth into old bare inner wood has no regrowth mechanism for that section. The active buds sit in the green shoot tips and foliage. Cut through to old brown wood and it stays brown.

The mistake typically happens when someone attempts to reduce a leylandii hedge that has grown beyond a manageable height and cuts it back to where they expect to find live wood, not realising the green zone is shallow relative to the overall depth of the hedge. It also happens on the top of a hedge where the leader was removed years ago without a side shoot being trained to take over the upward growth.

For a conifer hedge where the whole width needs reducing and there is minimal green growth at the target height, the options are limited and the expectations need to be honest. Sides can often be reduced while a brown top remains; a heavy one-sided reduction can remove dead sections at the cost of temporarily losing privacy on that face. Our conifer and leylandii hedge service page covers the realistic outcomes for this work.

A related point that comes up in this context: leylandii is the species most commonly cited in formal High Hedges complaints under Part 8 of the Anti-social Behaviour Act 2003. Canterbury City Council administers High Hedges matters for properties in Whitstable. The qualifying threshold is an evergreen or semi-evergreen hedge over 2 metres that is adversely affecting the reasonable enjoyment of a neighbouring property. If a leylandii hedge has been cut back hard into old wood and is now patchy or dying, that changes the practical dynamics of any ongoing neighbour dispute. See our neighbour high hedge guide for the full picture on the formal complaint route.

Fungal disease: Pestalotiopsis and Seiridium

Two fungal diseases cause canker-like dieback on conifer hedges in UK gardens: Pestalotiopsis and Seiridium cardinale. Both tend to appear as secondary problems following initial damage from aphids, drought stress or winter scorch that has weakened the plant and opened entry points. Pestalotiopsis in particular is common after wet winters, when spores spread from dead shoots to adjacent live growth. Seiridium cardinale is more serious on leylandii and produces distinct sunken stem cankers at branch junctions.

Look for small black pustule-like bodies on the dead shoot material: these are spore-producing structures and their presence distinguishes fungal disease from the other causes. Fungal damage typically produces more irregular browning patterns than aphid, and dead shoots often remain attached to the plant rather than dropping cleanly as they do with pure drought stress. The management for both is the same: prune out all dead and dying material and dispose of it (do not compost), improve air circulation through the hedge if it is very dense, and avoid cutting during wet conditions that spread spores. There are no registered fungicide treatments for either disease in domestic garden use.

When large sections are already dead

If a significant part of a leylandii hedge has been dead for more than one growing season, the practical options are limited. The dead sections are permanent, and if they face the boundary or a viewed area, there is no internal regrowth that will cover them from behind. The realistic choices come down to: accept the appearance and manage the remaining green growth; attempt a one-sided reduction to remove dead sections at the cost of temporarily losing that face of the hedge; or replace that section with a species better suited to the position.

On exposed Whitstable seafront plots, replacing like-for-like with leylandii is worth questioning. The species grows fast, but that vigour does not help in a salt-laden position where the hedge faces repeated wind-scorch cycles. Griselinia, escallonia and hawthorn establish more slowly but are considerably more reliable long-term on a coastal plot. We are direct about this when assessing a hedge that has browned in the same position through multiple winters.

Sources: Royal Horticultural Society guidance on conifers with brown patches; RHS/East Malling Research collaborative findings on cypress aphid (Cinara cupressivora) prevalence in UK conifer hedge brown-patch cases; Wildlife and Countryside Act 1981 s.1 (nesting season, active nest protection offence); Anti-social Behaviour Act 2003 Part 8 (High Hedges); Canterbury City Council High Hedges guidance.