Guides · Dover Hedges & Tree Services

Why has my conifer hedge gone brown? Diagnosing brown patches in Dover gardens

Brown patches on a leylandii, Lawson cypress or thuja hedge are one of the most common concerns Dover homeowners bring to us, and the answer is not always the same. There are five distinct causes, each with a different appearance, a different recovery prospect, and a different response. Getting the diagnosis right matters because the wrong action, cutting back harder into old wood to remove the brown, typically makes permanent damage worse. This guide covers how to tell the causes apart, and what is and is not worth attempting.

The five main causes of brown patches on conifer hedges

The RHS identifies the principal causes of brown patches on hedging conifers as: trimming into old wood; cypress aphid; adverse growing conditions including drought, frost and waterlogging; root disease; and occasionally the fungus Pestalotiopsis. Of these, trimming and cypress aphid account for the majority of cases in the domestic gardens of Dover and east Kent. Root disease is less common but more serious. Salt wind is a local factor along Dover's coastal strip that is absent from inland sites in Kent.

The most important single question is whether the affected wood is dead or alive. Scratch the bark of a brown stem with a fingernail. If the tissue beneath is pale green or white-green, the stem is alive and recovery is possible. If the tissue is grey, dry and fibrous with no green colouration, the stem is dead and will not regrow foliage.

Trimming into old wood: the permanent patch

With the exception of yew (Taxus baccata), conifers have little or no capacity to regrow foliage from old wood. Once a hedge trimmer has cut back past the living green foliage into the brown interior, the exposed wood will not resprout. This is the leading cause of permanent brown patches on domestic leylandii, Lawson cypress and thuja hedges in Dover.

The brown-to-green boundary is usually visible on the cut face of the hedge immediately after trimming. As a rule, each cut should take off no more than the current season's new green growth, stopping before the blade hits wood that was already brown before the cut. On a well-managed hedge cut twice a year, the safe cutting depth is typically 10 to 15 cm. On a hedge that has not been cut for two or three years, the safe cut is shallower relative to the total hedge depth because the green zone has extended outward on a longer stem.

There is no remedy for patches caused by cutting into old wood. The practical options are: allow the remaining green growth to extend laterally over the exposed area over time (effective for small patches but slow); plant a faster-growing screening shrub in front; or remove the affected section entirely and replant. See our guide to staged leylandii reduction if the broader issue is that the hedge has grown too large for its position and regular trimming is approaching old wood each time.

Cypress aphid: summer browning from the base

Cypress aphid (Cinara cupressivora) is a large, grey-green sap-sucking aphid that feeds on the inner stems of leylandii, Lawson cypress and thuja from late spring through to early autumn. It is associated with at least half of the brown patch cases investigated by RHS and East Malling Research. The feeding damage causes a characteristic browning of the inner and lower foliage, typically appearing between June and August, which is then often blamed on drought or a trimming mistake. The clearest indicator of cypress aphid is the sticky honeydew it secretes: look for a tacky coating on stems and lower leaves in the interior of the hedge, often accompanied by a black sooty mould. The aphids themselves can sometimes be seen on the inner stems in large colonies in May and June.

Unlike browning caused by cutting into old wood, cypress aphid damage can recover partially if the aphid population is controlled before too much foliage is lost. The surviving green shoots extend to fill the gaps over subsequent seasons. However, stems that have been stripped of all green growth will not regrow. See our detailed guide to cypress aphid on leylandii in Dover for treatment, cutting timing and prevention.

Drought on Dover chalk: tip browning that recovers

Dover's downland chalk above the town, at Kearsney, Lydden, Alkham, Temple Ewell and Barham, is exceptionally free-draining. Rainfall that would linger in the soil of the Dour valley floor drains away within hours on a shallow rendzina. A dry April-to-July period, which is not uncommon in east Kent, can cause significant moisture stress in conifer hedges rooted in thin chalk. The browning appears differently from aphid or trimming damage: it typically affects the shoot tips on the outer face of the hedge and works from the top downward on south-facing and west-facing exposures, rather than starting in the interior at the base.

Drought browning on stems that are otherwise alive and green is the most recoverable cause of the five. Deep watering at the base of the hedge (a slow trickle for 30 to 60 minutes per section, not a quick spray of the foliage) followed by a 5 to 8 cm bark mulch along the root zone makes a material difference on chalk rendzinas. The mulch should be kept clear of the actual stems to avoid collar rot. New growth from surviving green stems will cover small drought-browned patches within one to two seasons. For a newly planted conifer hedge on chalk at Aycliffe, Buckland Hill or the Western Heights, first-summer irrigation is not optional; a hedge allowed to dry out severely in its first July on thin chalk can suffer permanent damage that takes years to correct.

Root disease: when whole sections or trees die

Root disease has a different visual signature from surface causes. Rather than patches of browning scattered across the hedge face, root disease typically kills complete sections or individual plants in the row. The browning begins at the base and moves upward, and the affected stems show decay or dark staining in the bark at or just below soil level. Two root diseases affect Dover's conifer hedges: Phytophthora root rot and honey fungus.

Phytophthora is more likely on the Dour valley floor and in gardens with impeded drainage, whether from compacted subsoil, clay-cap imported topsoil in some post-war housing estates, or low-lying positions. Dover's chalk upland is not typically high risk for Phytophthora unless a clay-lined depression retains water around the roots. Honey fungus can affect any site and spreads underground via black rhizomorphs (bootlace-like cords) from infected woody material in the soil. In autumn, look for honey-coloured toadstools at the base of affected sections. See our guide to Phytophthora root rot in Dover hedges and our guide to honey fungus on hedges in Dover for management and replanting advice.

Root disease-affected sections almost never recover. Remove affected plants, treat the soil where possible and replant with resistant species; Lawson cypress and cherry laurel are susceptible to Phytophthora, but leylandii, hornbeam and beech are generally more tolerant.

Salt wind and frost: Dover's coastal exposure

Along Dover's clifftop, on Marine Parade, around the Western Heights and the Eastern Docks area, salt-laden wind from the Channel desiccates conifer foliage more aggressively than inland conditions. Salt scorch typically appears on the windward face of the hedge, moving from the top downward on exposed south-westerly and westerly aspects, usually after a sustained period of gale-force onshore wind in autumn or early spring. Lawson cypress (Chamaecyparis lawsoniana) is notably more susceptible to salt scorch than leylandii; thuja plicata is the most tolerant of the three common choices for coastal boundary work in Dover.

Hard late frosts in March or early April, after a warm spell has prompted new growth to break, cause a similar tip-browning on the emerging foliage of leylandii and Lawson cypress. The damage looks cosmetically similar to salt scorch but appears on all aspects rather than primarily on the windward face. Both salt-scorch and frost-damaged hedges on living wood can recover as new growth extends through spring and summer; the affected tips eventually disappear into the interior as the hedge is cut in subsequent seasons.

Common questions about brown patches on conifer hedges in Dover

Will a conifer hedge grow back after brown patches appear?

It depends entirely on the cause. With the exception of yew, conifers cannot regrow foliage from bare brown wood. If the brown patch was caused by trimming into old wood, that patch is permanent. If the cause was cypress aphid, drought, salt wind or frost on wood that is still alive and carrying some green growth, the hedge can recover partially as surviving green shoots extend outward over time. Root disease usually kills the affected sections entirely and those gaps will not recover. A quick test: scratch the bark of a brown stem with a fingernail. Green or pale-green tissue underneath means the wood is alive; grey or dry brown tissue means it has died and will not regrow.

How can I tell if my conifer hedge is brown from cypress aphid or from drought on Dover chalk?

Cypress aphid damage typically appears in summer (June to August), starts at the base or interior of the hedge, and is accompanied by sticky honeydew on the stems and a sooty black mould on the foliage. Look for the aphids themselves: large (up to 4 mm), grey-green and tending to cluster on the inner stems. Drought damage on Dover's shallow chalk rendzinas looks different: the browning is usually at the shoot tips on the outermost foliage across the whole hedge face, often affecting the upper sections first on south-facing or west-facing runs where solar gain is highest. Location of browning (base and interior versus tips and outer face) is the clearest distinguishing feature.

Is there anything I can do to fix a leylandii hedge that has been cut into old wood?

Realistically, no. Leylandii cannot regrow foliage from bare brown wood. If a section has been over-cut and exposed the brown interior, that section stays brown. The practical options are to screen it with a faster-growing species planted in front, to keep the remaining green growth clipped so it extends laterally over the bare area (which can cover smaller patches over several years), or to remove the section and replant. This is why stopping before hitting old wood is the fundamental rule for cutting leylandii: once you cross the green-to-brown boundary, you cannot undo it.

Can chalk soil make conifer hedges go brown faster in Dover?

Yes, indirectly. Dover's chalk rendzinas above the town, on the downland at Alkham, Lydden, Kearsney and Temple Ewell, are exceptionally free-draining and low in nutrients. During a dry April-to-July period, leylandii and Lawson cypress hedges on thin chalk can suffer significant drought stress, which shows up as tip-browning or premature inner foliage drop. The same hedge on the deeper loam of the Dour valley floor at River or Crabble would fare better. A bark mulch along the base of the hedge, 5 to 8 cm deep and kept clear of the stems, dramatically reduces moisture loss from the root zone on chalk and is the single most useful preventative measure on exposed rendzina sites.

Does salt wind cause brown patches on leylandii and Lawson cypress in Dover?

Yes, particularly on hedges facing the Channel on Dover's clifftop, Marine Parade, the Western Heights and around the Eastern Docks. Salt-laden wind desiccates conifer foliage faster than the plant can replace moisture, especially on westerly and south-westerly exposures during spring gales. The damage usually appears as browning on the windward face of the hedge, often from the top downward. Lawson cypress is more susceptible to salt scorch than leylandii. Thuja plicata is considered the most wind-tolerant of the three for coastal positions in Dover. For exposed clifftop boundaries, a temporary windbreak fence on the seaward side for the first two winters after planting dramatically reduces salt scorch on young growth.

How do I know if root disease is killing my conifer hedge rather than a surface problem?

Root disease (Phytophthora root rot or honey fungus) causes a different pattern from surface causes: typically one or more complete sections or individual trees die rather than patches of foliage across the face of an otherwise-living hedge. The dying begins at the base of the affected section and moves upward; affected stems show discolouration or decay at or below soil level. With Phytophthora, look for dark staining in the bark just below the soil line. With honey fungus, look for the distinctive bootlace-like rhizomorphs (black cords) in the soil around the roots, and the honey-coloured toadstools that can appear at the base in autumn. If a whole tree within the hedge dies suddenly rather than browning from the outside in, root disease is the more likely cause than drought or aphid.