Thuja Leaves Curling and Browning
Drought stress and root rot are the most common reasons thuja foliage turns brown and curls. This guide shows you how to identify each cause and keep arborvitae hedging dense, healthy, and green through every season.
1. Drought stress
Drought stress is the most common cause of foliage browning and die-back in thuja hedges in the UK, particularly in newly planted hedging during its first two growing seasons. Thuja occidentalis and Thuja plicata grow naturally in moist forest soils and riparian zones in North America and do not have deep root systems initially; container-grown specimens planted into garden soil have a very compact root mass that can dry out rapidly during hot, dry periods before lateral roots have developed sufficiently.
What to look for
Scale foliage at the shoot tips turns yellow, then brown, beginning on the outermost and most exposed growth. The browning has a bleached, papery tan quality and in early stages the foliage may feel dry and slightly crispy when rubbed between the fingers. The die-back progresses from the tips inward. In a recently planted hedge, entire plants may brown if drought is severe enough to kill the compact root ball. The soil beneath the plants feels very dry to depth. In an established hedge, drought typically causes patchy browning on the side facing the prevailing wind or afternoon sun.
What to do
Water newly planted thuja deeply twice a week throughout the first growing season during any dry spell, and once a week in the second year. Apply water slowly and let it penetrate to below the depth of the root ball. Install a soaker hose along the base of a new hedge line to deliver water efficiently and consistently. Apply a generous mulch 75 to 100 mm deep along the entire length of the hedge, keeping it clear of the stems, to conserve moisture and moderate soil temperature. Established thuja hedges rarely need watering once fully rooted but benefit from watering during extended dry periods of three weeks or more without rain, particularly during the first five years after planting.
2. Phytophthora root rot
Phytophthora root rot, caused by the water mould Phytophthora cinnamomi and related species, is one of the most serious diseases of thuja and can cause rapid and irreversible decline of individual plants or large sections of a hedge. The pathogen thrives in waterlogged or poorly draining soil and is impossible to eradicate once established, making prevention through appropriate site preparation essential.
What to look for
Foliage on one or more plants in a hedge turns grey-green, then yellow-brown, and then a uniform dark brown in a pattern that does not respond to watering. The browning typically affects the whole plant from the base up rather than starting at the tips as drought browning does. Scraping the bark at the base of an affected stem reveals reddish-brown rather than healthy cream-coloured tissue beneath. The roots, when exposed, are black and rotten rather than pale and firm. The problem is most common on sites with heavy clay soil, poor drainage, or where water collects.
What to do
There is no effective chemical treatment for Phytophthora in garden soil. Affected plants should be removed, including as much of the root system as possible, and the soil in that area should be replaced with fresh, well-drained soil or grit before replanting. Do not replant susceptible species in the same area. Prevent Phytophthora by ensuring excellent drainage before planting: incorporate coarse grit into heavy clay soil, avoid low-lying waterlogged positions, and do not overwater. A phosphonate-based fungicide applied preventively to adjacent healthy plants may slow the spread of the disease but does not cure affected plants.
3. Thuja aphid
The thuja aphid (Cinara juniperi and related species) is a significant pest of thuja hedging in the UK, with populations that can build very rapidly during spring and early summer. These aphids feed on the scale foliage and stems, causing browning, distortion, and in heavy infestations, significant plant stress. Unlike broad-leaved plant aphids, they are often overlooked because they are dark-coloured and feed on similarly textured foliage.
What to look for
Scale foliage at the shoot tips turns grey-brown and may feel slightly sticky. Examine the foliage closely: thuja aphids are dark brown or black, relatively large for aphids, and tend to cluster along the stems and on the undersides of scale foliage sprays. A sticky honeydew film on the foliage surface below aphid colonies leads to black sooty mould developing in dense infestations. Ants moving along the hedge stems and tending the colonies are a reliable indicator. Die-back that follows watering and appears otherwise inexplicable is often aphid-related rather than drought.
What to do
Natural predators including ladybirds and lacewings will control thuja aphid populations in most seasons without intervention. On newly planted or young hedges where infestation is severe, spray with horticultural soap solution or pyrethrin-based insecticide in spring before populations peak, ensuring thorough coverage of the entire foliage canopy. On established hedges, a preventive spray in April as aphids begin to hatch from overwintering eggs can significantly reduce summer populations. Avoid broad-spectrum systemic insecticides that persist in plant tissue and kill beneficial insects throughout the season.
4. Spider mites
Spider mites, particularly the spruce spider mite (Oligonychus ununguis), attack thuja and related conifers during hot, dry summers. They feed on the scale foliage, draining the cell contents and causing progressive bronzing, yellowing, and eventual browning of affected areas. Spider mite damage is most common during July and August and is consistently worse on plants already stressed by drought.
What to look for
Scale foliage develops a dusty, stippled, or slightly bronze appearance as mites damage the cells. Unlike thuja aphids, spider mites are extremely small and require a hand lens to see. Hold a piece of white paper beneath an affected shoot and tap it sharply; mites fall onto the paper and can be seen as slow-moving specks. In heavy infestations, very fine webbing may be visible on the foliage surface. The damage typically begins on the lower, older foliage and works upward and outward as the season progresses.
What to do
Keep plants well-watered and mulched; water-stressed thuja is significantly more susceptible. Misting the foliage with plain water in the morning reduces mite populations by increasing humidity. For established infestations, spray with a plant-oil-based miticide or fatty acid insecticide, ensuring good coverage of all foliage surfaces. Repeat after one week. Encourage natural predators by avoiding broad-spectrum pesticides. In particularly susceptible gardens, a preventive plant-oil spray in late June before populations peak can reduce the severity of summer infestations.
5. Wind and salt scorch
Thuja foliage is susceptible to desiccation from strong, cold, or salt-laden winds, particularly in exposed coastal positions or in gardens that funnel wind along their boundaries. Salt spray from roads treated with winter grit is a significant cause of hedge browning in gardens adjacent to busy roads, producing distinctive browning on the road-facing side of the hedge in early spring.
What to look for
Foliage browning is concentrated on one side of the hedge: the windward side, the road-facing side, or the side most exposed to prevailing weather. Road salt damage typically appears in February and March on the side of the hedge closest to the road, affecting all plants equally across the length of the hedge. Wind scorch produces a bleached, papery brown appearance similar to drought browning but is clearly directional and is associated with exposed conditions or cold east winds in late winter.
What to do
For road salt damage, rinse the affected foliage thoroughly with fresh water in late winter to dilute and remove salt deposits before they cause further damage. Erect a temporary screen of permeable windbreak netting on the exposed side of young hedges to reduce wind and salt exposure in their first two or three years while they establish. On established hedges, salt damage is an annual cosmetic nuisance; the browned foliage is pushed out of view as new growth develops through spring. In coastal gardens, Thuja plicata is significantly more salt-tolerant than Thuja occidentalis and is the better choice for exposed seaside positions.
Frequently asked questions
Why is the inside of my thuja hedge going brown?
Browning of the interior foliage of a thuja hedge is almost always natural and is not a disease. Thuja sheds its oldest inner foliage every year as part of its normal growth cycle; the brown, dead scale leaves in the interior of the plant simply lack adequate light and die as the outer canopy expands. A gentle shake or rake of the interior in autumn removes this dead material and improves air circulation. This process is completely different from outer foliage browning, which indicates a problem.
Is the bronze colour on thuja in winter normal?
Yes. Many thuja cultivars naturally turn bronze, copper, or brownish-green in winter in response to cold temperatures. This is a physiological response to cold and is not a sign of disease or winter damage. The cultivar Thuja occidentalis 'Rheingold' is particularly well known for its vivid bronze winter colour. The foliage returns to its normal green or gold tone in spring as temperatures rise. This bronzing is most pronounced after cold snaps and in exposed positions.
Why does my thuja hedge have brown patches that are not recovering?
Persistent brown patches that do not produce new growth are most commonly caused by Phytophthora root rot, which kills sections of the root system and causes progressive, irreversible dieback. Unlike drought browning which can recover with watering, Phytophthora-affected areas do not regenerate as the underlying roots are dead. Check for waterlogging at the affected plants and, if suspected, have the soil tested for the pathogen. There is no chemical cure; affected plants usually need to be removed.
How often should I water a newly planted thuja hedge?
Newly planted thuja hedging needs deep watering twice a week during any dry spell throughout its first growing season, and once a week in its second year during dry periods. Container-grown thuja has a very compact root ball that dries out quickly while it establishes, and underwatering in the first summer is the single most common reason for hedge failure. Apply a generous mulch along the hedge line to retain moisture, and water until it is clearly penetrating below the root ball depth.
Can thuja recover from browning?
Whether thuja recovers from browning depends entirely on the cause. Drought-browned foliage on otherwise healthy plants can recover if watered promptly, particularly on young growth; established brown tips typically do not regenerate but new growth covers them. Winter bronzing recovers fully in spring. Interior browning from natural leaf shed is harmless. However, foliage browned by Phytophthora root rot, severe drought that killed the roots, or persistent thuja aphid attack does not recover in the affected areas.
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