The arborvitae, a popular evergreen from the genus Thuja, serves as an excellent landscape plant, prized for its dense foliage and utility as a privacy screen. While it is generally robust, the sudden appearance of browning or thinning foliage can be alarming for any homeowner. This discoloration is a sign of serious stress, but the damage is often reversible if the underlying cause is correctly and quickly identified. Saving a stressed arborvitae involves a systematic approach, beginning with accurate diagnosis and moving through targeted interventions for water, pests, and environmental factors.
Identifying Visual Signs of Distress
The location of browning foliage provides the most immediate clue to the plant’s underlying problem. If browning is confined to the interior of the plant, near the trunk, it is usually a natural process called “shedding” or “flagging.” This occurs because dense outer growth prevents sunlight from reaching the older, inner needles, causing them to die and drop.
Browning on the outer tips or the entire canopy indicates a problem with water uptake or environmental damage. Browning on the tips, especially on the sunny or wind-exposed side, is a classic sign of underwatering or winter burn. Conversely, browning that begins from the bottom and progresses upward, or occurs when the soil is consistently soggy, may signal overwatering or root rot.
Browning that appears randomly on a few branches or is accompanied by fine webbing or small sacs suggests a pest or fungal infestation. Look closely for specific indicators on the affected foliage before proceeding to treatment. The precise pattern of damage determines the appropriate recovery plan.
Correcting Water Imbalances
Water issues are the most frequent cause of distress in arborvitae due to their shallow root systems. To determine if the plant is thirsty, use the “finger test” by sticking your finger two inches into the soil near the base; if the soil feels dry, the plant needs water. For established plants, deep watering is preferred, aiming to supply about one inch of water per week through rainfall or irrigation.
Use a soaker hose or a slow trickle to ensure water penetrates deeply into the root zone without runoff, encouraging deep root development. This is especially important during dry summer periods and in the late fall before the ground freezes to prevent winter desiccation. Arborvitae lose moisture through their foliage year-round, making consistent hydration necessary.
Overwatering is equally damaging and often leads to root rot, where suffocated roots begin to decay. Signs include yellowing foliage despite constantly moist soil, or a sour, mushy smell emanating from the base. Improving drainage is the primary remedy, which may involve aerating the soil or transplanting the arborvitae to a location with better-draining soil.
Addressing Pests and Fungal Issues
Biological threats, including insects and fungi, can rapidly compromise the health of a stressed arborvitae. Spider mites are common pests that feed on the foliage, leading to a dull, faded green color that progresses to browning. Confirm their presence by holding a white sheet of paper beneath a branch and sharply tapping the foliage; tiny, dark specks confirm a mite infestation.
Treatment for spider mites involves applying horticultural oil or insecticidal soap, ensuring thorough coverage, particularly on the undersides of the foliage. Another destructive pest is the bagworm, which constructs spindle-shaped casings out of silk and plant material. Manual removal of these sacs is the most effective immediate control, especially before eggs hatch.
Fungal diseases, such as Tip Blight, cause the tips of branches to turn tan or brown, sometimes with small black specks visible on the dead tissue. Cultural controls are the first line of defense, involving the removal of infected branches using sterilized pruning tools to prevent spore spread. Improving air circulation through pruning and avoiding overhead watering helps mitigate the moist conditions fungi thrive in.
Mitigating Environmental Damage and Soil Stress
Environmental factors, known as abiotic stressors, frequently cause browning often mistaken for disease. Winter burn occurs when foliage loses moisture to dry winter winds and sun, but frozen ground prevents roots from replacing that water. This damage is typically visible on the south or west-facing side and can be prevented by applying anti-desiccant spray in the late fall or wrapping susceptible plants in burlap.
Salt damage is a specific environmental threat, often resulting from de-icing salts used on nearby sidewalks and roads. The salt spray or runoff draws moisture out of the foliage and roots, causing browning and desiccation. In early spring, heavily watering the affected soil helps flush the salt downward and away from the root zone.
The soil’s chemistry and nutrition play a role in recovery; arborvitae prefer a slightly acidic to neutral soil. While testing the soil pH is helpful, avoid the temptation to over-fertilize a stressed plant, as this can cause fertilizer burn and worsen the decline. Recovery from significant environmental stress requires patience, and it may take a full growing season for new growth to mask the damaged foliage.