Arborvitae (Thuja species) are highly valued evergreens, prized for their dense foliage and ability to form screens and hedges. Despite their popularity, these plants are susceptible to decline, often displaying browning or thinning foliage when stressed. The apparent dying of an arborvitae is frequently a sign of an underlying issue that can be corrected with timely intervention. Restoring the plant’s health requires a systematic approach, starting with an accurate diagnosis, followed by targeted remediation of its environment and any biological threats.
Identifying the Cause of Decline
Determining the precise reason for the arborvitae’s distress is the first step before applying any treatment. Observing the location of the browning foliage is a key diagnostic indicator. Browning on the inner branches, close to the trunk, is often a normal process called needle senescence, where older foliage is shed to allow light and air circulation. However, browning on the tips or outer foliage signals an environmental stressor or a biological issue.
A simple “scratch test” on a seemingly dead branch can reveal whether the plant still has life. Gently scraping the outer bark should expose green tissue underneath if the branch is alive; brown or brittle tissue indicates the branch is dead. Another check involves examining the root flare, which is where the trunk widens at the base. If the root flare is buried under soil or mulch, it suggests improper planting depth, which can compromise the tree’s health and lead to root rot. If the arborvitae has been declining for multiple years, or if the trunk is rotting, the damage may be irreversible.
Adjusting Hydration and Root Conditions
Improper soil moisture is the most frequent cause of arborvitae decline, as these evergreens possess shallow root systems sensitive to extremes. Both overwatering, which leads to root rot, and drought stress can cause similar symptoms, such as browning or yellowing foliage. Check the soil moisture by probing a few inches down; if the soil is dry at a depth of two inches, the plant needs water.
When watering, deep soaking is more beneficial than frequent, shallow sprinkling, as it encourages deeper, more resilient root growth. For established plants, a deep watering once a week, or more during dry spells, is appropriate. Mulch application helps with moisture retention and temperature regulation, but it should be spread in a thin layer of two to three inches, kept several inches away from the trunk to prevent suffocation and disease.
Correcting the planting depth is also necessary. The root flare must be visible at or slightly above the soil line to prevent the trunk from being constantly moist, which can invite decay-producing fungi. If the plant was installed too deep, carefully removing the excess soil or mulch from around the base can expose the root flare and significantly improve the tree’s health. Ensuring the soil has good drainage is important because constantly soggy conditions inevitably lead to root rot.
Remediation of Pests and Pathogens
Beyond cultural issues, specific biological threats can cause rapid decline. One common pest is the bagworm, whose larvae create tough, silk bags covered in foliage particles that hang from the branches. These pests consume the foliage, leading to defoliation. They overwinter as eggs inside the bags, which hatch from late May to early June. Hand-picking and destroying the bags before the larvae hatch is an effective control method for small infestations.
Another threat is the spider mite, a tiny arachnid that feeds on the sap, causing the foliage to take on a dull green or grayish-brown appearance. Mites are difficult to see, but their presence can be confirmed by holding a white piece of paper under a branch and shaking it, looking for tiny specks. Horticultural oil or specialized miticides can be applied to control populations, especially when mite activity is highest in warm, dry weather. Fungal diseases like tip blight, caused by pathogens like Pestalotiopsis funerea, cause the tips of branches to turn brown or ash-gray. Infected branches must be pruned out and destroyed to prevent the spread of the disease, and fungicides may be necessary for severe cases.
Protecting Against Environmental Damage
Environmental stressors not related to pests or poor watering practices also contribute to arborvitae decline. Winter burn, or desiccation, occurs when the evergreen foliage loses moisture through transpiration faster than the frozen ground can replenish it. This damage appears on the side of the plant most exposed to the winter sun and wind, such as the south or southwest. Prevention involves ensuring the plant is well-watered in the late fall before the ground freezes, and applying anti-desiccant sprays to reduce moisture loss.
Physical barriers, such as temporary burlap screens, can be erected on the windward side to protect the foliage from drying winds and intense sunlight. Another concern is salt damage, which affects arborvitae planted near roadways or driveways where de-icing salts are used. Salt spray causes foliage browning, and runoff contaminates the soil, leading to root stress. Using plant-friendly de-icing alternatives or flushing the soil with water in the spring can help mitigate salt accumulation. Once a section of an arborvitae is completely dead, it should be pruned out, as the plant does not regenerate new foliage from old, dead wood.