Thuja species, commonly known as arborvitae, are popular in landscaping for their dense, year-round green foliage, creating effective privacy screens and borders. Despite their hardy reputation, browning and dieback suggest the plant is under severe stress. This decline is often reversible, but success depends on quickly and correctly identifying the root cause. This guide provides actionable steps for diagnosing the decline and implementing focused strategies to revive a struggling arborvitae.
Determining the Cause of Decline
The initial step in revival is a thorough diagnosis of the browning foliage, as the appearance of the damage indicates the underlying issue. Water stress is the most common culprit, manifesting as either overwatering or underwatering, which produces distinct browning patterns. Arborvitae suffering from drought stress typically show browning that begins at the outer tips of the branches and progresses inward. Conversely, browning that starts deep within the plant, especially if accompanied by a strong odor, often signals root rot caused by excessive moisture and poor drainage.
Environmental damage frequently contributes to browning, particularly during the winter months. Winter burn results from desiccation, occurring when the ground is frozen and the plant cannot draw water, while harsh winds and sun pull moisture from the foliage. Salt damage is another location-specific issue, where de-icing salts used on nearby roads and sidewalks spray onto the foliage or build up in the soil, dehydrating the plant tissue. Pests can also be responsible, indicated by the presence of fine webbing from spider mites or the distinctive spindle-shaped casings of bagworms attached to the branches.
Nutrient issues or soil pH imbalances can also contribute to a general decline in health. Soil that is too alkaline, or the lack of certain micronutrients, can impair the arborvitae’s ability to absorb necessary resources. A natural and harmless form of browning, known as seasonal needle drop, occurs in the autumn when the dense inner foliage that receives little light is shed. This interior browning is normal and should not be mistaken for a serious health problem.
Pruning and Assessing Plant Viability
Once a potential cause has been identified, the next step involves assessing how much of the tree can be saved. A simple method to check for living tissue on a seemingly dead branch is the “scratch test.” This involves lightly scraping the bark with a fingernail or knife to expose the layer beneath. If the tissue immediately under the bark is green and moist, the branch is alive; if it is brown, dry, and brittle, the branch is dead.
It is important to remove all completely dead, brown wood, cutting back to the nearest healthy, green tissue. Pruning dead material improves air circulation within the plant canopy and removes potential entry points for disease. Arborvitae do not typically regenerate growth from old, leafless wood. Therefore, any section that has turned completely brown and is confirmed dead by the scratch test will not fill in again and is permanently lost.
Correcting Water and Soil Issues
The most impactful revival measures involve adjusting hydration and improving the soil environment, since water stress is the primary cause of decline. Arborvitae prefer soil that is consistently moist but drains well. A common mistake is providing frequent, shallow watering, which encourages a weak, surface-level root system. Instead, water deeply and infrequently, aiming to saturate the soil to a depth of at least 12 inches to encourage deep root growth. Established trees typically need about one inch of water per week, either from rain or supplemental watering.
If overwatering and root rot are suspected, improving drainage is necessary, which might involve amending the surrounding soil with organic matter like compost. A 2- to 3-inch layer of organic mulch helps retain soil moisture and stabilizes soil temperature. It is crucial to keep the mulch several inches away from the trunk to prevent moisture buildup and decay at the base.
Fertilization should be approached with caution, even for a struggling plant. Excessive or high-nitrogen fertilizer can further stress a sick tree or exacerbate salt damage issues. If a soil test indicates a clear nutrient deficiency, a balanced, slow-release fertilizer can be lightly applied in the spring. If the diagnosis points toward severe pest infestation or a fungal disease like root rot, consulting a certified arborist for targeted, systemic treatments is the most effective course of action.
Preventing Environmental Stress and Recurrence
Long-term health requires proactive measures to protect the arborvitae from recurring environmental stressors, particularly during the winter. To prevent winter burn, the plant must be thoroughly watered in late fall before the ground freezes, ensuring the roots have a reserve of moisture. For younger or highly exposed trees, a burlap windbreak can be installed on the windward side to reduce the drying effects of winter winds. Anti-desiccant sprays, which are waxy coatings applied to the foliage in late fall, can also reduce the rate of moisture loss by up to 20%.
Protecting trees from de-icing salts requires placing a physical barrier, such as a sturdy screen or burlap wrap, between the tree and the road, or switching to less harmful de-icers like calcium chloride. If salt damage has occurred in the soil, a deep, thorough watering in the early spring can help leach the salt below the root zone. Proper planting depth is also fundamental for long-term health; the root flare, where the trunk meets the roots, must be planted slightly above or level with the surrounding soil to prevent root suffocation and decline.