Arborvitae, belonging to the genus Thuja, are a popular evergreen choice for creating screens and hedges in the landscape. Their dense, scale-like foliage offers year-round color, making it alarming when sections begin to turn brown. This browning is often a sign of environmental stress, but it does not automatically mean the plant is lost. With prompt diagnosis and proper intervention, many brown arborvitae can be successfully nursed back to health, though recovery depends entirely on the extent and location of the damage.
Identifying the Cause of Browning
Determining the cause of discoloration is the first step toward saving the plant. One common issue is winter burn, or desiccation, which occurs when the ground is frozen and roots cannot absorb water while the foliage continues to lose moisture through wind and sun. This damage typically appears on the exposed, windward, or south-facing side of the shrub, affecting the branch tips.
Browning that starts deep within the shrub or at the bottom often signals a moisture problem, either drought or root rot. Drought stress causes the plant to shed older, interior foliage to conserve water for the outer growth. Conversely, root rot, often caused by the Phytophthora fungus in overly saturated soil, prevents the roots from functioning, leading to a similar lack of water uptake and overall discoloration.
Pest activity also leaves identifiable clues, such as the feeding damage from spider mites. These tiny arachnids suck cell content from the foliage, resulting in a fine, bronze-colored stippling or flecking, often accompanied by fine webbing. Another visible culprit is the bagworm, whose larvae create spindle-shaped bags from the arborvitae’s own needles, leading to defoliation on the infested branches.
Assessing the Damage and Salvage Potential
After identifying the likely cause, the next step is to determine if the affected branches retain any living tissue. Arborvitae, unlike many deciduous shrubs, do not possess dormant buds on their old wood and cannot regenerate foliage from bare stems.
The “scratch test” is a reliable method to check for life. Using a thumbnail or a small knife, gently scrape away a tiny section of the bark on an affected branch. If the tissue immediately beneath the outer bark, the cambium layer, is moist and bright green, that portion of the branch is still alive. If the tissue is brown, dry, and brittle, that section is dead and will not recover.
The possibility of salvage depends on how much of the main structure remains green. If the browning is confined to the tips or the interior needles, the plant is likely viable. However, if the main branches show no green cambium when scratched, or if more than half of the plant is dead, the structural damage is too severe for the arborvitae to ever regain an attractive form.
Immediate Steps for Recovery
Once the plant is assessed as salvageable, the first action is to prune all dead, brown foliage. This should be done by cutting the dead wood back until you reach a point where the cambium is green and actively growing. Removing this dead material improves air circulation and directs the plant’s energy toward healthy growth.
Correcting the watering technique is necessary to ensure deep root hydration. Use a soaker hose or allow a garden hose to trickle slowly at the base of the plant for 15 to 30 minutes. This method ensures that the water penetrates 8 to 12 inches into the soil, promoting the deep root growth necessary for long-term health, rather than shallow watering that leads to surface roots. Ensure the soil is moist but never soggy, especially in cases where root rot is suspected.
If a pest infestation is confirmed, specific products can provide immediate relief. Spider mites can be treated using a horticultural oil or insecticidal soap, which works by suffocating the tiny pests. These products must be applied to thoroughly cover the mites and should be used during cooler times of the day to prevent foliage burn. For bagworms, hand-picking the bags off the branches before the larvae hatch is an effective immediate measure.
Preventing Future Browning
Long-term health begins with proper cultural conditions and planting location. Arborvitae thrive when planted in well-drained soil and receive adequate sunlight, typically a minimum of six hours per day. Planting them too close together or in overly shaded areas increases stress and leads to the internal browning of needles due to lack of light and poor air circulation.
Proper mulching around the base of the shrub is another important preventative measure to regulate soil temperature and retain moisture. Apply a layer of organic mulch, such as shredded bark, that is two to four inches deep. Critically, ensure the mulch does not touch the trunk, leaving a two- to three-inch gap to prevent moisture buildup that can lead to bark rot.
Preparing the plant for winter is essential to avoid desiccation damage. Continue deep watering until the ground freezes completely, allowing the foliage to enter dormancy fully hydrated. In late fall, when temperatures are between 40 and 50 degrees Fahrenheit, an anti-desiccant spray can be applied to the foliage. This product creates a waxy coating that reduces the amount of moisture lost through the needles, providing a crucial barrier against drying winter winds.