The Arborvitae (Thuja) is a popular evergreen used widely for privacy screens and landscape accents. Its dense, scale-like foliage normally maintains a vibrant green color throughout the year, so brown spots are alarming for homeowners. Browning is almost always a symptom of underlying stress rather than an immediate death sentence. Understanding the specific nature of the discoloration allows for a targeted response to restore the plant’s health.
Environmental Stressors Causing Browning
Browning on Thuja is most frequently caused by non-infectious environmental conditions that create physiological stress for the plant. One common issue is natural inner needle shedding, where older foliage closer to the trunk yellows and drops in the fall. This is the plant shedding two- to three-year-old growth that no longer receives adequate sunlight, and the browning is contained entirely within the interior of the canopy.
The location of the browning can help distinguish other environmental causes, such as desiccation injury, or winter burn. This damage appears on the side of the plant exposed to prevailing winter winds or intense, direct sunlight, often the south or southwest face. The browning starts at the tips of the foliage and progresses inward because the frozen ground prevents the roots from replacing water lost through the needles.
Drought stress, or a lack of available water during the growing season, typically causes browning that starts at the top of the tree and progresses downward, affecting the newest growth first. Conversely, excessive moisture from poor drainage can lead to root rot. Root rot causes browning or yellowing that begins on the lower branches and works its way up the tree from the base. The inability of damaged roots to uptake water results in symptoms similar to drought.
If the Thuja is planted near a roadway or driveway, browning can indicate salt damage. Salt spray or runoff from de-icing materials causes foliage to brown on the exposed side, often from the ground up to about six feet. The salt draws moisture out of the plant cells, creating a “chemical drought” that mimics desiccation. Identifying the affected side is the most reliable way to diagnose this problem.
Biological Threats: Pests and Fungal Infections
When browning appears on the exterior tips, the cause may be a biological threat from pests or fungal pathogens. Spider mites, which are nearly invisible, cause damage by sucking the cell contents out of the needles, resulting in a stippled, yellowish-brown discoloration. To diagnose them, shake a branch over white paper. If tiny, moving specks appear—sometimes leaving a reddish streak when smeared—spider mites are likely present. They are often most problematic during hot, dry periods.
Bagworms also cause significant browning. Their larvae create distinct, spindle or cone-shaped sacs made of silk and pieces of Thuja foliage. These protective structures are attached to the branches and can be up to two inches long when the pest is fully grown. Bagworms skeletonize the foliage, and a severe infestation can kill the tree.
Fungal infections, such as Tip Blight (often caused by Pestalotiopsis), also manifest as browning on the branch tips. This disease is opportunistic, primarily infecting trees already weakened by stress like drought or winter injury. The infection starts on the foliage tips, turning them yellowish before progressing to a dark brown or almost black color. A defining symptom is the appearance of small, black, pinhead-sized fungal fruiting structures, or pycnidia, dotting the surface of the dead tissue.
Immediate and Long-Term Recovery Strategies
The immediate response to browning is to stabilize the plant by addressing moisture stress. Proper water management is foundational to recovery. Deep, infrequent watering is the ideal approach, ensuring the soil is moist but never saturated to prevent root rot. A thorough watering in late fall before the ground freezes is especially important to prevent winter desiccation.
Removing damaged foliage improves the plant’s appearance, prevents the spread of fungal spores, and promotes better air circulation. Dead, brown material should be pruned out in mid-spring after the threat of hard freezes has passed and new growth has begun to emerge. For bagworms, physical removal by hand-picking the sacs off the plant before the eggs hatch in early summer is an effective control method.
Long-term health involves optimizing the plant’s environment. This starts with using an organic mulch layer, which conserves soil moisture and regulates temperature. Mulch should be spread a few inches deep but kept several inches away from the trunk to avoid moisture buildup that can invite pests and disease. Ensuring the planting site has good soil drainage is paramount, as Thuja will not tolerate soggy conditions.
For confirmed biological infestations, targeted interventions may be necessary after cultural practices are optimized. Dormant oil is a non-toxic option for controlling overwintering spider mite eggs and scale insects. This application should occur in late winter or early spring before new growth appears. Apply the oil on a day when temperatures are above 40 degrees Fahrenheit for at least 24 hours to ensure proper drying. Fungal blights are best managed by cultural practices like pruning, but if symptoms persist, a preventative fungicide application, such as one containing copper, may be warranted during wet spring weather.