The Thuja, often called Arborvitae, is a popular evergreen tree valued for its dense foliage and suitability for hedges and privacy screens. When its vibrant green scales begin to turn brown, it signals that the plant is under stress and requires prompt attention. Identifying the exact cause of this discoloration is the first step toward recovery, as the browning pattern provides a direct clue to the underlying problem. The causes range from simple environmental issues to complex infestations or diseases that demand specific treatments. Understanding these distinctions helps homeowners quickly diagnose and address the health of their Thuja.
Browning Caused by Water and Environmental Stress
Water management is the most frequent cause of browning in Thuja, as both too much and too little moisture can be detrimental. Drought stress occurs when the plant cannot draw enough water, causing uniform browning across the entire tree as it sacrifices outer growth to survive. A newly planted Thuja is especially susceptible, requiring consistent moisture to establish a root system capable of supporting its dense canopy. Feeling the soil six inches deep is the most reliable way to determine if thorough watering is necessary.
Conversely, excessive moisture from overwatering or poor soil drainage can lead to root rot, where roots are deprived of oxygen and become susceptible to soil-borne pathogens. The foliage often yellows and wilts, mimicking drought stress because the damaged roots cannot absorb water, eventually leading to large brown areas. Allowing the soil to dry slightly between watering sessions and ensuring proper drainage are preventive measures.
Environmental extremes also contribute to browning, particularly during winter. Winter burn, or desiccation, happens when the ground is frozen, preventing roots from absorbing water, while cold, dry winds or intense sunlight pull moisture from the foliage. This damage appears as browning on the exposed sides of the plant or at the tips of branches. Road salt runoff can also cause chemical burn and stress, leading to browning on plants situated near sidewalks or driveways.
Identifying Damage from Insect Pests
Certain insect pests feed on Thuja foliage, causing discoloration and eventual browning that follows a distinctive pattern. Spider mites, which are tiny arachnids, are common culprits, particularly in hot, dry conditions. Their feeding activity causes the green foliage to develop fine stippling, leading to a dull, grayish, or rusty brown appearance. You can confirm their presence by looking for fine webbing among the scales or by shaking a branch over a white piece of paper to see the mites fall.
Bagworms are another pest that causes noticeable damage, forming distinctive, spindle-shaped sacs that hang from the branches. These sacs are constructed from silk and bits of Thuja foliage. The larvae inside feed on the needles, leading to the rapid browning and defoliation of the attached branches. Hand-picking and destroying these bags before the eggs hatch in late spring is the most effective control method.
Recognizing Fungal Diseases and Pathogens
Browning can also be the result of various fungal diseases that thrive in moist, humid environments, often targeting stressed trees. Phomopsis tip blight is a common issue that causes the tips of one-year-old branches to die and turn brown or ash-gray. This fungus can spread to girdle larger branches; tiny black, pimple-like fruiting structures are often visible on the dead tissue. Pruning and destroying infected branches immediately helps reduce the spread of the spores.
Another concern is root rot, which, while often linked to poor drainage, is caused by water mold pathogens like Phytophthora. These pathogens attack the feeder roots, compromising the plant’s ability to absorb water and nutrients. The resulting browning starts from the top of the tree and progresses downward, causing rapid decline, especially during warm weather. Improving soil drainage and avoiding waterlogged conditions are the primary preventative measures against this destructive disease.
Distinguishing Natural Shedding from Serious Distress
Not all browning is a sign of a health problem, as Thuja naturally sheds its older, interior foliage in a process called “flagging.” This occurs in late summer or fall when the plant discards scales that are no longer photosynthetically efficient due to shading from new outer growth. The browning in this normal process is confined to the innermost parts of the branches closest to the trunk, while the outer tips remain green and healthy.
Understanding the location of the browning is the quickest way to diagnose the cause. If the browning starts at the tips of the branches and moves inward, or if the browning is patchy across the exterior of the plant, it indicates a problem like disease, pests, or environmental stress. Natural shedding, conversely, involves the inner foliage turning a dry brown color before dropping off.
Ongoing maintenance prevents stress-related browning and protects the plant from opportunistic pests and pathogens. This includes ensuring the Thuja is planted at the correct depth and applying a layer of mulch around the base to help regulate soil temperature and retain moisture. Consistent, deep watering, particularly during extended dry periods, helps the plant maintain its vigor and resist infection.