The Green Giant Arborvitae (Thuja plicata x standishii) is a popular, fast-growing hybrid, widely chosen for creating dense privacy screens and hedges. Homeowners often anticipate a maintenance-free plant due to its reputation for vigor and disease resistance. While this cultivar is generally robust, it is still susceptible to various biological and environmental issues that can disrupt its health and appearance. Understanding these common problems and implementing proactive care strategies is necessary for maintaining a healthy, uniform green barrier.
Pest-Related Damage
One of the most destructive pests is the bagworm, whose larvae create spindle-shaped bags of silk and plant debris, which they carry as they feed. This feeding results in defoliation and branch dieback, often starting in the upper canopy. For light infestations, handpicking the bags before the eggs hatch in late spring or early summer is the primary control method. When infestations are heavy, a biological insecticide containing Bacillus thuringiensis (Bt) should be applied when the caterpillars are young and actively feeding, usually between late May and mid-June.
Tiny arachnids known as spider mites thrive in hot, dry conditions and can cause significant discoloration. Their feeding activity results in fine yellow speckling or stippling on the foliage, which eventually turns bronze or brown, often accompanied by fine webbing. A simple diagnostic test involves shaking a suspected branch over a white piece of paper to check for tiny moving specks. Treatment often involves a strong jet of water to physically dislodge the mites or the application of horticultural oil or insecticidal soap, taking care to use miticides rather than general insecticides which can harm beneficial insects.
Scale insects, such as Fletcher scale, represent a less common threat, appearing as small, immobile, turtle-like bumps on twigs and branches. These sap-feeding pests excrete a sticky substance called honeydew, which can lead to the growth of black sooty mold on the foliage below. Management includes pruning infested twigs, manually removing the scales, or applying horticultural oil during the tree’s dormant season to smother overwintering eggs.
Common Fungal and Root Pathogens
The most frequent cause of Green Giant Arborvitae mortality is root rot, a condition triggered by water molds like Phytophthora that flourish in overly saturated soil. Symptoms often resemble drought stress, displaying as a sudden browning, wilting, and overall decline of the foliage, but the tree will not recover when watered. Root rot results from planting in poorly draining, heavy clay soil or from excessive irrigation. Preventing root rot relies on proper site selection and soil management, ensuring the soil is consistently moist but never waterlogged.
Specific fungal diseases can attack the above-ground portions of the tree, particularly when the plant is already under stress. Seiridium canker manifests as “flagging,” where individual branches turn yellow to reddish-brown among healthy foliage. Affected branches reveal sunken, often resin-oozing cankers in the bark, usually located closer to the main trunk. There is no chemical cure for canker diseases; the only action is to prune the diseased branches several inches below the visible canker using sterilized tools to prevent the fungus from spreading.
Fungal tip blights, caused by pathogens like Pestalotiopsis, typically appear as the tips of the foliage turn from green to yellow, then to a dark brown or black. This disease is opportunistic, often attacking foliage that has been weakened by winter injury or other environmental stresses. Controlling tip blight involves pruning the dead tips and ensuring good air circulation by avoiding overcrowding.
Environmental and Cultural Stress
Many issues that appear biological in nature are the result of environmental or cultural stresses. Winter burn, or desiccation damage, occurs in cold months when the foliage loses water to drying winds and sun faster than the frozen ground can replenish it. The browning, golden, or bleached foliage is most noticeable on the side of the tree facing the winter sun and prevailing winds. Deeply watering the tree in late fall before the ground freezes is the primary preventative measure.
Drought stress is common, especially in newly planted trees that require consistent, deep watering for the first year to establish their root systems. Symptoms include dull, weak foliage and browning of the inner needles, which is the tree’s response to conserving water. Improper planting depth is a common cultural error; the top of the root ball must be set level with the surrounding grade. Planting the tree too deeply restricts oxygen flow to the roots, which can lead to decline, stunting, and eventual death.
Managing Structural Integrity
The rapid growth rate and dense, columnar habit of the Green Giant Arborvitae can lead to physical damage, particularly during winter weather. Heavy loads of snow or ice accumulate easily on the foliage and can cause the branches to splay outward or even split the main trunk. This risk is pronounced in trees that have not been adequately pruned or maintained to encourage a strong central leader.
To mitigate damage from snow, gently remove the accumulation with an upward sweeping motion using a soft broom. Leave ice to melt naturally, as forcing it off the branches can cause them to snap. Regular, light pruning is advisable to maintain a conical shape and strengthen the main trunk, which reduces the likelihood of the tree splitting under a heavy load. Deciding whether to hedge it for a uniform screen or allow it to grow naturally is important for long-term maintenance planning.