The Green Giant Arborvitae (Thuja plicata x standishii) has become a popular choice for homeowners seeking a fast-growing, dense privacy screen. This evergreen hybrid is generally recognized for its robust nature and adaptability. Despite its reputation for hardiness, the Green Giant Arborvitae is not immune to common issues that can compromise its appearance and health. Homeowners often encounter problems stemming from insects, pathogens, or environmental stress. Understanding these frequent issues and the appropriate responses is necessary for maintaining a healthy and continuous privacy screen.
Insect Infestations
Several insect pests target the Green Giant Arborvitae, with damage often appearing as browning or thinning foliage. Spider mites are tiny arachnids that cause significant discoloration, particularly during hot, dry weather. Their feeding results in a fine stippling or yellowish speckling on the foliage, which can progress to widespread bronzing or browning. A fine webbing on the branchlets or a quick tap over a white sheet of paper can confirm their presence.
Control for spider mites often begins with non-chemical methods, such as using a strong jet of water to physically wash them off the foliage. If an infestation is heavy, applying horticultural oil or insecticidal soap is effective, as these products smother the mites. Treatments should be applied at the first sign of infestation, usually in late spring or early summer, and repeated as necessary.
Bagworms are another highly destructive pest, identified by the distinctive, spindle-shaped “bags” they construct from silk and pieces of the arborvitae foliage. The larvae inside these bags feed on the needles, leading to defoliation that typically begins at the top of the tree and moves downward. Because the bags are made from the plant’s own foliage, they are initially difficult to spot until the damage becomes severe.
For light infestations, the most direct control method is hand-picking the bags off the branches and destroying them, ideally before the eggs hatch in late May or early June. If manual removal is impractical, especially on taller trees, an application of a pesticide containing Bacillus thuringiensis (Bt) can be used when the larvae are actively feeding, typically in early summer. Scale insects, such as Fletcher scale, are a secondary threat that appears as small, immobile bumps on the twigs and stems. These sap-feeding pests excrete a sticky substance called honeydew, which can lead to the growth of sooty mold.
Fungal and Pathogen Issues
Fungal diseases can cause symptoms in the foliage that are frequently mistaken for environmental damage. Tip Blight, often caused by fungi like Pestalotiopsis funerea, is a common issue that manifests as browning at the ends of branches, moving inward. Close inspection of the affected areas may reveal small black dots, which are the fungal fruiting bodies.
This fungal disease often attacks tissue already weakened by stress or injury. Management involves pruning the infected branches immediately to prevent the fungus from spreading to healthy tissue. In cases of repeated or severe infection, a preventative fungicide application may be necessary, often timed to protect new growth.
A more severe issue is root rot, caused by decay-producing fungi like Phytophthora or Pythium. The symptoms are visible in the canopy as widespread yellowing, wilting, and browning of the foliage, eventually leading to the collapse and death of the plant. Root rot is fundamentally a biological response to poor cultural conditions, attacking the roots and preventing them from transporting water and nutrients. The presence of mushrooms or perpetually soggy soil near the base of the tree can indicate active decay.
Environmental and Maintenance Stressors
The most frequent causes of Green Giant Arborvitae decline are related to environmental and maintenance errors, which create conditions that stress the tree. Poor drainage is a leading stressor, as these trees do not tolerate standing water or soil that remains saturated for extended periods. Planting the arborvitae in an area where water puddles after rain or where the soil is continually squishy will almost certainly lead to root rot conditions. Amending heavy clay soil with organic material and ensuring the planting area is not low-lying is a necessary preventative measure.
Water stress is a duality, encompassing both overwatering, which creates the conditions for root rot, and drought stress. While the Green Giant is drought-tolerant once established, it requires deep, infrequent watering, especially during the first few years and during extended dry periods. Allowing the soil to dry slightly between waterings is important for established trees. Watering deeply in the fall before the first hard freeze is also a preventative step, as it helps the roots store moisture for the dormant season.
Winter burn, or desiccation, is a common non-infectious problem that occurs when the foliage loses moisture faster than the roots can replace it from frozen soil. This often results in browning on the side of the tree exposed to harsh winter winds or intense sun. Prevention involves ensuring the tree is well-hydrated going into winter and using physical barriers like burlap screens to block drying winds. Anti-desiccant sprays applied to the foliage in late fall can also help reduce moisture loss during the winter months.
Improper planting depth can also hinder a tree’s long-term health. Planting the root flare—the point where the tree trunk widens at the base—too deep restricts oxygen exchange and contributes to root stress. Additionally, physical damage from deer browsing is a widespread issue, as deer find the foliage of arborvitae highly palatable, especially in winter when other food sources are scarce. Installing fencing or regularly applying commercial deer repellents can deter this type of damage.