The American Pillar Arborvitae (Thuja occidentalis ‘American Pillar’) is a popular evergreen cultivar known for its narrow, columnar habit and rapid vertical growth, making it ideal for dense privacy screens. While generally resilient, this specific arborvitae is susceptible to a range of issues that can compromise its long-term health and aesthetic appeal. Understanding the most common insect, disease, and environmental problems is crucial for maintaining a healthy and uniform hedge.
Common Insect Pests
Bagworms (Thyridopteryx ephemeraeformis) and Spruce Spider Mites are the most frequent arthropod threats to the American Pillar Arborvitae. Bagworms are identified by the spindle-shaped, silken bags they construct, which are camouflaged using pieces of the host plant’s foliage. These bags house feeding larvae that strip needles and leaves, causing localized defoliation that can eventually kill the shrub if untreated.
Management depends on the season and the insect’s life stage. The most effective control is physical removal and destruction of the bags during late fall and winter, as each female bag contains hundreds of eggs. If an infestation is discovered after larvae hatch in late spring or early summer, a targeted insecticide application is necessary. Treating with a product containing Bacillus thuringiensis (Bt), a biological control, is effective when the caterpillars are small and actively feeding.
The Spruce Spider Mite (Oligonychus ununguis) is an arachnid that feeds by piercing the foliage and sucking out cell contents. This feeding causes stippling or bronzing of the foliage, often going unnoticed until damage is extensive. To test for mites, shake a suspected branch over white paper; mites will appear as tiny crawling specks.
Spider mites thrive in hot, dry conditions, leading to rapid population growth during summer droughts. Treatment involves a forceful spray of water to dislodge them or the application of horticultural oil. Applying a dormant oil spray in late fall or early spring can also smother overwintering eggs before the growing season begins.
Fungal and Pathogen Diseases
The American Pillar Arborvitae is susceptible to diseases caused by fungal pathogens. Tip Blight, often caused by Phomopsis or Pestalotiopsis, affects the newest growth, causing branch tips to turn brown or gray before dying back. This blight is distinct from insect damage, as it starts at the tips and progresses inward, sometimes leaving visible cankers on small stems.
Tip Blight thrives where there is poor air circulation and prolonged wet foliage, allowing fungal spores to germinate. Management requires immediately pruning out infected tips and disposing of the debris to prevent spreading. Fungicide application may be necessary in severe cases, timed to protect new spring growth.
Root Rot is a serious, often fatal, condition resulting from planting in soil that retains too much moisture. This cultural problem is caused by poor drainage that suffocates the roots. Symptoms include the gradual decline of the entire plant, with foliage turning yellow or brown. Improving soil drainage or replanting the arborvitae with the root flare slightly above the soil grade is the only effective long-term solution.
Environmental Stress and Cultural Damage
Environmental factors and poor cultural practices cause much of arborvitae decline, often mistaken for disease or pests. Winter Burn is common desiccation damage occurring in cold climates when the ground is frozen, preventing water absorption. Strong winter winds and bright sun cause the evergreen foliage to lose moisture (transpire), which cannot be replenished. This results in the foliage turning rusty brown, often observed on the side facing the prevailing wind and sun.
Drought Stress causes similar symptoms—foliage turning brown or yellow—but occurs during dry summer and fall months. Newly planted arborvitae are vulnerable because their root systems are not yet expansive enough to access deep moisture. Browning often begins on the interior or lower branches first. Consistent, deep watering during dry spells is required to prevent this stress.
Deer browsing is a frequent problem, even though this cultivar is considered more resistant than other arborvitae varieties. Deer feed on the foliage when other food sources are scarce. Deer damage is characterized by ragged, torn branch tips, typically occurring at an accessible height. This mechanical injury is often confined to the outer edges of the plant.
Proactive Care and Prevention
Preventative maintenance and proper planting practices mitigate the majority of problems affecting the American Pillar Arborvitae. Selecting a site with full sun and protection from harsh winter winds reduces the risk of Winter Burn and promotes dense growth. When planting, set the arborvitae so the top of the root ball is slightly higher than the surrounding soil. This ensures proper drainage and prevents water from pooling around the trunk.
Consistent watering is necessary, especially during the first two years, to establish a robust root system. Providing deep, soaking irrigation in late fall before the ground freezes is important to minimize winter desiccation risk. Apply a two-to-three-inch layer of organic mulch around the base to conserve moisture and regulate temperature. The mulch must be kept several inches away from the trunk to prevent moisture buildup that promotes disease.