Arborvitae trees (Thuja species) are valued for their dense, year-round foliage, making them popular for privacy screens and hedges. When these evergreens decline, the damage is highly visible. Decline or death typically stems from a combination of mobile pests, aggressive fungal diseases, and environmental stress. Understanding these specific threats is the first step in protecting these landscape staples.
Insect and Mite Damage
Several tiny pests cause significant decline by feeding on the foliage and sap. Arborvitae leaf miners tunnel inside the scale-like leaves, eating the interior tissue. This internal feeding causes the foliage tips to turn yellow, then brown, creating a scorched appearance that reduces the tree’s ability to photosynthesize.
Spider mites, particularly the spruce spider mite, are minute arachnids that pierce plant cells and suck out fluids. This feeding causes fine yellow or bronze stippling on the foliage, leading to widespread bronzing and premature needle drop. Mite populations surge during hot, dry periods and severe infestations may leave fine webbing.
Boring insects, such as cedar and cypress bark beetles, often attack trees weakened by other stressors. Their larvae tunnel beneath the bark, disrupting the flow of water and nutrients. This tunneling can effectively girdle the tree and cause rapid dieback of entire sections.
Fungal Pathogens and Root Issues
Fungal infections, often facilitated by poor cultural conditions, pose a serious threat. The most catastrophic disease is root rot, which thrives in excessively wet, poorly drained, or anaerobic soils where roots are deprived of oxygen. These conditions are often created by overwatering or poor site selection.
Infected roots turn dark, become soft, and lose their ability to absorb water and nutrients. This causes the foliage to look drought-stressed, wilt, and turn dull green or yellow before the entire tree collapses.
Needle blight is another common issue, typically infecting foliage weakened by winter injury or mechanical damage. This disease causes browning that starts at the tip and progresses toward the base. It often results in small twig dieback and a scorched look, particularly on inner, lower branches where air circulation is limited.
Environmental and Cultural Stressors
Environmental factors and improper care are the most frequent causes of arborvitae failure. Water stress is a primary culprit, as these trees require moist but well-drained soil; both drought and overwatering lead to problems.
Insufficient water, especially in newly planted trees, prevents root establishment. Conversely, overwatering creates saturated conditions that invite lethal root rot.
Winter desiccation is a significant non-disease threat to the foliage. This occurs when leaves lose moisture through transpiration on sunny or windy winter days, but frozen ground prevents the roots from replenishing water. This results in the exposed sides of the tree turning bronze or brown, becoming fully apparent when temperatures warm in the spring.
Arborvitae are also highly sensitive to road salt. Damage occurs either through direct spray onto the foliage or by salt accumulating in the soil, interfering with water absorption.
Finally, deer browsing can strip enough foliage to fatally weaken the plant. Since the inner, bare wood of arborvitae does not readily regenerate new growth, this damage is often permanent.