Problems With Arborvitae

Arborvitae, also called thuja, are evergreen trees with flat, scale-like shoots and shaggy, reddish-brown bark. They grow from 10 to 60 feet and are used as ornamental trees and hedges. Their leaves and branches are eaten by moth larvae and deer. Arborvitae are generally hardy and relatively easy to grow but are susceptible to some problems.

Arborvitae is susceptible to foliar damage from dehydration in dry, windy winters. Shrubs and trees enduring summer and autumn drought and plants situated in open, unsheltered areas are especially vulnerable to drying winter winds. Damage includes burning, browning and drying of foliage and even woody tissue in severe winters. Foliage that is damaged can be clipped off in spring, and the plant will put out new growth; but plants that sustain sever drying and burning into woody stems, branches and trunks will likely die. To avoid winter wind damage, water arborvitae well just before the first hard freeze and protect shrubs and trees with burlap windbreaks or snow fencing. Snow and ice also damage arborvitae in winter, causing branches and trunks to break from the weight of heavy snow and ice. Carefully brushing snow off arborvitae shrubs and trees before breakage occurs prevents heavy buildup.

Bagworms are destructive larvae of moths that feed on foliage while expanding their nest made of webbing. Bagwom eggs overwinter in foliage and become active in late spring. Bagworms should be addressed as soon as they are observed to prevent them from killing the plant and spreading to other nearby foliage. If the infestation is small, clip off affected branches, and burn them or seal them in plastic bags and dispose of them. Monitor the plants to ensure there is no reoccurrence. If the infestation is large, you may lose the plant. Bacillus thuringensis kurstaki is a bacterial disease that can control bagworms if applied at the larval stage.

Scale is an insect that is round and flat in the larval stage, growing into small crawling insects for a short time in the spring. They are very small and only diagnosed with microscopic examination. Scale insects travel to other parts of the plant and to nearby plants when in the crawling stage and also by wind dispersal. Feeding scale populations turn foliage yellow, then brown, as the foliage dies. Although the damage from scale is usually minimal, larger infestations cause leaf dieback, prevent new growth and can kill plants under optimal conditions. Dormant oil spray in spring after a microscopic diagnosis is an effective control.

Vole damage can kill arborvitae. The animals eat the bark and soft tissues underneath during the winter months, which prevents the plant from drawing fluids up through its trunk and branches and causes it to die. Protecting the trunks with hardware cloth before snowfall will control vole damage.

Blight is a fungal infection of leaves, branches and trunks that kills tissues with cankers, or unhealthy, abnormal growths. Foliage bronzes and turns brown while dying off from blight, with fuzzy fungus usually visible with a magnifying glass. Blight takes hold in damaged and injured areas of plants. Several different kinds of blight can affect arborvitae. Control includes removing injured parts of plants, well-spaced plantings to allow enough sunlight and air circulation and fungicide applications.