In general arborvitaes exhibit few serious insect and disease problems. Unfortunately American and Oriental Arborvitae are considered ice cream by deer and are quite vulnerable to deer browsing especially in the winter months.
Leaves Drop Off In Spring or Fall
Normal Leaf Senescence All evergreens drop some of their foliage every year, usually in the fall, but it can occur in the spring as well. Do not be concerned if you notice some brown, dead leaves on arborvitaes at the same time the deciduous trees and shrubs are displaying their fall colors. This normal reddish-brown discoloration occurs on branches closest to the trunk. Subsequent leaf drop may occur annually or every second or third year. However, if brown foliage appears at other times of the year or on leaf tips, the plant may have spider mites or be suffering from an environmental problem.
Plant Loses Its Shape
Old Age Harsh weather may cause some branches and even trunks of older arborvitaes to break off, making unsightly gaps in the form of the shrub. If pruning does not improve the shrub's appearance it may be time to replace it.
Twigs, Leaves Turn Brown During Drought
Sunscald or Sunscorch A shortage of water in summer may cause twigs to turn brown and eventually drop off. During the summer, soak the soil to a depth of about two feet once every two weeks or so. If the symptoms develop in late winter or early spring, they may be caused by drying winds and hot sun. They are hard on the previous season's growth and recently transplanted shrubs in exposed locations are most severely affected. In hot sun water evaporates from the leaves faster than the root system can replace it, which causes the leaf discoloration. Minimize damage by mulching and thoroughly soaking the ground around the shrubs before the ground freezes in winter. Spray the foliage of arborvitae in exposed sites with an anti-transpirant spray in the fall to protect it from drying out during the winter. Follow label instructions.
Foliage Browns, Trunk Splits Near Soil means Freeze Injury
Freeze Injury Normally arborvitae gradually acclimate to increasing colder temperatures as fall yields to winter and are able to withstand winter successfully. In some cases a prolonged warm fall, followed by a sudden cold spell or a warm spell in the middle of winter prevents normal gradual acclimatization and plant tissues freeze and die, killing it. While there is nothing to be done about the weather, you can control the arborvitae's environment somewhat. Avoid feeding with high nitrogen fertilizers late in the season and be sure that arborvitaes are planted in well-drained soil. For more information see the file on Dealing With Winter Injury To Trees and Shrubs
Foliage Chewed, Striped From Twigs
Deer, Moose, Rabbits Unfortunately arborvitae foliage is tasty to several critters. As the pressure of development restricts their habitats even more severely than ever before, wildlife is forced to feed in residential landscapes. Here they often find plants that are tastier than the wild ones they normally forage on. There are many animal repellent spray products for plant foliage available that discourage the casual browser by either smell or taste. They tend to be effective for only a short time, until the rain washes them off or the animal gets used to them. Often they are most effective if several are used alternately. Animals under severe population pressure and desperate for food can only be discouraged by a barrier of some kind. An effective, unobtrusive fencing material is black polynetting that can be easily installed around the area where the arborvitaes are planted, or around the entire yard. For more information see the file Controlling Deer or on Dealing With Rabbits
Small Silken Bags Hang From Twigs
Bagworms The bagworm caterpillar builds a silken cocoon, or bag, with silk and bits of leaves attached to the outside. It carries its bag with it as it feeds. These small spindle-shaped bags hanging from your arborvitae's branches like Christmas tree ornaments indicate its presence. See the file on Controlling Bagworms.
Leaves Webbed Over, Turn Gray or Brown.
Spider Mites Spruce spider mites or red spider mites spin webs and cause a graying or browning of arborvitae leaves. For more information see the file on Controlling Mites
Leaves Curled and Distorted.
Aphids The arborvitae aphid is a reddish brown, soft-bodied, pear-shaped sucking insect about the size of the head of a pin. Aphids suck plant sap from foliage, retarding or distorting arborvitae growth. Affected leaves may turn yellow or brown, wilt under bright sunlight, or sometimes curl and pucker. For more information see the files on Controlling Aphids
Sawdust at Base of Shrub, Poor Growth
Cedar Tree Borers In its larval stage, the Cedar Tree Borer bores into the inner bark and wood, frequently girdling the arborvitae, making it more susceptible to heat, drought, and disease. A mass of gummy sawdust ("frass") at the base of an injured arborvitae shrub signals the presence of borers, the larvae of the cedar tree borer. For more information see the file on Controlling Borers
Leaves and Branches Encrusted With Small Bumps
Scale Insects Scale insects lurk under waxy shells, forming groups of small bumps or blister-like outgrowths on arborvitae stems and leaves. The shells may be white, yellow, or brown to black, and are about 1/10 to 2/5 inch in diameter. These bumps and discolored upper leaf surfaces, followed by leaf drop, reduced growth, and stunted shrubs, suggest a scale attack. For more information see the file on Controlling Scale
Leaf Margins Notched
Arborvitae Weevils These weevils are small and black, covered with metallic green scales and fine short hairs. Their larvae (grubs), which are white to pink with brown heads, attack arborvitae roots from June or July to midwinter or the following spring. Then they emerge from the soil to feed on leaves from May to July. Notches appearing in the margins of arborvitae leaves probably means these weevils are at work. The adults usually are active at night and hide in soil and trash during the day. Adult weevils will "play dead" when disturbed, folding their legs and dropping off plants to the ground. For more information see the file on Controlling Weevils
Twig Tips/Leaves Turn Brown Or Yellow, Die Back
Fungal Diseases In some cases the disease causes branch tips to turn brown and die back until the entire branch dies. Leaf spot causes leaves of affected shrubs turn straw yellow or brown and are thickly dotted with small black fruiting bodies. These diseases mostly attack foliage or shrubs already weakened by stress from heat, lack of water or other environmental problem. Sometimes the entire shrub may be involved. For more information see the file on Controlling Fungal Disease
Foliage Burned
Dog Urine Dog urine may discolor arborvitae foliage and even kill branches. For more information see the file Dealing With Dogs
