Ranging from 2 to 50 feet tall, arborvitae (Thuja spp.) are evergreen shrubs and trees with varying colors, including blue, yellow and green. Although they're not deciduous plants, arborvitae must form new foliage over time as they shed older growth. Preferring U.S. Department of Agriculture plant hardiness zones 4 through 7, these decorative plants do turn colors in the fall, but not all the coloring is caused by natural shedding.
Some arborvitae drop their needles every autumn or every three to four years, depending on the variety and climate. Natural shedding appears brown, yellow or even red as it glows in the autumn light. Foliage near the trunk drops from the plant; you should not see stem tips turning color. Arborvitae retains its outer green growth for maximum photosynthesis. Because of their dense growth habit, arborvitae bushes drop their internal needles to conserve energy. This hidden foliage cannot photosynthesize well to support the tree or shrub. Dropping the old foliage allows the plant to direct energy into new needle growth.
Cold Weather Reaction
Arborvitae prefer locations in full sunlight with well-drained soil. Because of arborvitae's exposure, cold weather often causes the foliage to turn colors. As nights lengthen, needles cannot photosynthesize as effectively during the day. As a result, chlorophyll dies back and your arborvitae appears brown or yellow. To reduce the impact of this fall and winter eyesore, carefully prune your tree or bush in the spring and summer. With a tidy, pruned appearance, your arborvitae still turns colors in cold weather, but quickly returns to green as spring arrives.
Fungal and Pest Issues
If your arborvitae is stressed from overwatering, fungi can attack needles and create brown stem tips. With careful inspection, you may also see black fungi along the browning areas. Consistently water your arborvitae, but do not maintain wet soil conditions. Pests such as spider mites also look for stressed arborvitae as potential homes. Browning from spider mite pest infestations occurs year-round. Periodically spray needles with water to physically remove the mites to avoid unnatural browning.
Late summer drought conditions cause arborvitae to turn colors in fall as a result of dying foliage. The needles lose moisture quickly in full sunlight locations, making them drop off or turn brown. Using a soil moisture meter, maintain damp soil conditions to a 24-inch depth. Arborvitae roots remain within the top 2 feet of soil for ample nutrients, moisture and oxygen. Spread mulch around your arborvitae as well to conserve soil moisture and reduce drought stress.