Narrow, dense, pyramidal conifers, Thuja occidentalis (American arborvitae) trees turn brown naturally in fall and in response to stress and infection. Growing 20 to 40 feet tall and 10 to 15 feet wide, American arborvitae trees are hardy in U.S. Department of Agriculture plant hardiness zones 2 through 7. Compact and dwarf cultivars are available, and American arborvitae tolerate annual pruning to restrict their size, making them useful hedging and screening plants.
Turning brown is part of American arborvitae's normal growth cycle, and often trees require no treatment. Although American arborvitae are evergreen, the trees' inner branches turn brown and drop during fall and winter. Red-brown, mature bark sheds at the same time. American arborvitae outer foliage also turns yellow-brown, which may look unattractive but requires no treatment. Brush heavy snow buildup from branches to prevent breakage, and the trees' color will improve when new, fresh leaves appear in spring.
Protection from cold winds and drought treats browning in stressed American arborvitae. Strong, winter winds cause brown, burned leaves in American arborvitae growing in exposed sites, and foliage also turns brown and dies during prolonged drought, which these trees don't tolerate. Grow American arborvitae in sheltered, full-sun sites in moist, well-drained soil. Trees benefit from light afternoon shade in areas with hot summers and grow best in neutral or slightly alkaline soil. Water trees regularly so the soil remains moist but not sodden. Completely brown patches are probably dead and won't resprout, but new green growth may grow to cover them over time.
Leaf-tip blight turns American arborvitae leaf tips brown, and treatment involves pruning or spraying with fungicide. Symptoms of leaf-tip blight (Pestalotiopsis funerea) include tan or brown twig tips and small, black, pimple-like, spore-producing dots. Prune trees suffering from leaf-tip blight with pruning shears sterilized by wiping the blades with rubbing alcohol. Prune all affected areas, cutting 3 inches from the affected area into healthy, living tissue, and sterilize the pruning shears again after use. Alternatively, put on safety goggles and protective clothing, and spray foliage with a fungicide containing 58 percent copper salts of fatty and rosin acids, diluted at a rate of 1 tablespoon per gallon of water, or according to the manufacturer's instructions. Repeat applications every seven to 14 days or as required. Fungicides should be stored out of reach of children and wash hands after using.
American arborvitae suffering from twig blights turn brown and require pruning. Kabatina twig blight (Kabatina thujae) infects 1-year-old American arborvitae branches, and phomopsis twig blight (Phomopsis juniperovora) can infect any branches. Infected tissue turns brown or ash gray as it dies, and dead branches remain on the tree for months. Small, black spots appear where dead tissue meets living wood. Prune American arborvitae suffering from twig blights with sterilized pruning shears, removing twigs and branches at least 3 inches inside healthy tissue. Bag and destroy all pruned branches. Fungicides to treat these twig blights aren't available for home use and can only be applied by professionals.