Arborvitae, known botanically as Thuja, is a genus of needle evergreen trees and shrubs belonging to the cypress family of plants. Like all coniferous evergreens, arborvitae require very little and infrequent pruning, but can tolerate some trimming for grooming or shaping purposes. When the arborvitae is badly overgrown, you should trim it in stages to avoid cutting down to bare wood, as the tree cannot regenerate green foliage growth at that point. Overall, trimming cuts should always be thoughtfully considered and implemented with a light touch.
Trim your overgrown arborvitae in the spring before new green growth appears, but after the last hard frost occurs in your region.
Cut away dead, discolored or diseased foliage, plus twigs and small branches from the interior and perimeter of the tree or shrub. Compost or discard the clippings.
Trim the terminal tips of all the branches to reduce the overall size of the plant, using long-bladed scissor shears or electric shears. Work around the entire plant, evenly removing up to, but no more than, 1/3 of the arborvitae's overall volume of green needle foliage.
Repeat the 1/3 shearing process for three consecutive years to gradually reduce the size of the plant. Light shearing will also spur new branching at the tips, and boost the density of the green foliage, making the plant more attractive while filling in holes created from the pruning.