Arborvitae (Thuja) is an evergreen conifer widely used in landscaping for its dense foliage and ability to form effective privacy screens and hedges. Valued for its upright, columnar growth habit, it provides year-round structure and color. Regular trimming is necessary to maintain the desired shape, density, and overall health. Successful pruning requires adherence to specific rules to avoid causing permanent damage.
The Fundamental Rule of Trimming Arborvitae
The fundamental rule when pruning Thuja is to cut only into the green, leafy growth. The interior, near the trunk and older branches, consists of brown, woody material that lacks a mechanism for regrowth. Unlike many deciduous shrubs, arborvitae branches do not have regenerative structures or latent buds on their older wood.
If a trim penetrates past the visible green foliage into the woodier center, the cut area will not produce new shoots. This results in a permanent brown patch or hole in the plant’s profile. Since growth is concentrated at the tips, surrounding foliage will not grow inward to cover the exposed wood.
This biological limitation means arborvitae cannot be severely cut back or rejuvenated like plants such as boxwood or privet. Radical size reduction exposes the non-regenerative inner structure, compromising the plant’s uniform appearance indefinitely. Therefore, all trimming must be shallow, focusing only on the current season’s growth to maintain the outer green shell.
Optimal Timing for Maintenance and Shaping
The best time for primary trimming is late spring or early summer, following the first significant flush of new growth. Pruning then, typically late May through June, allows the plant to quickly recover and produce a subsequent layer of growth. This timing ensures the new foliage has time to mature and harden off before cold weather.
A lighter, secondary touch-up trim can be performed in mid-to-late summer, around August, to refine the shape. Trimming must be completed well before the first expected hard frost, ideally by early September. Pruning too late stimulates new, tender growth that does not have sufficient time to acclimate before winter dormancy. This unhardened growth is highly susceptible to desiccation and winter burn, causing the new tips to brown and die back.
Performing the Trimming Cut
Proper technique involves making light, surface-level cuts to encourage density and maintain the hedge line without violating the green growth rule. For shaping, use sharp, clean shears to remove no more than one-third of the new growth from the branch tips. This light shearing stimulates the existing buds closest to the cut to branch out, resulting in a thicker, denser layer of foliage.
Structurally, the arborvitae should be shaped with a slight taper, ensuring the base is slightly wider than the top. This trapezoidal shape allows sunlight to reach the lower branches, preventing the natural shading and subsequent browning often seen at the bottom of vertically-sided hedges. Maintaining light penetration to the lower canopy is essential for keeping the entire plant uniformly green.
In addition to shearing for shape, occasional thinning cuts should be made deeper inside the canopy using hand pruners to remove any dead, diseased, or damaged branches. These cuts improve air circulation throughout the plant, which helps reduce the risk of fungal disease and encourages overall vigor.
Always use sharp tools, such as bypass pruners or hedge trimmers. Wipe the blades with a disinfectant solution, like a diluted bleach or alcohol mixture, between plants to prevent the spread of pathogens.