Pruning Arborvitae
When and how — Thuja occidentalis
Prune your arborvitae in May, June and September — the optimal month is usually June.
You're in the pruning season right now — grab the secateurs.
When to prune?
The tree arborvitae is pruned in May, June and September.
Prune trees for structure and health, not productivity.
Tree pruning is almost always about crown shape and health, not flowering or fruit. Good tree pruning starts in the first ten years: you set the framework with three to five strong scaffold branches that leave the trunk at an open 45–60° angle. After that, prune mainly to remove dead, diseased or crossing wood. Heavy renovation pruning later in life triggers masses of watershoots and weakens the tree — better to do light corrective pruning every two or three years than one drastic intervention per decade. Timing follows the sap flow: deciduous trees during winter dormancy (December to February, except birch and walnut which 'bleed'), conifers any time of year except during frost.
How to prune arborvitae
Arborvitae requires minimal pruning, but regular light trimming keeps hedges tidy and encourages dense growth. Prune in May, June, or September—avoid cutting during frosty weather or high summer heat. Late spring and early autumn are ideal because the plant is actively growing and will recover quickly. Use sharp bypass secateurs or hedge shears. For formal hedges, trim back new growth by about one-third, cutting just above green foliage. Never cut into old, brown wood; arborvitae does not regenerate readily from bare stems, and you risk leaving permanent gaps. Always leave some green growth on every branch. If you're maintaining an established hedge, a light trim once or twice a year—typically in late May and again in September—is sufficient. For specimen trees, pruning is largely unnecessary. Remove any dead, damaged, or diseased branches as soon as you notice them, cutting back to healthy wood or the main trunk. If a leader (main central stem) is damaged, select a strong lateral shoot, tie it vertically to a cane, and train it as the new leader. Thin out any crossing or rubbing branches to maintain good air circulation and an attractive shape. Avoid heavy pruning or drastic reshaping. Arborvitae responds poorly to hard cutbacks, and recovery is slow. If a hedge has become overgrown and bare at the base, it's often better to replace it than attempt renovation. Light, regular attention from an early age is far more effective than infrequent severe cuts.
Common mistakes
✗ Cutting flush to the trunk
Remove branches just outside the branch collar (the swelling at the base), not flush to the trunk. The collar contains the cells that seal the wound — cut those off and the wound won't heal, giving rot a clear path in.
✗ Topping to limit height
Drastically shortening the leader triggers massive watershoot growth and permanently weakens the tree. Want a smaller tree? Choose a smaller species at planting time, or replace the tree.
✗ Painting wounds with sealant
Once standard, now outdated: wound paint traps moisture and actually encourages rot. A clean cut at the right moment heals on its own.
