Arborvitae (Thuja) are evergreen conifers valued in landscaping for their dense, conical growth, often used as privacy screens or hedges. As they mature, they frequently outgrow their intended space, leading homeowners to seek ways to reduce their size. While physically possible to cut a large portion off, successful reduction requires understanding the plant’s unique biology to ensure its survival and appearance.
The Biological Limits of Arborvitae Pruning
The ability to cut an arborvitae in half is limited by its inability to regenerate from old wood. Unlike many deciduous shrubs, arborvitae, like most conifers, do not possess dormant buds along their older stems. New foliage only grows from younger, green wood.
Cutting past the green, living foliage into the brown, interior core creates a permanent bare patch. This exposed, leafless wood will not sprout new growth, which is why older arborvitae often have a “dead zone” in their interior. Severe reduction cuts that expose this interior will leave a visible, brown scar on the exterior that will never fill in.
Aggressive pruning should be limited to removing no more than one-third of the plant’s total foliage in a single season. Exceeding this limit places the plant under stress and increases the risk of permanent, unattractive bare spots. The pruning strategy must be designed to avoid cutting into the leafless, old wood.
Safe Strategies for Reducing Vertical Height
Reducing the height of a tall arborvitae is best done using a reduction cut, which is preferable to simply “topping” the main trunk. This involves cutting the main vertical stem, or leader, back to a healthy side branch. The lateral branch selected to become the new top must be at least one-third the diameter of the leader being removed.
This ratio helps prevent dieback and discourages the formation of multiple, weak, vertical shoots, often called water sprouts. While this cut successfully reduces height, it permanently alters the tree’s natural pyramidal form. The tree will no longer grow vertically at the same rate, instead becoming bushier at the top.
If a significant height reduction is necessary, perform the process gradually over two to three years. Removing a small amount of height annually, always cutting back to a strong lateral branch with green foliage, allows the plant time to recover. This measured approach minimizes shock and makes the resulting flat top less noticeable.
Safe Strategies for Reducing Width and Outward Growth
Controlling the width and density of an arborvitae is accomplished through shearing and selective pruning. Shearing involves using hedge trimmers to lightly cut the tips of the foliage, which is suitable for maintaining a formal hedge shape. This method encourages the exterior foliage to become denser.
For a substantial reduction in width, selective pruning is necessary. This technique involves reaching inside the canopy with hand pruners and cutting longer branches back to a side branch closer to the center. The goal is to make the cut deep inside the shrub while ensuring green foliage remains on the outer surface to cover the cut branch.
When reducing width, maintain a slightly narrower top than the base of the plant. This shape allows sunlight to reach the lower branches, preventing them from becoming thin and bare due to shading. Failing to maintain this wider base will cause the lower foliage to thin out and expose the woody center.
Timing and Essential Post-Pruning Care
The optimal time for performing a severe size reduction is in late winter or early spring, just before the plant breaks dormancy. Pruning during this window allows the plant to use stored energy for recovery and new growth as the growing season begins. This timing maximizes the plant’s ability to compartmentalize the wound and fill in the pruned areas.
Avoid making significant cuts in late summer or fall. Pruning then can stimulate a flush of tender new growth that does not have enough time to harden off before the first hard frost. This soft growth is highly susceptible to winter damage, which can weaken the entire plant.
Following a severe cut, providing adequate moisture is important for recovery. The arborvitae’s reduced root-to-foliage ratio makes it susceptible to drought stress, so consistent watering is necessary during dry periods. Applying a balanced, slow-release fertilizer in the spring after pruning can support new growth and aid recovery.