The arborvitae, a popular evergreen (Thuja genus), is frequently used for privacy screens and hedges due to its fast-growing, dense foliage. When these conifers become overgrown or exceed a desired height, homeowners often “top” them to manage vertical growth. This involves cutting the main terminal leader to stop upward expansion. Successfully reducing the height requires careful timing, the right technique, and subsequent shaping.
Understanding the Impact of Topping
Topping an arborvitae differs fundamentally from pruning a deciduous tree. Arborvitae, like many conifers, do not possess dormant buds on old, leafless wood. This means any cut made into the brown, interior section of the plant will not regenerate new foliage. The height reduction is permanent, as the plant cannot grow a new central leader once the existing one is removed.
This procedure should be reserved for essential height control or repairing storm damage, not for routine shaping. Topping immediately changes the tree’s natural pyramidal or columnar form, resulting in a flat, unnatural top. The plant will continue to grow outwards from the highest remaining side branches, creating a boxy profile that requires ongoing maintenance.
Preparing for the Cut: Timing and Tools
The most appropriate time for major height reduction is during the dormant season, typically late winter or early spring before new growth begins. Pruning then minimizes stress and allows the plant the entire growing season to recover and seal the wound before winter. Avoid significant cuts in late summer or fall, as this stimulates tender new growth that may not harden off before cold weather.
Use the correct tools to ensure clean cuts, which is important for the plant’s health. For the main leader on larger trees, a sharp hand saw or pole saw is necessary. For surrounding side branches, use sharp bypass loppers or hand pruners, depending on the branch diameter. Safety is important when working at height, so use a stable, well-secured ladder or elevated platform to reach the terminal growth.
Step-by-Step Guide to Reducing Height
The first step is to clearly determine the final desired height, understanding that this decision is irreversible. The cut must be made only into the younger, green-foliage-bearing section where live lateral branches are present. Cutting into the bare, brown wood of the tree’s interior will create a permanent, dead patch that will not fill in.
Locate a healthy, vigorous lateral (side) branch positioned just below the intended final height. This chosen branch will effectively take over the role of the terminal leader, dictating the tree’s future vertical growth. Make the actual cut cleanly and precisely just above the junction of this selected lateral branch and the main trunk.
When making the final cut, angle it slightly away from the chosen lateral branch. This angle encourages water to shed away from the wound, promoting faster healing and reducing decay. If the arborvitae has multiple leaders, select the strongest and most centrally located one to become the new top, removing the others to maintain a single growth point.
Post-Topping Care and Shaping
After the main leader is removed, the arborvitae will initially have a noticeably flat top. To encourage the selected lateral branch to become the new leader, you may need to train it vertically. Gently tie the branch to a temporary stake secured to the main trunk to help restore a more natural, upright profile over time.
Consistent maintenance of the sides is necessary for long-term health. The plant must maintain a shape that is wider at the base than at the top. This tapered form is necessary because it allows sunlight to penetrate down to the lower branches. If the top becomes too wide and shades the lower canopy, the bottom branches will thin out and die.
Following the stress of a major cut, support the plant’s recovery with proper hydration and nutrients. Ensure the arborvitae receives deep watering during dry periods to keep the soil consistently moist. Applying a balanced fertilizer in the early spring can help provide the necessary energy for recovery and new growth.