The Green Giant Arborvitae (Thuja standishii x plicata) is an evergreen hybrid known for its rapid growth rate, reaching three to five feet per year once established. This pyramidal tree is highly valued for creating dense, year-round privacy screens and windbreaks due to its thick, dark green foliage. Regular pruning is necessary to control its height, maintain a uniform, dense shape, and promote overall health. Pruning encourages the plant to fill out, preventing thin, open growth.
Optimal Timing and Necessary Tools
The optimal window for major structural pruning is during late winter or very early spring, just before the first flush of new growth begins. This timing allows the plant to heal quickly and dedicate spring energy to producing new growth that will conceal the cuts. Maintenance shearing, if needed, can be done later in the season, such as mid-summer, once the initial spring growth has hardened off.
Avoid pruning in the fall or early winter because fresh wounds may not callus before freezing temperatures arrive. This vulnerability can lead to desiccation and tip dieback, increasing the risk of winter injury. Tools should include sharp bypass pruners for individual branches and manual or powered hedge shears for general shaping. Always wear protective gloves and goggles, and use a stable ladder for height safely.
Specific Techniques for Shaping and Height Control
Height control involves “tipping the leader,” which halts upward growth and redirects the plant’s energy laterally. The leader is the single, main vertical stem at the top of the tree; removing it caps the tree at its current height. Make a clean cut just above a strong, outward-facing lateral branch or bud at the desired height.
This strategic cut redirects apical dominance, causing the side branches immediately below the severed leader to grow more vigorously and become denser. For trees with multiple leaders, which are prone to breaking in heavy snow or ice, select one as the main leader. Cut the others back severely to subordinate them, then tip the main leader at the desired height.
Lateral shaping is performed by shearing the sides to maintain a tapered, pyramidal, or conical form. Ensure the base of the tree remains slightly wider than the top, creating a gentle slope from the ground up. This shape allows sunlight to reach the lower branches; if the top flares out, the lower foliage will be shaded and eventually die.
When shearing the sides, only remove a small amount of material, specifically no more than one-third of the current season’s new growth. This light, regular shearing encourages the formation of dense, tight foliage, which is the goal for a solid privacy screen. Consistent, shallow cuts are more beneficial than infrequent, deep cuts, which can expose the inner, bare wood.
Preventing Damage and Ensuring Healthy Recovery
The most important rule is to avoid cutting into the “old wood,” which is the interior section of a branch that has lost its green needles. Arborvitae species do not possess dormant buds on this older, leafless wood that can sprout new growth. Cutting past the green foliage into the brown, bare wood will result in a permanent, unsightly brown patch or dead area that will not regenerate.
When shearing, always aim to leave a layer of green foliage on the branch to ensure the plant can recover and continue photosynthesis. After pruning, especially if the weather has been dry or warm, providing a deep watering is beneficial for recovery. Adequate moisture reduces stress and helps the tree channel energy into healing the cuts and producing new, dense growth.