Arborvitae (Thuja) are popular landscape conifers valued for their dense, scale-like foliage and characteristic conical or columnar growth habits. These evergreens are frequently utilized for hedges, privacy screens, and as specimen plantings, providing year-round structure and color. While generally low-maintenance, regular shaping is necessary to maintain healthy density, control size, and preserve their desired appearance. Without routine attention, Arborvitae can become overgrown or develop a sparse look in the lower canopy.
Preparation: Timing and Tools
The optimal time for major shaping or heavy cuts is early spring, just before the plant breaks dormancy and begins its new growth cycle. Pruning then minimizes stress on the Thuja and allows the plant to quickly heal. Light trimming for size maintenance can also be performed from late spring to mid-summer, after the initial flush of growth has matured.
Avoid heavy pruning in late summer or fall, as this stimulates tender new growth vulnerable to winter damage. Extreme heat in mid-summer should also be avoided for significant cuts, as the resulting stress can lead to browning or sunscald. Essential tools for shaping include sharp bypass hand pruners for selective cuts, hedge shears (manual or electric) for formal shaping, and loppers for removing thicker branches.
Routine Pruning Techniques for Size Control
Routine pruning primarily involves cuts made within the plant’s green, active growth area to encourage branching and maintain a tidy form. For a precise, formal hedge appearance, shearing is the preferred technique, involving uniformly clipping the outer growth tips. Hand pruning, which involves selective cuts deeper inside the canopy, is better suited for maintaining a more natural, less rigid shape.
When trimming the sides, maintain a slight taper so the base of the plant remains wider than the top. This structural design, often a slight angle of 5 to 10 degrees, is important because it allows sunlight to reach the lower branches. If the top is wider, the lower foliage will be shaded out, causing it to thin and die, resulting in a sparse, woody appearance. Cuts should be made at the junction of new shoots, keeping the pruning within the green wood to ensure successful regrowth.
Addressing Overgrowth and Structural Damage
For Arborvitae that have become too tall, “heading back” can reduce the height by cutting the central leader. This cut should be made to a strong, healthy lateral branch, which becomes the new terminal point. Removing the central leader stops the upward growth of the main trunk and redirects the plant’s energy into lateral growth, often resulting in a bushier top.
Structural issues, such as branches damaged by heavy snow or wind, should be addressed by selectively removing the damaged material. Dead, diseased, or broken wood can be cut back to the main stem or a healthy side branch at any time of year to prevent the spread of pathogens and improve air circulation. Rejuvenation pruning, which involves cutting the plant back severely, is generally not recommended for Thuja species, as they often fail to recover or regrow evenly from drastic cuts into old wood.
Post-Pruning Safety and Care
The most important limitation when shaping Arborvitae is that they do not possess dormant buds on old, brown wood. Pruning cuts must always be made within the green foliage layer. Cutting back into the leafless, woody interior will result in a permanent, noticeable bald spot. Arborvitae are conifers, and their growth points are concentrated on the younger, green tissue.
After any significant pruning, especially during active growth, deep watering is recommended to help the plant recover from the stress of tissue removal. This practice encourages the roots to grow deeper, enhancing the plant’s overall stability and health. It is also beneficial to clean up all pruned debris from around the base of the plant, as this material can harbor pests or fungal spores.