The Thuja, commonly known as arborvitae, is a popular evergreen choice for creating dense privacy hedges and ornamental plantings, valued for its scale-like foliage and year-round color. Trimming is necessary to maintain a desired shape and density, preventing the shrub from becoming overgrown and sparse. While arborvitae are resilient, they demand a specific pruning approach that differs significantly from many other landscape shrubs. Understanding the plant’s unique growth habit is the first step in successful maintenance.
The Critical Limitation of Arborvitae Pruning
The core difference between arborvitae and many other evergreens lies in the location of its regenerative growth capacity. Unlike plants that can sprout new growth from dormant buds along old, woody stems, arborvitae branches lack these latent buds on the older, leafless wood closer to the trunk. This creates an internal “brown zone” of permanently dormant tissue.
New growth, which is responsible for the plant’s dense outer appearance, only emerges from the green, actively growing tips of the foliage. Cutting back past these green needles and into the brown, woody interior will reveal a permanent bare spot. This exposed old wood will not produce new shoots to cover the bald patch, a condition sometimes referred to as “reversion.”
Removing too much of the green foliage, particularly more than one-third of the total canopy, can stress the plant by reducing its ability to photosynthesize. When pruning cuts are made deep into the brown zone, the resulting permanent patch can take years to fill in, if it ever does, by surrounding branches growing over the void. Therefore, all shaping and size-reduction efforts must respect this biological limitation by never cutting into wood that is devoid of green needles.
Seasonal Timing and Shaping Techniques
Timing is a significant factor in ensuring that pruning cuts encourage healthy, rapid regrowth. The optimal time for maintenance trimming is in the late spring or early summer, after the initial flush of new growth has hardened slightly. Pruning at this time allows the plant to quickly put out a second, smaller flush of growth, which helps heal the cuts and maintain a dense exterior.
Trimming later in the season, such as late fall, should be avoided for major cuts. Late pruning can stimulate new, tender growth that may not have time to harden off before the first frost. This soft growth is susceptible to winter damage, which can weaken the arborvitae and lead to browning. Removing dead, diseased, or broken branches can be done at any time of the year.
For routine maintenance and shaping, a technique known as “tipping” or light shearing is the safest method. This involves removing only the outer one-third of the current season’s new growth, which is identified by its lighter green color. This shallow cut encourages the remaining growing tips to branch out, promoting a denser screen while ensuring the cut remains within the regenerative green foliage.
When size reduction is necessary, especially for height or width, it must be performed gradually over multiple seasons. To reduce the plant’s height, the main leader can be cut back, but the cut must be made just above a healthy, well-placed lateral branch that can take over as the new terminal point. For width reduction, cuts should be made back to a healthy side branch or just above a tuft of green foliage, ensuring the cut is made into the green zone. Hand pruners are best for selective, precise cuts into green tissue, while sharp hedge shears are suitable for light, overall surface shearing.