Arborvitae (Thuja) are popular conifers used in landscaping for creating dense, year-round privacy screens or hedges. Their rapid growth frequently leads to them becoming overgrown, exceeding the desired height and width. Reducing the size of these large evergreens without causing permanent damage requires a specific, careful approach that respects the plant’s unique biology. The goal of a drastic size reduction is to restore a manageable size and shape without creating noticeable, unsightly brown patches.
Essential Timing and Necessary Tools
The most effective time for a major size reduction is during the late winter or very early spring, just before the plant breaks dormancy. Pruning during this dormant period minimizes stress and allows the plant to direct stored energy toward recovery and new growth. Avoid pruning heavily in the late summer or fall, as this stimulates tender new growth. This new growth will not have enough time to harden off before winter, leading to potential frost damage.
A successful pruning effort requires sharp, clean tools to ensure precise cuts that heal quickly and reduce the risk of introducing pathogens. Sharp bypass hand pruners are appropriate for general trimming and smaller branches. Loppers are needed for cutting branches up to an inch or two in diameter, and a small pruning saw may be necessary for removing thick central leaders. Sanitize all cutting tools with a disinfectant solution before beginning work to prevent the spread of plant diseases.
Understanding The Biological Limits of Arborvitae Pruning
Arborvitae pruning differs significantly from trimming deciduous trees because these conifers do not possess latent buds on their old, interior wood. This biological limitation means that once a branch is cut back past the green foliage and into the brown, woody “dead zone,” that portion will not regrow new green shoots. The resulting brown spot will remain permanently bare, as the plant cannot regenerate growth from that point.
To ensure the plant’s survival and subsequent regrowth, always make cuts into the green, actively growing tissue, or just above a junction where green growth is present. To maintain health and minimize stress, remove no more than 20 to 35 percent of the plant’s total foliage mass in a single season. If the arborvitae requires a more drastic reduction, spread the process out over two or three years rather than attempting a single, aggressive cut.
Executing the Size Reduction Pruning
Reducing the overall height of an overgrown arborvitae requires identifying the central vertical stem, known as the leader, and cutting it back to the desired final height. This cut, called a heading cut, should be made just above a healthy, vigorous lateral branch growing outward. Cutting to a side branch transfers the dominance of vertical growth to that lateral branch, which will become the new leader. Reducing the height in this manner ensures the cut is covered by existing foliage and prevents a flat, unnatural top from forming.
Managing the plant’s width is accomplished by maintaining a tapered shape. The base of the arborvitae must be slightly wider than the top, forming an “A” or pyramidal shape. This slight slope, ideally around a 5 to 10-degree angle, ensures that sunlight penetrates the lower branches, preventing the bottom sections from thinning out and turning brown.
For major width reduction, avoid simply shearing the outer layer, which only results in a dense shell of foliage that shades out the interior. Instead, make selective thinning cuts deep inside the canopy, targeting individual branches extending too far. Each thinning cut should remove a branch back to a smaller, healthy side shoot or a secondary branch growing in the desired direction. This method maintains the plant’s natural appearance while opening the canopy to light and air circulation, promoting denser growth.
Immediate Aftercare and Long-Term Shaping
Immediately following the major pruning, the arborvitae will be under stress, making proper aftercare important for recovery. The plant will benefit significantly from deep watering to rehydrate the remaining foliage and support the root system. Water the soil slowly and deeply around the base of the plant, especially if the weather has been dry, to encourage deeper root growth.
While the plant is recovering from a hard prune, it is generally recommended to avoid heavy fertilization, as this can force excessive, weak growth that may not survive. If the plant appears highly stressed, a light application of a balanced, slow-release fertilizer can be used, typically in early spring, following the manufacturer’s directions. Mulching around the base of the plant with a two- to three-inch layer of organic material will help retain soil moisture and regulate temperature.
In subsequent years, the arborvitae should only require light, annual maintenance to keep it at the reduced size. This ongoing care involves simply shearing or tip pruning the new, green growth that appears each season. Consistent trimming into this new growth will encourage the plant to branch out, resulting in a denser, healthier screen that maintains its shape without becoming overgrown again.