Arborvitae (Thuja) are popular evergreen conifers valued for their dense foliage. Understanding their mature width is fundamental for successful planting projects. Failing to account for the ultimate lateral spread leads to overcrowding, restricting air circulation and sunlight, and compromising the plant’s health. The final size is determined by genetic makeup, environmental conditions, and active management techniques.
Width Varies by Cultivar
The mature width of an arborvitae is primarily dictated by its specific cultivar, which determines its genetic potential for lateral growth. Cultivars range dramatically from narrow columns to wide, globe-shaped shrubs, a difference that can span many feet.
For instance, the popular ‘Emerald Green’ arborvitae (Thuja occidentalis ‘Smaragd’) is prized for its slender, pyramidal form, reaching a mature width of only three to five feet. This narrow habit makes it an excellent choice for tight spaces or formal hedges. In contrast, the hybrid ‘Green Giant’ (Thuja plicata x standishii) possesses a vigorous growth habit, often spreading between 10 and 18 feet at full maturity.
Dwarf varieties like ‘Hetz Midget’ develop a dense, rounded or globe shape. These smaller shrubs typically mature to a width of three to five feet, making them suitable for foundation plantings or rock gardens. Selecting the appropriate cultivar based on its maximum genetic width is the first step in ensuring a healthy, proportionate landscape.
Environmental Factors That Affect Spread
While genetics establish the potential mature width, external growing conditions determine whether a plant will reach that full size. Sunlight is a significant factor, as arborvitae grown in full sun (at least six hours of direct light daily) will develop denser, wider canopies. Trees planted in excessive shade often become sparse, with reduced lateral growth as the plant struggles to reach for light.
The quality of the soil and water availability also directly influence the plant’s ability to expand. Arborvitae thrive best in consistently moist, well-drained soil; poor drainage can stunt growth and lead to root issues. Drought stress limits the energy a tree can dedicate to lateral branch extension. A plant with ideal conditions will reach its maximum width potential more quickly and maintain a fuller, healthier spread.
Pruning Techniques for Width Management
Active pruning is the primary method for controlling or reducing the lateral spread of an arborvitae. There are generally two approaches: shearing and selective thinning. Shearing involves trimming the outer layer of foliage to maintain a formal, geometric shape, which is common for hedges.
Selective thinning is a more precise technique where individual branches are cut deeper within the canopy. This reduces overall bulk and allows more light and air into the plant’s interior. When pruning for width, it is essential to only cut into the green, live foliage and avoid the brown, interior wood.
Arborvitae wood lacks dormant buds in this “dead zone,” meaning cuts made into older, leafless wood will not regenerate new growth, resulting in permanent bare patches. Major structural pruning to reduce width is best performed in late winter or early spring before the new season’s growth begins.
Calculating Proper Planting Spacing
The mature width of the chosen cultivar is the direct calculation point for determining correct planting spacing. The desired outcome—whether a continuous hedge or stand-alone specimens—will dictate the final distance between trees. To create a dense, uniform privacy hedge, arborvitae should be planted at a distance equal to approximately half of the cultivar’s mature width. For example, a cultivar with a mature width of four feet should be planted two feet apart to ensure the branches form a solid, continuous screen.
This close spacing results in faster visual privacy but requires more frequent light shearing to maintain the desired width. For stand-alone specimens, the trees should be spaced at their full mature width, or slightly more, to allow each plant to fully develop its natural shape. This wider spacing also ensures ample air circulation, which is important for preventing fungal diseases.