Arborvitae, belonging to the genus Thuja, are popular evergreen conifers appreciated for their rapid growth rate and thick, scale-like foliage. They are versatile, serving effectively as dense privacy screens, windbreaks, or standalone specimen trees. Understanding the specific cultural needs of these hardy evergreens is the foundation for maintaining their health and density.
Site Selection and Planting
The success of an arborvitae begins with selecting the right location. These conifers thrive best with at least six hours of direct sunlight daily, though partial shade is tolerated but may result in less dense growth. A well-draining soil structure is necessary to prevent root saturation, as standing water quickly leads to root rot and plant decline.
When planting, excavate the hole two to three times wider than the root ball, but only as deep as the ball itself. This width allows roots to spread easily into the surrounding loosened soil, promoting rapid establishment. Ensure the root flare remains slightly above or level with the native soil line after backfilling, and space plants according to their mature width for proper air circulation.
Routine Hydration and Feeding
Consistent moisture is crucial for arborvitae health, especially immediately following transplantation. Newly planted specimens require watering two to three times per week during the first growing season. Once established, arborvitae prefer deep, infrequent watering that saturates the top 12 to 18 inches of soil, encouraging roots to grow downward.
Drought stress, particularly in late summer and late fall, can impact the plant’s winter hardiness and foliage color. Monitoring soil moisture before the ground freezes is important, as a well-hydrated plant better withstands desiccating winter winds. Applying a two to three-inch layer of organic mulch across the root zone helps moderate soil temperature and retain moisture. Keep the mulch several inches away from the trunk to prevent moisture buildup and potential collar rot.
Arborvitae benefit from a light application of fertilizer in early spring as new growth begins. A balanced, slow-release granular fertilizer formulated for evergreens, such as a 10-10-10 ratio, supplies necessary macro-nutrients. Avoid over-fertilization, as excess nitrogen can lead to weak growth that is more susceptible to disease and pest infestation.
Managing Growth Through Pruning
Pruning arborvitae encourages dense growth and maintains the desired shape. The optimal time for this structural work is late spring or early summer, after the initial flush of new growth has hardened off. Lightly shearing the exterior foliage stimulates lateral branching, contributing to the plant’s thick, screen-like quality.
When shaping, avoid cutting back into the older, inner wood, often termed the “dead zone.” Unlike many deciduous shrubs, arborvitae will not generate new growth from brown, leafless wood. Cuts should be limited to the current season’s green growth, removing no more than one-third of the new growth at any one time.
If the goal is to reduce the overall height of a specimen, the central leader, which is the main vertical stem, can be cut back to a lateral branch. This technique redirects the plant’s energy into horizontal growth, effectively capping the height. Consistent, gentle shaping is more beneficial than aggressive, corrective pruning, which can leave the plant looking sparse and damaged.
Common Ailments and Winter Protection
Pests
Even with proper care, arborvitae can occasionally fall victim to pests. Bagworms are a common issue, identifiable by the distinctive, spindle-shaped silk bags they construct on the foliage, which house the feeding larvae. Small infestations can be managed through hand-picking and destroying the bags before the larvae hatch in the spring.
Another frequent pest, the Spruce Spider Mite, is a cool-season arachnid that causes stippling and yellowing of the foliage, often concentrated on the inner branches. Horticultural oils or insecticidal soaps are effective treatment options when applied according to label directions. Addressing environmental stress, such as “winter burn,” is equally important, as this browning is caused by frozen ground and desiccating winter winds drawing moisture from the needles.
Winter Protection
Preventing winter burn involves deeply watering the plants throughout the late fall until the ground freezes solid. Applying an anti-desiccant spray in late autumn can coat the foliage, reducing moisture loss. Physical protection from larger threats, such as deer browsing, is often necessary during winter months when food sources are scarce.
Defense against deer includes wrapping individual plants or hedges in burlap or netting, which creates a physical barrier. Heavy snow loads or ice accumulation can cause upright branches to splay outward, leading to structural damage. Tying the main branches loosely together vertically with soft twine helps the plant withstand the weight of precipitation.