Arborvitae, members of the Thuja genus, are widely favored landscape plants, especially for creating dense privacy screens and hedges. Over time, however, these evergreens can lose their natural density, becoming sparse, thin, or “leggy,” which diminishes their appeal and screening capability. Restoring a healthy, full appearance requires a combination of precise mechanical actions and consistent environmental support. The process involves encouraging the plant to produce new growth where it is currently lacking.
Strategic Pruning to Encourage Bushiness
The most direct way to stimulate new, dense growth is through intentional pruning, which manipulates the plant’s natural hormone distribution. For arborvitae, the technique is to cut back the terminal ends of the branches to encourage lateral branching, which fills out the plant. This process is often called shearing, but it must be done carefully to avoid creating permanent bare spots.
When shaping the shrub, it is important to maintain a pyramidal or “A-shape,” where the base remains wider than the top. This specific contour allows sunlight to reach the lower branches, preventing them from thinning out and browning due to shading from the upper canopy. If the top is wider, the lower foliage will eventually die off, and this growth will not regenerate.
The most important rule in pruning arborvitae is to never cut into the “dead zone,” which is the old, brown wood that lacks green needles. Unlike many deciduous plants, arborvitae do not have dormant buds on this old wood, meaning a cut made here will not produce new growth, leaving a permanent gap in the foliage. Cuts should only be made where there is still green, active growth, ideally trimming back to a lateral branch or node.
Pruning in late winter or early spring, just before the new growth flush begins, is the best time for more significant shaping or size reduction. Consistent, light trimming of the branch tips throughout the growing season, removing no more than 20% to 35% of the plant’s foliage annually, will encourage continuous density. This consistent removal of the terminal buds forces the plant to divert growth hormones to dormant buds lower on the stems, promoting the desired bushiness.
Optimizing Soil and Nutrient Supply
Once new growth is stimulated through pruning, the plant requires adequate nutrition to fuel the vigorous development of new foliage. Arborvitae benefit from a slow-release fertilizer that is relatively high in nitrogen, such as a formulation with a ratio like 10-5-5 or 12-6-4, which supports lush, green leaf production. Applying a balanced fertilizer in early spring, before new growth appears, provides the necessary nutrients for the season ahead.
The soil composition itself also plays a role in the plant’s ability to absorb nutrients. Arborvitae prefer soil that is slightly acidic to neutral, generally within a pH range of 6.0 to 7.0. A soil test can confirm whether amendments are needed to adjust the pH or correct any specific nutrient deficiencies.
Applying a 2- to 4-inch layer of organic mulch around the base of the plant, keeping it a few inches away from the trunk, helps regulate soil temperature and conserve moisture. As the mulch breaks down, it provides a slow, steady release of micronutrients, further supporting the plant’s overall health. Avoid fertilizing in late summer or fall, as this can encourage tender new growth that will not harden off before winter, making it susceptible to cold damage.
Managing Common Stressors That Cause Thinning
Environmental factors often inhibit the plant from filling in or cause existing growth to thin out and brown. Proper watering is important, requiring deep and infrequent saturation rather than shallow, frequent sprinkles. Shallow watering promotes surface roots, making the plant highly susceptible to drought stress, which leads to browning.
Arborvitae need adequate sunlight, ideally a minimum of six hours per day, to maintain density throughout the canopy. If the interior or lower sections are thinning, it may indicate insufficient light penetration, requiring the removal of nearby shading branches or structures. In winter, browning can be caused by desiccation, often called “winter burn,” which occurs when the foliage loses moisture faster than the frozen roots can replenish it.
Specific damage from road salt or pet waste can cause localized browning and thinning. If salt is the cause, thoroughly rinsing the foliage and deeply watering the soil in the spring can help leach the sodium ions away from the root zone. Pests like spruce spider mites can also cause thinning, resulting in a fine yellowish speckling, or stippling, on the foliage that progresses to bronzing.
Spider mites thrive in hot, dry conditions, so maintaining proper watering helps to deter them. Confirming their presence can be done by tapping a branch over a white sheet of paper to check for tiny, crawling specks. If pests are identified, treating the plant with a horticultural oil or insecticidal soap can manage the infestation, allowing the arborvitae to dedicate its energy to producing dense, new growth.