The Northern White Cedar, Thuja occidentalis, is a highly popular evergreen in North American landscapes, often known by its trade name, Arborvitae. Its dense, scale-like foliage and naturally pyramidal or columnar shape make it an excellent choice for privacy screens and formal hedges. While this species is relatively low-maintenance, regular pruning is necessary to control its size, maintain a dense form, and promote overall health. Proper pruning ensures light penetration to the interior of the plant and prevents the development of unsightly brown patches.
Optimal Timing for Pruning Northern White Cedar
The timing of pruning should align with the cedar’s natural growth cycle to minimize stress. The primary window for heavy or structural pruning is in the late winter or early spring, just before new growth begins. Pruning during this dormant period allows the tree to recover quickly once the growing season starts.
A secondary, lighter pruning for maintenance and aesthetic shaping can be performed in mid-summer, typically from late June to August. Summer pruning encourages new growth to fill in the cuts and helps maintain a tidy shape. Only trim back the current season’s new growth during this period. Avoid pruning too late in the season, such as late fall, as this stimulates tender new growth that may not harden off before frost. This vulnerable growth is susceptible to winter damage.
Essential Pruning Techniques and Tools
The primary rule when pruning Northern White Cedar is to never cut into old, bare wood, also known as the “dead zone.” Thuja occidentalis does not readily develop new buds on old stems. A deep cut into brown wood results in a permanent bare spot that will not regrow. All cuts for size reduction must be made back to a lateral branch or a visible green bud.
There are two main methods of pruning: shearing and thinning. Shearing involves using hedge trimmers to cut all the branch tips back to a uniform plane, which is the preferred method for maintaining a formal, dense hedge. When shearing, the tree should be shaped so that the base is slightly wider than the top, creating a trapezoidal shape. This shape ensures that sunlight reaches the lower branches, preventing them from becoming sparse and brown.
Thinning involves selectively removing entire small branches deeper within the canopy, cutting them back to a main branch or the trunk. This technique is beneficial for allowing light and air to penetrate the interior of the tree. This promotes inner foliage health and reduces the likelihood of disease. When reducing the overall size of the plant, known as heading back, never remove more than about 20% of the plant’s total height in a single season to avoid stressing the tree.
Dead, diseased, or damaged branches should be removed at any time of the year to maintain plant health. Dead wood appears brown and brittle and should be cut back to the nearest healthy, green growth or to the branch collar. For tools, sharp hand pruners are suitable for smaller twigs, while loppers handle thicker branches. Hedge shears, manual or motorized, are useful for maintaining large hedges. Always clean your pruning tools with rubbing alcohol before and after use to prevent the spread of plant pathogens.
Post-Pruning Care and Health
Immediately following a major pruning session, provide adequate care to help the Northern White Cedar recover. If pruning occurred during a dry period, especially in summer, ensure the tree receives sufficient water. Deep, thorough watering mitigates the stress of the cuts and supports the energy needed for new growth.
Monitoring the plant for signs of stress or pest activity is necessary after pruning, as newly cut areas can be entry points for diseases or insects. If you choose to fertilize, use a balanced, slow-release fertilizer. Avoid applying high-nitrogen fertilizers right after heavy pruning, as this can force a flush of weak, rapid growth vulnerable to damage. A layer of mulch around the base of the tree helps conserve soil moisture and moderate soil temperature, further aiding recovery.