Arborvitae, often mistakenly called “cedars” in Minnesota, are a popular choice for creating dense, evergreen privacy hedges. These hardy trees, particularly varieties of Thuja occidentalis, provide year-round screening against the backdrop of a harsh climate. Because of Minnesota’s extreme seasonal temperature swings, timing the trimming process is paramount to the plant’s health and survival. Pruning at the wrong time can stimulate vulnerable growth that cannot withstand the intense cold, turning a lush green hedge into a brown, damaged eyesore by spring.
Spring and Early Summer: Routine Maintenance Trimming
The best time for routine trimming and light shaping of arborvitae is after the threat of hard frost has completely passed, which typically means late spring, around May, and continuing into early summer. This timing allows the plant to immediately begin sealing the minor wounds created by the pruning cuts. Arborvitae are continuous growers, meaning they will push new foliage throughout the warm season, making this the ideal window for maintaining their desired shape and density.
This type of light maintenance should focus on shearing the soft, new growth at the branch tips. Removing the outer few inches encourages the tree to branch out and become denser. A second light shearing can be performed in mid-summer, generally by early July, to clean up any rapid growth.
Arborvitae do not readily sprout new growth from old, brown wood. Therefore, routine shaping must never cut back past the current year’s green growth and into the woody interior. Cutting into this dead zone will create permanent, unsightly brown voids that the tree cannot fill in.
When to Schedule Major Size Reduction
When an arborvitae hedge has become significantly overgrown, requiring a substantial reduction in height or width, the timing must shift to the dormant season. Heavy pruning, which involves removing large sections of a branch, is best performed in late winter or very early spring, typically between March and April in Minnesota. Pruning while the tree is fully dormant minimizes the physiological stress on the plant, as it is not actively expending energy on growth.
The immediate benefit of dormant pruning is that the arborvitae will direct its stored energy into recovering and producing new growth as soon as the spring growing season begins. This allows the plant the maximum amount of time to heal and recover from the severe cuts. For significant reductions, remove no more than one-third of the plant’s overall size in a single season to prevent excessive stress.
Major cuts should follow the branch structure to minimize the appearance of the pruning, and they should be made just above a lateral branch or bud. While arborvitae are generally resilient, hard pruning into old wood should still be avoided if possible. If necessary, performing this reduction during dormancy gives the best chance for recovery.
The Critical Danger of Late-Season Pruning in Minnesota
Cutting arborvitae late in the growing season, roughly after mid-August, is the most significant pruning error a homeowner can make in Minnesota. This timing directly conflicts with the tree’s natural cycle of preparing for the extended northern winter. Any pruning after this point, and definitively by September 1st, is highly discouraged.
Pruning stimulates the arborvitae to produce a flush of tender, new foliage in response to the injury. If this growth occurs too late in the year, the new tissue does not have enough time to undergo the process of “hardening off.” Hardening off is the physiological change where plant cells prepare for freezing temperatures by reducing water content and increasing solutes.
When the first hard freeze arrives, this unhardened, water-rich new growth is extremely susceptible to freeze damage and desiccation, or “winter burn.” The tips of the branches turn a characteristic reddish-brown color, which remains visible until the following spring. Avoiding late-season trimming ensures the foliage that will face the intense winter conditions is mature and properly prepared to survive the cold, drying winds and sub-zero temperatures.