What causes tree bark to split during New Brunswick winters?

Tree bark splitting during New Brunswick winters is primarily caused by frost cracking — a phenomenon where rapid temperature fluctuations cause the bark and outer wood to expand and contract at different rates, creating vertical splits that can run several feet along the trunk. This is especially common on the southwest-facing side of trees, where winter sun warms the bark during the day to well above freezing, then temperatures plummet rapidly after sunset — a cycle that NB's continental-influenced climate produces regularly from December through March.

Young and thin-barked tree species are most vulnerable in New Brunswick. Maples (particularly sugar and red maple), fruit trees (apple, cherry, plum), lindens, willows, and mountain ash are the most commonly affected across the province. Trees planted in exposed locations without wind protection or in urban settings where reflected heat from buildings intensifies daytime warming are at highest risk. New transplants and trees under 10 years old suffer most because they have not yet developed thick, insulating bark.

Frost cracks typically appear as long vertical splits on the south or southwest side of the trunk, sometimes accompanied by an audible crack during sudden temperature drops. The wound may partially heal during the growing season, forming a callus ridge along each side of the crack, but often re-opens the following winter in the same location, creating a recurring wound that can persist for the life of the tree. While frost cracks are unsightly, they rarely kill an otherwise healthy tree — though they do create entry points for fungal diseases and wood-boring insects.

Prevention is straightforward and highly effective. Wrap the trunks of vulnerable trees with commercial tree wrap (white or light-coloured plastic tree guards or crepe-paper tree wrap) from late October through late April. The wrap reflects winter sun and moderates temperature swings, dramatically reducing the risk of cracking. This is especially important for the first 5-7 years after planting. Apply wrap from the base of the trunk to just above the first major branch. In Fredericton, Woodstock, and other inland NB locations (Zone 4a-4b) where winter temperature swings are most extreme, wrapping young trees should be considered essential fall maintenance.

If a frost crack has already occurred, resist the urge to fill it with pruning sealer or tar — research has shown these products actually slow healing and can trap moisture and disease organisms inside the wound. Instead, keep the tree healthy and vigorous through proper watering, mulching (2-3 inches in a ring around the base, not touching the trunk), and appropriate fertilization so it can compartmentalize the wound naturally. If the crack is extensive, gaping, or showing signs of decay (soft, punky wood or fungal growth), consult a certified arborist ($150-400 for an assessment in NB) to evaluate whether cabling, bracing, or more involved treatment is warranted.

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