Sunscald and Frost Cracks: Why Calgary Bark Splits Open in Winter

A long vertical split running down the sunny side of a trunk, or a patch of dead, sunken, peeling bark that was not there last fall, catches a lot of Calgary homeowners completely off guard. There was no impact, no lawnmower, no obvious cause, and yet the bark is clearly damaged. The answer, almost every time, is winter sun rather than summer heat, which is why it feels so mysterious.

Sunscald and frost cracks are close cousins, both driven by the same brutal temperature swings that define a Calgary winter. They look different, one is dead bark and the other is a split, but the underlying cause is the same. And the encouraging part is that both are largely preventable once you understand the simple physics behind them.

The Physics of a Winter Bark Split

On a cold but brilliantly sunny winter day, the sun heats the south and west side of a dark trunk enough to wake up the living tissue just beneath the bark, even while the air stays below freezing. That tissue comes partly out of dormancy and loses its cold hardiness. Then the sun drops behind a fence, a house, or the horizon, and the temperature at the bark crashes back to deep cold within minutes.

That freshly active tissue, caught completely off guard by the sudden refreeze, is killed. Repeated across a winter of sunny afternoons, it leaves an area of dead, discoloured, sunken bark, and that is sunscald. Frost cracks are the related event: the outer wood of the trunk contracts faster than the inner wood as the temperature plunges, and the resulting stress splits the trunk open, sometimes with a sound like a rifle shot.

Chinooks Turn It Up to Eleven

This is yet another tree problem that our chinooks make dramatically worse, and it is worth appreciating just how extreme the swings can be. A single midwinter chinook can push a shaded Calgary street from bitter, deep cold to shirtsleeve warmth and then back to bitter cold again, all within one day. That whiplash is precisely the sequence that damages bark.

Cities further east that settle into a steady, stable cold for the winter see far less of this kind of injury, because their trees are not repeatedly warmed and refrozen. Calgary's dramatic day-to-day and even hour-to-hour temperature swings are exactly what set our trees up for sunscald and frost cracking, which is why local trees need local precautions rather than generic advice.

Which Trees and Which Trunks

Young, thin-barked trees are by far the most vulnerable, because their bark does not yet insulate the living tissue underneath very well. The usual Calgary casualties are newly planted maples, lindens, fruit trees like apple and pear, mountain ash, and other smooth-barked species. Older trees with thick, corky, deeply furrowed bark are much better protected and rarely suffer.

The damage nearly always appears on the south or southwest face of the trunk, the side that catches the low winter sun as it swings across the southern sky. That predictable location is a useful diagnostic clue in itself. If you find a split or a dead patch on the shaded north side of a trunk instead, something other than sunscald is usually going on and it is worth investigating separately.

Preventing the Damage

The cheapest and most effective fix is almost embarrassingly simple: a light-coloured trunk wrap, or a coat of diluted white latex paint, applied in late fall. The pale colour reflects the winter sun so the bark never heats up in the first place, and if it never heats up, it never crashes. That one step interrupts the whole warm-then-refreeze cycle before it can start.

Wrap young and thin-barked trees for their first several winters, until the bark has thickened enough to protect itself. Just make a point of removing the wraps every spring, because a wrap left on through the growing season traps moisture against the trunk and can shelter insects and disease, trading a winter problem for a summer one.

When the Bark Is Already Damaged

If you already have sunscald or an established frost crack, resist the strong urge to paint, seal, tar, or fill the wound. It feels helpful, but it is not. Trees close over damage naturally from the edges inward through their own compartmentalization, and wound sealants tend to trap moisture against the wood and actually slow that natural healing rather than speeding it up.

Instead, keep the tree healthy and well watered so it has the energy to wall off and grow over the damaged area, and keep an eye out for decay or insects moving into the exposed wood. Aardvark Tree Care can judge whether a given split is merely cosmetic or a genuine structural concern, and help you protect your young trees properly before the next Calgary winter does the very same thing all over again.

The winter sun warms the bark, then vanishes behind a fence, and the tissue dies in the sudden refreeze.