Sunscald and frost cracks

What you are looking at

Two injuries that turn up on the same trees for the same underlying reason, and that most people meet in spring rather than in the winter that caused them.

The first is a patch of bark on the trunk that has gone sunken and dull, often running a metre or more up the stem and confined to one side. Through spring it dries, cracks and starts lifting at the edges, and by summer it has fallen away to leave bare wood. If the patch runs most of the way round, the branches above it wilt in the first hot spell.

The second is a split. A clean vertical crack up the trunk, sometimes short and sometimes several metres, occasionally opening with a report loud enough to hear from indoors on a very cold night. Old ones close over into a raised ridge of callus running up the stem, and reopen in the same place winter after winter.

Both fall on young trees with thin smooth bark far more than on old rough-barked ones, and both fall on freshly planted and recently pruned trees hardest of all.

Winter sun, not winter cold

The patch is on the side of the trunk that catches the low afternoon sun in winter. In the northern hemisphere that is the south-west, which is why North American growers call it southwest injury. South of the equator it is the north-west. Either way it is the side that gets the last and warmest light of a short day.

On a clear, still, freezing day, dark bark facing that sun absorbs enough heat to sit ten or even twenty degrees above the air temperature. The living cambium beneath it warms with it, comes out of dormancy over a few hours, and loses the cold hardiness it had built up through the autumn.

Then the sun goes behind a cloud, or behind the horizon, and the temperature of that strip of bark collapses back to the air temperature within minutes. The tissue is caught deacclimated and it freezes. Ice forms inside the cells, the cells burst, and a strip of cambium dies.

So the damage needs a bright winter rather than a severe one. A grey cold winter does far less of this than a sunny cold one, so the injury bites harder in continental climates with clear winter skies than in cloudy maritime ones at the same latitude.

Why the trunk splits

A frost crack starts from a different mechanism and often on the same side.

Wood shrinks as it cools, and the outer wood of a trunk cools faster than the core. On a night when the temperature falls sharply, the outside is contracting around an inside that has not caught up, and the tension builds until the wood fails along a radial line. That is the crack, and the bang.

The failure almost always starts at a weak point. Old pruning wounds, branch stubs, dead knots and last year’s sunscald are where they begin. A crack that has opened once is a permanent weakness, so the same trunk splits in the same place each hard winter and lays down a thicker rib of callus each summer trying to close it.

Trees carrying a frost rib often live for decades with it. The risk is what gets in through the wound, since decay fungi and wood-boring beetles both use it, and a rib on a large tree near a building is worth having someone qualified look at.

The trunk that was shaded until last year

A separate case catches people out in summer rather than winter. Bark that has grown up shaded, whether by the tree’s own lower branches, by a neighbouring tree or by a hedge, has never developed the thickness and pigmentation that exposed bark has.

Take that shelter away in one go, by felling the neighbour or pruning the tree up into a clean stem, and the newly exposed bark scalds in the first hot bright spell. The damage looks the same and arrives in July.

Fruit takes the equivalent injury. Apples, tomatoes and peppers suddenly exposed by heavy summer pruning or by a collapsed plant develop pale, papery, sunken patches on the side that faces the sun. That is the same failure in thinner tissue, and the answer is the same: do not strip the cover off in one go.

What to do about it

Leave the lower branches on a young tree for its first few years.

Wrap or shade the trunk of a newly planted thin-barked tree through winter, or paint it white, and take the wrap off in early spring.

Plant in spring on a cold site and water into the autumn.

Stop feeding by midsummer so the wood hardens before the cold.

Take a shelter belt down over several seasons rather than in one winter.

Mulch the root area and keep the mulch clear of the bark.

Pare loose dead bark back to firm tissue with a clean knife and shape a smooth vertical ellipse, and use no wound sealant.

Leave frost cracks alone and keep the tree well watered and mulched.

Check the wound margins each spring for borer holes and bracket fungi, and have a large split assessed.

What it gets mistaken for

Apple canker sinks the bark in rings around a wound or a bud and carries small red or white fruiting bodies through the winter.

Bacterial canker on cherry and plum runs amber gum out of the dead margin.

Coral spot dots dead wood with pink cushions, and it moves into sunscald damage rather than causing it.

Rabbit, deer and vole damage strips bark in a band at a fixed height, right round the stem, with tooth marks or shredded fibres.

Strimmer damage sits at ground level, is ragged, and goes round rather than up.

Lightning strips a narrow spiral of bark from the crown to the ground and usually splits the wood behind it.