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On a clear winter day, the bark of a fruit tree can heat up 20 °C more than the surrounding air, and by evening cool down sharply. Such temperature fluctuations can damage the trunk, especially in young trees, which is why gardeners whitewash the bark before strong frosts arrive.
Agroforum reports this.
White trunks in autumn orchards are not just a tradition. The light coating reflects sunlight, causing the bark to heat up less during the day and experience smaller temperature swings at night.
The greatest danger occurs on sunny frosty days. The south and south-west sides of the trunk can heat up very much, even when the air temperature remains low.
When the sun sets or clouds cover it, the bark cools rapidly. Due to abrupt temperature changes, the tree tissues can be damaged, and longitudinal cracks may appear on the trunk.
Why tree bark cracks in winter
Under the influence of winter sun, the tree tissues can partially exit a state of deep dormancy. If after heating the temperature drops quickly, the cells become more vulnerable to frost damage.
Another cause of cracks is uneven cooling of the outer and inner trunk tissues. They contract at different rates, causing the bark to crack.
Such damage is called winter sunscald. It poses the greatest threat to young fruit trees, whose bark is still thin and smooth.
The risk also increases after heavy pruning. If previously shaded portions of the trunk or main branches end up under direct sunlight, they can heat up significantly more.
Why is the trunk whitened specifically white
The white color reflects a significant portion of solar radiation. As a result, the whitened bark heats up less than the dark surface of an untreated trunk.
Measurements confirm that white coating can substantially reduce the surface temperature of the tree. In a study on stone fruit crops, trunks coated with white latex paint were, under certain winter conditions, up to 30 °C colder than untreated ones.
This difference is not constant for all trees and weather conditions. Heating is influenced by the intensity of sunlight, wind, trunk diameter, and other factors.
Importantly, whitewashing does not protect against frost as such. Its main purpose is to reduce daytime bark heating so that it does not experience as sharp a drop in temperature in the evening and at night.
Does whitewashing help protect trees from spring frosts
Sometimes it is believed that whitened trees bloom later and, because of this, better tolerate spring frosts.
The white coating indeed can reduce the heating of the bark and cambium—the layer of tissues through which the tree thickens. There is data that under certain conditions this can somewhat delay spring development.
However, whitewashing does not guarantee later flowering and cannot on its own protect blossoms from frost. Its main purpose is to prevent winter damage to the trunk.
Does painting kill pests and disease agents
Whitewash is often credited with the ability to kill insects that overwinter in the bark and to protect trees from diseases. However, there is no convincing widely accepted evidence that ordinary whitewash can replace plant protection products.
Its benefit in this regard is mostly indirect. Sunscalds and cracks create damaged areas through which some pathogens can penetrate tree tissues.
If whitewash helps reduce the number of such damages, it may also lower the risk of secondary infection of the tree.
At the same time, white coating is not a universal remedy against pests and does not replace necessary garden care.
What to whitewash fruit trees with and when it is best to do it
Traditionally, a mixture of slaked lime and water is used for whitewashing. However, lime is caustic, so protective equipment is needed during preparation and application.
Some specialists recommend white latex-based paint for interior work, diluted with water, for trunk applications.
Oil-based paints are not suitable for this purpose. Exterior paints are also not recommended due to possible additives in their composition.
Apply the coating before the onset of strong winter cold, choosing a time when it can dry.
First of all, protect the south and south-west sides of the trunk, which are most affected by winter sun. Young fruit trees with thin bark should be evenly covered around the entire trunk.
If during winter rain or snow washes away the whitewash significantly, the coating can be refreshed as needed.
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