The practice of coating the lower trunks of trees with a white substance is a time-tested horticultural technique, not a purely decorative choice. This distinct visual marker, often seen in orchards and landscapes, is a simple, proactive measure focused on maintaining a tree’s health and longevity. The light-colored coating serves as a protective barrier against environmental stresses that can compromise the integrity of the bark. It is a low-cost, effective method for preventing damage, especially in young trees whose bark is thin and vulnerable.
The Main Purpose Protection from Sunscald
The primary reason for painting tree trunks white is to prevent sunscald, or southwest injury, which typically occurs during the winter months. Sunscald is damage caused by rapid and extreme temperature shifts on the bark of a tree. On a cold, sunny winter day, the dark bark facing the sun (often the south or southwest side) absorbs solar radiation and warms significantly. This warmth can activate living tissue just beneath the surface.
This warmth prematurely draws the tree’s cells out of dormancy, causing sap to flow in that localized area. When the sun sets or is rapidly obscured, the temperature of the bark plummets back down to freezing or below. The activated, non-dormant tissue is then flash-frozen and killed by the sudden, extreme cold. This results in dead, discolored, and sunken bark, which may eventually crack, peel, and form an elongated canker.
The white paint works by reflecting the intense winter sunlight away from the trunk, similar to how light-colored clothing reflects heat. By reflecting the radiation, the white coating keeps the bark temperature stable and cooler throughout the day. This prevents the premature activation of the underlying cambial tissue, ensuring the cells remain dormant and protected from the subsequent nightly freeze. This thermal regulation is beneficial for thin-barked species like fruit trees, maples, and lindens.
Secondary Benefits Pest and Disease Deterrence
Beyond thermal regulation, the white coating provides several secondary benefits that contribute to overall tree health. The physical application acts as a minor barrier against certain wood-boring insects, such as flatheaded borers. These pests are often attracted to stressed or damaged trees, and the reflective coating may help mask natural cues they use to find a host.
The coating can also help seal off small, pre-existing surface wounds or minor cracks in the bark. By covering these entry points, the paint reduces the likelihood of fungal spores and bacterial pathogens gaining access to the inner wood. Gardeners sometimes incorporate animal repellents into the paint mixture to discourage rabbits and rodents from gnawing on the bark during the winter months.
Materials and Best Practices for Application
Choosing the Right Material
Choosing the correct material is important, as applying the wrong type of paint can severely harm a tree. Standard household paints, especially oil-based or exterior paints, should be avoided completely. They form an impenetrable film that prevents the bark from exchanging gases, essentially suffocating the tree.
The appropriate material is a diluted mixture of water-based (latex) paint, often white interior latex paint, mixed with water in a ratio of about one part water to one part paint. Traditional whitewash, made from hydrated lime and water, is also an acceptable and time-honored option.
Application Guidelines
The paint should be applied to the lower trunk, extending from the ground up to the first set of main branches. It should cover at least the highest point that receives direct winter sun exposure. While the southwest side is the most susceptible area, full-trunk coverage is often recommended for maximum protection. The best time to apply the coating is typically in the late fall or early winter, ensuring the protective layer is present before the period of highest sunscald risk.