Applying a white coating to the lower trunk of trees is a common technique in horticulture and orchard management. This visible white band is not decorative; it functions as a protective shield for the tree’s living tissues. The treatment, often called whitewashing or trunk painting, is typically applied to young trees with thin bark or newly transplanted specimens lacking a mature, insulating layer. This simple, non-toxic method guards against environmental stressors and biological threats, primarily through light reflection and temperature stabilization.
Preventing Temperature Damage
The main reason for coating a tree trunk in white is to prevent sun scald, also known as southwest injury. This damage occurs most frequently during winter when the sun is low and shines intensely on the bark. On cold days, the dark bark on the south or southwest side of the trunk absorbs significant solar radiation. This absorption causes the bark’s temperature to rise dramatically, sometimes 15 to 20 degrees Celsius above the ambient air temperature.
This solar heating can prematurely activate the underlying cambium layer, the thin living tissue responsible for the tree’s growth and nutrient transport. When the sun sets or a cloud passes, the bark temperature plummets rapidly, often dropping back below freezing. This sudden temperature fluctuation causes the activated cambium cells to freeze and rupture.
The resulting damage manifests as vertical cracks or splits in the bark, creating open wounds that struggle to heal. White paint mitigates this risk because of its high albedo, which measures how much light a surface reflects. The white surface reflects 70 to 90 percent of incoming solar radiation, preventing the dark bark from absorbing heat. By reflecting sunlight, the coating keeps the trunk temperature stable and closer to the air temperature, preventing damaging freeze-thaw cycles.
Deterring Pests and Small Mammals
Beyond temperature regulation, the white coating offers protective benefits against insects and small animals. The paint creates a physical barrier that discourages wood-boring insects from laying eggs or entering existing crevices. This is helpful because many wood-borers are attracted to stressed or damaged wood left vulnerable by sun scald injuries.
The thick, painted layer can also mask the chemical scents that pests use to locate a host tree. Some commercial or traditional mixtures contain additives that are naturally repulsive to insects, enhancing the repellent effect. This barrier helps maintain the integrity of the bark, which is the tree’s first line of defense against disease.
The paint application is also an effective deterrent against gnawing mammals like rabbits, voles, and mice, especially during winter. When natural food sources are scarce, these animals often resort to chewing the soft bark of young trees. The white paint, particularly when mixed with animal repellents, makes the bark unappetizing or abrasive, discouraging feeding. Preventing this feeding is crucial: if animals chew the bark completely around the trunk (girdling), it severs the phloem tissue, leading to the death of the tree above the damaged ring.
Selecting and Applying Tree Paint
For this practice to be beneficial, it is essential to use only materials safe for the tree. The most common material is water-based (latex) paint, which must be diluted to prevent the coating from being too restrictive. Oil-based paints, varnishes, or any paint containing volatile organic compounds must be avoided. These materials prevent the bark from exchanging gases and moisture, causing harm to the tree.
A typical, safe mixture involves diluting white interior latex paint with an equal part of water, creating a 50/50 ratio that allows the bark to remain porous. Traditional alternatives, such as a lime wash made from hydrated lime and water, provide the same reflective benefits. Some growers also mix one-third joint compound with the paint and water for a thicker, more durable coating.
The paint should be applied to the entire circumference of the trunk, starting from the soil line and extending upward. A common application height is 18 inches to 2 feet, or up to the lowest scaffold branches. The ideal timing for application is late fall or early winter, before the period of most significant sun scald risk. This ensures reflective protection is in place when the sun’s angle and cold temperatures pose the greatest threat.