Applying a white coating to the lower trunk of trees is a proactive horticultural measure used by arborists and orchardists. It supports the health of young and thin-barked trees, particularly in climates with harsh winters or intense sun exposure. The material used is specific because applying the wrong product can severely damage or kill the tree by sealing the bark and preventing necessary gas exchange. Understanding the purpose reveals what kind of coating is required to be effective and harmless.
Horticultural Reasons for Trunk Coating
The main reason for applying a light-colored coat is to prevent sunscald, or southwest injury. This damage occurs during late fall and winter when the dark bark absorbs solar radiation on sunny days, causing the temperature of the underlying cambium layer to rise significantly. Temperatures on the south or southwest side of a trunk can be much warmer than the air temperature.
When the sun sets, the warmed tissue quickly cools, sometimes below freezing, causing the activated cells to burst. This fluctuation creates vertical cracks or splits in the bark, allowing entry points for diseases, fungi, or wood-boring insects. The white coating reflects sunlight, minimizing temperature swings and keeping the bark surface cooler and more stable.
The coating also offers secondary protection against pests. The physical barrier can deter wood-boring insects from laying eggs. It can also discourage rodents like rabbits, mice, and voles from gnawing on the bark. Protecting the bark is important because damage that extends all the way around the trunk, known as girdling, interrupts the tree’s ability to transport nutrients, which can lead to tree death.
Selecting Non-Toxic, Breathable Coatings
The material of choice for safely coating tree trunks is a water-based, 100% acrylic latex paint. This paint is non-toxic and remains permeable, allowing the bark to continue breathing and exchanging gases with the atmosphere. A white or light color is advised because of its superior ability to reflect sunlight and heat away from the trunk.
The paint must be diluted with water before application to prevent it from creating an impermeable film that seals the bark. A common and effective ratio is one-to-one (1:1), mixing 50% paint and 50% water. This mixture maintains the reflective properties needed for sunscald protection while lowering the concentration of ingredients that might interfere with bark function.
Avoid oil-based paints, solvent-based paints, or any paint containing volatile organic compounds (VOCs). These products form a hard, non-porous seal that prevents the bark from respiring, which can suffocate the living tissue underneath and cause severe harm. Specialized commercial tree paints are also available, often formulated with kaolin clay or natural oils, designed to be breathable and provide sun protection.
Preparation and Application Guidelines
The tree trunk requires minimal preparation before applying the reflective coating. The surface should be free of loose debris or large flakes of detached bark, but aggressive scrubbing is unnecessary and can damage healthy bark. The best time to apply the coating is in the late fall or early winter, just before the cold season when the risk of sunscald is highest.
The goal is to cover the vulnerable sections of the trunk with a thin, even coat of the diluted white mixture. Coverage should extend from the soil line up to the first major scaffold limbs or branches. This area is the most susceptible to temperature fluctuations and sun exposure, especially the south and southwest sides of the trunk.
Apply the paint using a paintbrush, roller, or sprayer, avoiding thick drips or pooling that could compromise the coat’s breathability. A single, well-applied coat is sufficient to provide protection throughout the winter season. If the tree has existing damage or wounds, avoid painting directly over them unless advised by a professional arborist, as sealing these areas can trap moisture and hinder healing.