The sight of white paint covering the base of a tree trunk, often called whitewashing or trunk painting, is a deliberate horticultural technique. Used for centuries, particularly in orchards, this method safeguards vulnerable trees from environmental damage and pests. The application of a reflective white coating serves protective functions that contribute to the tree’s health and longevity. It is a simple, cost-effective way to manage environmental stresses that can weaken young or newly planted trees.
Protecting Trees from Sun Scald
The primary reason for applying a white coating is to prevent sun scald, or winter injury, which is a form of bark damage caused by rapid temperature fluctuations. Dark-colored bark absorbs significant solar radiation, especially during winter when the sun is low in the sky. This absorption causes the temperature on the sun-exposed side, typically the southwest, to rise dramatically, sometimes 15 to 20 degrees Celsius above the ambient air temperature.
This solar warming reactivates the tree’s cambium layer, the thin, living tissue beneath the bark, causing sap flow to begin prematurely. When the sun sets, the temperature quickly plummets back to freezing. This rapid contraction of active cells causes abrupt thermal stress, leading the bark to crack and split, creating a vertical wound.
The white paint acts as a reflective barrier, significantly increasing the bark’s albedo (light-reflecting capacity) by up to 70 to 90 percent. By reflecting sunlight, the coating keeps the bark temperature stable and cooler throughout the day. This moderation prevents the cambium from warming prematurely, eliminating the damaging freeze-thaw cycles that cause bark splitting. This protection is important for young trees with thin bark and recently transplanted trees.
Approved Materials and Application Methods
The material used for whitewashing must be carefully selected to ensure it protects the tree without causing harm. Standard, oil-based house paints are avoided because they form an impermeable layer that prevents the bark from respiring and exchanging gases, which can suffocate the tree. The coating must be breathable to allow the natural passage of moisture and air.
The most common modern material is diluted, water-based, white interior latex paint. A typical recommendation is a 1:1 mixture of paint to water, though ratios up to 1:4 are sometimes suggested to ensure permeability. This dilution creates a thin, reflective film that adheres well while still allowing the tree to breathe.
Alternatively, a traditional lime wash, also known as whitewash, is used, which is a mixture of hydrated lime (calcium hydroxide) and water. This traditional mixture is naturally porous and is often preferred by organic growers. Zinc sulfate is sometimes added for fungicidal properties. Application is typically done with a paintbrush for better penetration into the bark crevices, covering the trunk up to the lowest scaffold branches.
Secondary Uses and Contexts
Beyond thermal protection, the white coating offers several secondary benefits. The bright color can help deter certain pests, such as wood-boring insects, which are attracted to volatile compounds released by stressed trees. The paint is thought to mask the natural visual and chemical cues these pests use to select a host for egg-laying or feeding.
The physical presence of the paint can also act as a slight barrier against small gnawing animals like rabbits and voles, especially in winter. Gardeners sometimes mix a rodent repellent directly into the formulation for added defense. Furthermore, a lime-based wash can offer resistance against fungal and bacterial diseases, as the alkaline nature of the lime may inhibit pathogen growth.
In non-horticultural settings, white paint on tree trunks can serve as a visual marker. It is often used for boundary demarcation in forests, along municipal roads, or in parks. These markings indicate protected trees, serve as guideposts, or signal a boundary line. These non-protective uses are distinct from the primary agricultural purpose of safeguarding the tree’s health from sun scald and pest infiltration.