Painting the lower trunks of trees with a white coating, often called whitewashing or trunk painting, is a common horticultural technique used in orchards and newly planted landscapes. This practice is not merely decorative but serves multiple protective purposes, primarily safeguarding the tree from environmental stresses and biological threats. The application of a reflective white layer provides a low-cost, effective method for long-term tree health management.
Protection Against Sunscald and Temperature Fluctuation
The primary purpose of painting a tree trunk white is to prevent sunscald, a form of winter injury caused by severe temperature swings. Dark bark absorbs solar radiation, especially during clear, cold winter days when the sun is low. This absorption causes the bark’s surface temperature, particularly on the south and southwest sides, to rise dramatically. Temperatures can sometimes exceed 68 degrees Fahrenheit (20 degrees Celsius) above the ambient air temperature.
This rapid warming prematurely activates the thin, living layer beneath the bark called the cambium. When the sun sets, the bark temperature plummets quickly to below-freezing temperatures. This sudden shift causes the awakened cambium cells to freeze and contract, leading to vertical cracks or splits in the bark, known as frost cracks. Young trees with thin bark are particularly susceptible to this damage.
The white paint functions by significantly increasing the bark’s albedo, which is its ability to reflect light. White surfaces can reflect up to 70–90% of incoming solar radiation, preventing the trunk from overheating. By reflecting sunlight, the paint keeps the trunk’s surface temperature stable and closer to the surrounding air temperature. This minimizes the damaging freeze-thaw cycles that compromise the tree’s integrity. This reflective barrier is most beneficial for trees in exposed locations or newly transplanted trees lacking a full canopy.
Deterrence of Pests and Borers
Beyond thermal protection, the white coating offers a secondary defense against various biological threats, including insects and scavenging mammals. The paint creates a physical barrier that helps deter wood-boring insects, such as flatheaded borers or bark beetles. These pests seek out damaged or stressed trees to lay their eggs. A thick application makes it more difficult for these pests to penetrate the bark and infest the tree’s living tissue.
The bright color may also confuse certain pests naturally drawn to dark, textured bark. Some boring insects are believed to be attracted to the visual contrast of a dark trunk against a light background. The white surface disrupts these visual cues, making the tree a less appealing target for breeding or feeding.
The treated trunk also helps protect against damage from scavenging animals like rabbits and voles during winter. These animals often gnaw on the tender bark at the base of young trees when food sources are scarce. This gnawing can girdle the tree and disrupt the flow of nutrients. Adding a non-toxic repellent directly into the paint mixture provides an extra layer of deterrence against foraging damage.
Safe Materials and Application
The materials used for trunk painting are specific, as standard household paints are not suitable. Oil-based paints should never be used because they seal the bark completely, preventing the tree from exchanging gases and moisture. This sealing can suffocate the living tissues. The coating must be porous and non-toxic to ensure the tree can respire properly.
The recommended material is a water-based, breathable white latex paint, typically diluted with water. A common mixture involves combining equal parts of white interior latex paint and water (a 50/50 ratio) to achieve a porous, paintable consistency. Alternatively, a traditional lime-based whitewash, made from hydrated lime and water, is also used. Zinc sulfate is sometimes added to this mixture for fungicidal properties.
The application should focus on the lower trunk, the area most vulnerable to sunscald and pests. It is recommended to paint from the soil line up to the first scaffold branch, typically 18 to 36 inches high. Although sunscald risk is highest on the south and southwest sides, covering the entire circumference ensures complete protection against thermal stress and boring pests.