The bark of a tree functions as its primary defense, a protective layer against environmental threats, injury, and disease. It is composed of outer layers that guard the living tissues underneath and the inner bark, which contains the phloem responsible for transporting sugars produced during photosynthesis. When this outer layer begins to peel, crack, or flake off, it naturally raises concerns about the tree’s health. While this shedding can be a sign of significant stress or damage, it is important to understand that for many species, peeling bark is a completely normal process of growth and self-maintenance.
When Bark Peeling is Natural and Expected
For certain tree species, shedding their outer bark is a genetically programmed feature that is not a cause for alarm. This process, known as exfoliation, is the tree’s way of accommodating trunk expansion and replacing old, rigid tissue with new, flexible bark. The appearance of this natural shedding is often uniform, occurring over the entire trunk and major limbs.
Trees like the American Sycamore shed large, irregular plates of bark, revealing a smooth, mottled pattern of inner bark underneath. The River Birch and Paperbark Maple exhibit a distinctive peeling that curls into papery sheets or flakes. The Shagbark Hickory is famous for its long, thick strips of bark that peel away, giving it a characteristic shaggy appearance.
A key indicator that the peeling is natural is the health of the tissue revealed beneath the shed layer. If the exposed inner bark is fresh, firm, and a light, healthy color, the tree is simply undergoing a normal developmental process. This natural exfoliation usually occurs without any accompanying signs of decline, such as yellowing leaves or branch dieback.
Environmental Stressors and Physical Injuries
When bark peeling is not characteristic of the species, it is often the result of abiotic factors, which are non-living environmental stresses or physical injuries. These events compromise the integrity of the bark and the delicate cambium layer directly beneath it, which is responsible for creating new wood and bark cells. Damage is frequently localized, rather than uniform, and can expose the underlying wood.
One common issue is sunscald, which typically occurs on the south or southwest side of thin-barked trees during winter or early spring. On sunny winter days, the dark bark absorbs solar radiation, activating dormant cambial cells. When temperatures drop rapidly after sunset, the active tissues are killed by the freezing cold. This leads to sunken, discolored areas that eventually crack and peel away.
Rapid temperature fluctuations can also cause frost cracks, which are vertical splits in the bark and wood of the trunk. This phenomenon happens when the inner wood shrinks faster than the outer bark, often following a sudden cold snap.
Mechanical damage is another frequent cause of peeling, resulting from impacts with lawnmowers, string trimmers, or construction equipment. These wounds strip the bark and cambium, creating entry points for disease and insects. The tree may peel the damaged tissue away in an attempt to compartmentalize the injury.
Biotic Causes: Pests and Pathogens
When peeling is accompanied by other symptoms like sawdust, oozing, or discoloration, the cause may be biotic, meaning it is due to living organisms such as insects or disease. These biological factors directly attack or indirectly stress the tree, leading to the failure of the bark tissue. The presence of these organisms often indicates a deeper underlying health issue.
Fungal or bacterial cankers are a major category of bark disease that result in localized, sunken, or discolored lesions. These pathogens invade the bark and cambium, often through existing wounds or points of stress, killing the tissue and causing the bark to separate and slough off. The resulting peeling may reveal dark, dead wood underneath and is often a sign that the tree is already weakened by other factors like drought or poor health.
Insect pests, particularly wood borers and bark beetles, cause peeling through their feeding and tunneling activities beneath the outer bark. These insects disrupt the phloem and xylem vessels, which are the tree’s vascular system for moving water and nutrients. Extensive tunneling separates the bark from the wood, leading to detachment and peeling, frequently accompanied by small entry/exit holes, sawdust-like frass, or sap flow.
Assessing the Severity and Seeking Help
When examining peeling bark, the depth and extent of the damage are the most important factors for assessing the tree’s prognosis. The cambium layer must be intact for the tree to heal. If the peeling exposes dry, dead wood and the cambium is completely destroyed, the tree cannot grow new bark in that area.
A particularly serious condition is girdling, which occurs when a wound or peeling encircles the entire circumference of the trunk or a large branch. Because the phloem tissue is located just beneath the bark, complete girdling cuts off the flow of sugars from the leaves to the roots, which is fatal to the tree. If the peeling covers more than 25% of the trunk’s circumference and the cambium is destroyed, professional help is generally recommended.
Practicing good cultural care, such as providing adequate water during dry periods and applying mulch properly to regulate soil temperature and moisture, can mitigate stress and increase the tree’s natural defenses. For extensive peeling, signs of active infestation, or if the diagnosis remains uncertain, consulting a certified arborist is the most reliable course of action. These professionals can accurately diagnose the underlying problem and recommend specific treatments, such as targeted pest control or structural support, to maximize the tree’s chances of recovery.