Ash trees (Fraxinus genus) are widely distributed across North America, but they face a severe threat from the invasive Emerald Ash Borer (EAB), which kills nearly all infested trees. Correct identification is crucial for preserving the remaining population and managing the beetle’s spread. The bark is often the most accessible and reliable feature, especially during the dormant winter season when leaves are absent. Understanding the bark’s texture, color, and how it changes with age is the first step toward effective monitoring and protection.
The Defining Pattern of Mature Ash Bark
The bark of a fully mature Ash tree, particularly White Ash (Fraxinus americana), develops a distinct and deeply furrowed texture that serves as a signature feature. This pattern is often described as a series of tight, interlacing ridges that form a recognizable diamond or “alligator hide” pattern along the trunk. The furrows are deep and tend to run vertically, crossing over each other to create the geometric, netted appearance.
The color of the mature bark is typically a dull gray or brownish-gray, though the precise hue can vary depending on the species and the presence of lichens. White Ash bark generally presents the most prominent diamond pattern, while Green Ash (Fraxinus pennsylvanica) has less pronounced furrows. These deeply etched characteristics are the most reliable visual cues for identifying a large, established Ash tree.
How Ash Bark Changes with Tree Age
Ash bark undergoes a significant transformation as the tree grows from a sapling to a mature specimen. Young Ash trees typically have bark that is relatively smooth, thin, and light gray or grayish-brown. This smooth bark may sometimes feature small, horizontal markings called lenticels.
As the tree trunk expands in diameter, the outer layer of bark begins to crack and split due to the internal pressure of growth. These initial cracks are shallow vertical fissures that start to break up the smooth surface. Over many years, these fissures deepen and intersect, gradually forming the complex, tight diamond-shaped pattern characteristic of mature Ash. This progression means that a medium-aged tree will show an intermediate bark texture.
Identifying Common Bark Look Alikes
Several other common tree species have bark that can be confused with Ash. Mature Black Walnut (Juglans nigra) bark, for instance, is deeply furrowed and dark, but its ridges are typically blocky and plated, lacking the tight, cohesive diamond cross-hatching of Ash. The vertical furrows on Black Walnut often appear wider and more separated than the interlacing pattern found on Ash.
Certain species of Maple, such as Sugar Maple (Acer saccharum), also develop deeply furrowed bark as they age. However, their fissures usually run in long, irregular vertical strips rather than forming a uniform geometric pattern. Norway Maple (Acer platanoides) bark can sometimes mimic the texture, but it often has shallower ridges. Shagbark Hickory (Carya ovata) is easily distinguished because its bark peels away in long, loose vertical strips that curl outward, a feature completely absent on Ash.
Bark Signs of Emerald Ash Borer Infestation
The presence of the Emerald Ash Borer (EAB) often leaves specific, observable signs on the bark that indicate an infestation. The most definitive sign is the D-shaped exit hole, created by the adult beetle upon emerging from the tree. These holes are approximately 1/8 inch wide, and the flat edge of the “D” shape is distinctive, distinguishing them from the round exit holes of native borers.
Another common symptom is “blonding,” which refers to patches of light-colored, stripped-off outer bark caused by woodpeckers foraging for EAB larvae beneath the surface. Infested trees may also exhibit vertical splitting or flaking of the bark, which occurs as the tree attempts to grow callus tissue over the feeding tunnels. If the outer bark is removed, the S-shaped or serpentine larval galleries, packed with frass (sawdust and excrement), become visible underneath the bark layer. Additionally, the tree may produce secondary shoots, known as epicormic sprouts or “water sprouts,” low on the trunk or at the base as a stress response to the damage.
Confirming Identification with Other Tree Features
Confirming an Ash identification requires checking a few other easily recognizable tree features. Ash trees are one of the few species with an opposite branching pattern, meaning branches, buds, and leaves grow directly across from each other on the stem. This opposing arrangement is most easily observed on the twigs and smaller branches.
The twigs feature distinct terminal buds that are often dark and conical, sitting just above a shield-shaped leaf scar where the previous season’s leaf was attached. White Ash leaf scars look like the letter “C” turned on its side, while Green Ash scars resemble a “D” shape. The leaves themselves are compound, composed of five to eleven leaflets arranged along a central stem.