Ash trees possess opposite compound leaves without teeth, winged seeds and gray grooved bark. They also have branches and leaves that lie directly across from one another on the main limb or branch, known as opposite branching.
The leaves of most ash trees are pinnately compound. Compound leaves are those leaves that consist of several leaflets. In ash trees, the leaflets number from five to nine. Pinnately compound leaves have leaflets that attach along the length of the leaf, rather than radiating from a single, central location as in palmately compound leaves.
The seeds of ash trees, known as samaras, are winged and clustered. Some people refer to these seeds as helicopters or keys. Ash bark is gray and appears smooth in young trees, but it develops ridges with age. The grooves of ash bark have a vaguely diamond-shaped pattern. The tips of ash limbs resemble pitchforks once the leaves drop in fall and winter.
The emerald ash borer is a nonnative, invasive beetle, native to Asia, that is detrimental to North American ash populations. The larva of the beetle live under the bark of ash trees, which typically die fewer than five years after establishment of larvae.
Mountain ashes, also known as rowans, are unrelated to true ash trees.