Most ash trees grow in the temperate climates of the Northern Hemisphere, and all are members of the genus Fraxinus, part of the olive family. Many types of ash trees grow in North America, in an array of sizes and in different parts of the continent depending on the species. The identification of ash trees can be difficult because the many species often have very similar features.
The Michigan State University Extension website says that one key to identifying ash trees is to always remember that the branches will grow opposite each other on the limbs. In North America, this occurs only on species such as ash, maple, dogwood and horsechestnut. Looking at an ash tree, you will notice how the branches develop on both sides of a limb, which is often easier to see when the trees are bare in the winter.
Ash trees feature compound leaves, another identifying aspect you may employ to recognize them. The leaves consist of a long stem (rachis) that will attach to the twigs. On this stem are several small leaflets that make up the compound leaf. The size of the rachis and the length and number of the leaflets vary among ash species. For instance, white ash has a leaf rachis as long as a foot, with from five to 13 leaflets, each between three and five inches long, attached to it. Black ash has a rachis as long as 16 inches and from seven to 13 leaflets, all about four inches long.
Female ash trees have unique fruit, which the Ohio Department of Natural Resources site says hangs in clusters. Called samaras, these fruits contain a seed that attaches to the tree and possesses a thin covering that has a wing shape. When the samaras fall from the ash trees, the breeze will spin them toward the ground beneath the tree or into the surrounding area. The male trees lack these samaras, but do develop flowers as the females do; the flowers of both sexes come out before the leaves but are small and inconspicuous.
The height of an ash species can sometimes help you to tell one from another where different kinds of ashes grow together. The white ashes and green ashes grow to maximum heights of around 80 feet, while blue ash seldom will get bigger than 60 feet. Texas ash grows to 50 feet, black ash to 90 feet and Carolina ash to 40 feet. Some of the smaller ash types are singleleaf ash at 20 feet high and velvet ash with a maximum height of about 30 feet.
Knowing what types of ash trees grow where you live in North America can be of great help in identifying these trees. In the eastern United States--from New England to northern Florida and westward to the Great Plains--you will find white ash and green ash sharing a similar range. Blue ash grows in the Midwest, black ash in the Northeast and pumpkin ash along the Atlantic Coast and along the coasts of Gulf states such as Alabama and Louisiana. Singleleaf ash, velvet ash, Texas ash and fragrant ash are all species located in the Southwest; Oregon ash grows on the Pacific Coast, according to the "Trees of North America" guide.