The ash trees (Fraxinus) are part of the Olive family and share similar characteristics that you can use to identify them. However, even though most of the ashes in the United States have compound leaves--a series of smaller leaflets arranged around a central stem--the different species are often hard to distinguish from each other. You must consider the many aspects of the ash trees and look at them carefully to recognize the differences that distinguish one type of ash from another.
Gauge the heights of ash trees. Some species, such as the black ash tree, grow as high as 90 feet tall. Others are barely bigger than a shrub, with the singleleaf ash a good example. This ash tree rarely grows higher than 20 feet.
Measure the length of the ash leaf's rachis to determine the species to which it belongs. You will measure from where the rachis attaches to the branch all the way to its end. The central stem of the green ash averages between 6 and 9 inches in length. Those of the pumpkin ash can grow to 18 inches in length. Velvet ash, a species of the Southwest, has small leaves with a rachis that rarely gets longer than 6 inches.
Measure the length of the leaflets attached to these longer leaf stems. These leaflets also vary in size between ash tree species. You will discover that the leaflets on a white ash are about 3 to 5 inches long, while those on a pumpkin ash can reach lengths of 8 inches. Texas ash, with 1 to 3 inch long leaflets, has some of the smallest leaflets of the American ash trees.
Examine the shapes of the leaflets to discern the ash species. The white ash has oblong leaflets with a point at one end and a rounded base. Those of green ash are not as wide as white ash and the base is not as rounded. The shape of the leaflets on Texas ash is more oval and the point is not as distinct as other ash types. The leaves of singleleaf ash, which are not compound leaves but instead are a single simple blade, are broadly oval, making them nearly round.
Inspect the samaras of the ash trees. The samara is the fruit of the ash tree. It resembles an insect wing with a thin membrane and contains the seeds of the tree. The samaras hang off the ash trees after evolving from the flowers, floating down to the ground in late summer or the beginning of the autumn. The elongated samaras of the white ash are between 1 and 2 inches long. The samara of a black ash is from 1 to 1.8 inches long, according to the "Trees of North America" field guide. Carolina ash has 3-inch long samaras and the ones on a singleleaf ash are rounded and less than an inch in length.