While it might seem easy to identify the leaves of poplar trees, the fact that many species of poplar exist in the United States complicates the task. Poplars are all part of the willow family and are called cottonwoods, after the cottony seeds that disperse from the trees in the summer months. You can recognize the leaves of the various types of poplar trees by looking at the many distinct traits of the foliage.
Study the size of the poplar leaves. Measure the leaves and determine how long they are at average and how wide the leaves are. You will notice differences between species in leaf size among the poplars that can aid you in identifying specific species. For example, the eastern cottonwood has leaves as long as 6 inches and as wide as 5 inches, while Fremont poplar is a kind of poplar with leaves as long as 2.5 and as wide as 3 inches.
Examine the shape of the poplar leaves you find. All poplar leaves are single-blade leaves, but some, like narrowleaf cottonwood, are longer like those of a weeping willow. The leaves of Fremont cottonwood have a triangular appearance; the leaves of black cottonwood possess a broad base and taper to a point so they resemble the head of a spear. Categorize these leaves by shape and you will be closer to identifying them.
Look at the colors of poplar leaves in the spring and summer months. Determine what shades of green they are on both the upper and lower surfaces of the leaf. Notice that some are two-toned, such as swamp cottonwood, which is darker green above but a duller pale green on the lower surface of the leaf, with a yellow midrib branching through it. Other poplar leaves vary in coloration, like the black cottonwood tree of the Pacific Coast. It has dark green on the upper surface, but can be pale green, rust-colored or silvery-white underneath.
Watch the fall foliage on poplars to see what colors the leaves change. Eastern cottonwoods turn to yellow in the fall months, as does Lombardy poplar, an introduced species from Europe. White poplar leaves go reddish in autumn.
Inspect the edges of the poplar leaves, looking to ascertain what types of serrations exist on them. All poplar leaves have "teeth" along their edges, but some, like Plains cottonwood and Eastern cottonwood, own teeth that are coarsely rounded, while others like the black cottonwood have teeth that are much less obvious.