Confusion often reigns when it comes time to identify the leaves of the many species of hickory that occur in North America. This is because to the untrained eye, the leaves of the multiple types of hickory look the same, despite there being many differences. Identification of the leaves belonging to the hickories is manageable when you carefully inspect the foliage and look for specific aspects of it.
The leaves of hickory fall under the classification of being odd-pinnately compound. This means that rather than a single bladed leaf, like in species such as maple, oak or birch, each hickory leaf consists of an odd number of smaller leaflets arranged upon a large central stem. The leaflets grow opposite each other on the stem, with the exception of one that grows at the very end of the long stem.
The overall size of the central stem, known in botanical terms as a rachis, is one way to help identify the various hickory leaves. Those of the pignut hickory, for instance, will be from 6 to 10 inches in length. "Trees of North America" notes that the rachis of the shagbark hickory can grow as long as 14 inches. Rachises of this length are common, with some being much longer, such as the potentially 22-inch long rachis on a shellbark hickory.
Leaflet length is another facet of the hickory leaf that allows you to recognize those of one species from those of another. The leaflets at the end of the rachis, the one away from where the rachis attaches to the branch, are typically larger than those closest to the branch. The "odd-man-out" growing at the end of the rachis is usually the biggest. Considering a few hickories for leaflet size, the leaflets of the swamp hickory vary in length from between 2 inches long to 5 inches long, states the "National Audubon Society Field Guide to Trees." Black hickories leaflets can attain lengths of 6 inches, while those of the mockernut hickory may be as long as 8 inches.
The number of leaflets growing on a hickory leaf's rachis is an important key in identifying the species. Some hickories have as few as five leaflets on this centralized stem, including shagbark and shellbark hickory. Others have as many as 11 or 13, such as bitternut hickory and water hickory respectively.
The hickory leaves are all some shade of green, with shellbark hickory, for instance, being a shiny dark shade of green above with a more pale hue on the undersides. The Ohio Department of Natural Resources website reports that the shagbark hickory leaf's color falls between a medium tint of green to a mixture of yellow and green. Some hickories can change to attractive colors come autumn, making them useful as ornamentals. Swamp hickory may turn golden bronze, while bitternut hickory leaves frequently change to shades of yellow. All hickories are deciduous, raining down their foliage to the earth before winter, which in turn can create quite a mess under the tree.