Oak trees have some features in common, including the fact that the trees have female and male flowers on the same tree, the leaves grow alternate on the twigs and branches, and the fruit is an acorn. While most American oak trees lose their leaves before winter, making them deciduous, others retain their foliage and are evergreen, usually in Southern states. Oaks of different species grow to varied heights and have leaves of diverse sizes and shapes. You can make oak tree identification a little easier by centering on the many facets of the tree.
Scrutinize range maps of different oak species to first determine which types of oaks grow in your area. Look at online sites such as the University of Connecticut Plant Database or books like the National Audubon Field Guide to Trees to see where in the United States certain oaks do and do not grow. For example, if you live in Maine, you will learn that species such as post oak are not native but northern red oak is.
Observe an oak to distinguish if it is a member of the red oak family or from the white oak family. Red oaks typically have blackish furrowed bark and leaves with bristles on the lobes. White oaks have light gray scaly bark and the lobes of the leaves lack the bristles common in red oaks. Once you determine what family of oak you have, you can then further narrow down the possibilities.
Study the leaves of an oak tree, looking at their size and overall shape to tell species apart. Take white oak as an example. This species has leaves with from five to nine lobes with round ends. These leaves are as long as 8 inches and sometimes as short as 4 inches. New spring leaves will have a shiny gray-green color while older summer foliage looks blue-green and is much duller.
Examine the acorns in oak trees. Focus on such features as their size and their caps. Overcup oak, for instance, is a species of oak that you can identify often just by its unique acorns. They will be as long as an inch, are nearly round and their cap encompasses almost the entire acorn, hence the tree's name.
Watch the colors of oak leaves in autumn. They can give you good clues to the identity of the species. Some will turn a deep red in autumn, like the scarlet oak. Others, such as laurel oak, change to yellow or red-brown in the fall before coming off the tree.