Walking in an oak forest in the autumn brings you into contact with plenty of acorns. Black, brown and green, fuzzy and smooth, elongated and stout, acorns display different characteristics, depending on the oak tree species. According to the University of Tennessee Extension, you can tell red from white oaks -- the two types of oak trees -- based on the fuzziness inside the cup: Red oaks have it, white oaks don't. Other acorn characteristics can help you go even further toward identifying oaks.
What distinguishes the bur oak (Quercus macrocarpa) acorn from other acorns, according to the Ohio Public Library Information Network, is the fringe that you find on the edge of its goblet-shaped cup. This fringe occurs on the outside of the cup and can be so dramatic that the acorn appears to be wearing a fuzzy cap that covers most of the acorn. The bur oak produces the largest acorns of any North American oak, measuring 1-1/2 inches in diameter, according to the Living Lands and Waters' Milliontree project. If you need other clues, look to the trees' leaves, which have rounded lobes, typical of the white oak group to which this species belongs. One end fans out, narrowing as it moves toward the stem. Although not among the tallest of oaks, the bur oak develops an impressively wide branch spread.
Northern Red Oak
In the red oak group, the northern red oak (Quercus rubra) forms egg-shaped acorns that can exceed an inch in length. The Virginia Department of Forestry describes them as reddish-brown with a blunt tip. The bowl-shaped cup covers less than one-third of the acorn and contains tightly overlapping scales. Sinuses between the lobes on the leaves do not extend more than halfway to the center of the leaf, and the lobes reach a sharp point characteristic of red oaks.
The white oak (Quercus alba) also drops acorns that often exceed an inch in length and form an egg shape. The Virginia Department of Forestry also describes the white oak's cup as containing "warty" scales covered with small hairs. The bowl-shaped cup covers one-third or less of the nut, according to the Peterson field guide "Eastern Trees." The white oak acorn begins green and matures to brown or gray. If you can find a leaf with the acorn, you can identify the white oak by its deep sinuses and evenly sized, rounded lobes.
Swamp White Oak
Some acorns you find lying on the ground have a piece of stem that remains attached to the top of the cup. You can distinguish the acorns of the swamp white oak (Quercus bicolor) by these stalks, which are up to 3 inches long. In addition, the bowl-shaped cup with scales and fine hairs covers just under half of the acorn. Swamp white oak acorns often occur in pairs, according to the Milliontree Project. Leaves of the swamp white oak lack the dramatic lobes characteristic of oak trees, appearing wavy instead, and leaves are silvery and hairless on their undersides.
Eastern Black Oak
A member of the red oak family, the eastern black oak (Quercus velutina) produces small, round acorns that reach about 3/4 inch in length. Large, rough scales cover the bowl-shaped cup, along with fine gray hairs. The acorn has a sharp point on the uncovered end, and the cup encloses about half of the acorn. Like other red oaks, the eastern black oak produces lobed leaves that reach sharp points.