Picture an oak tree, and you probably picture a large, majestic tree with distinctively shaped leaves. This describes many oak species, however, including the live oak (Quercus virginiana) and the water oak tree (Quercus nigra). To tell the difference between the two species, instead look at the tree's location, shape, trunk and leaves.
Location, Size and Shape
Live oak trees thrive in U.S. Department of Agriculture plant hardiness zones 7 through 10. Like many oak trees, they can live to very old ages -- hundreds of years, in fact -- and grow to great sizes. Live oaks in particular can grow to an average maximum height of 80 feet with a 100-foot spread. These trees are usually much wider than they are tall. Water oak trees, on the other hand, grow best in USDA zones 6 through 9 and have a maximum average height and spread of 80 feet; they are equally tall and wide.
So take a look around. Chances are, if your oak tree is in the Deep South, it's a live oak. If your oak tree is in the Southeast, Northwest or West, there's a higher probability of it being a water oak. Growing areas can overlap, however, so look at other characteristics as well.
Next, take a gander at the tree's trunk and branches. Mature live oaks have very distinctive, twisting branches and thick trunks. The branches often sprout from the trunk very close to the ground, twisting and turning as they reach up and out. These are the trees of the bayous, dripping with Spanish moss. Water oaks, on the other hand, usually produce tall, slender trunks, with much straighter branches.
Leaves, Acorns and Bark
For all its showy form, live oak trees drop the ball somewhat when it comes to their leaves, producing simple leaves that are green on top and paler beneath. Long and elliptical, they range in length between 2 and 5 inches. Most importantly, their appearance is uniform all over the tree. In the fall, the leaves turn dull brown and, in most climates, cling to the tree until pushed out by new leaves. In Northern climates, they often drop in the fall.
The water oak, on the other hand, is much different. Perhaps the most distinguishing feature of this tree is the fact that the shape of its leaves varies widely throughout the tree: from smooth-edged and oblong to lobed, bristle-tipped and spatulate. In short, if your tree has differently shaped leaves from one branch to another, it is probably a water oak. In the fall, the leaves turn bright yellow, providing a brief show of golden color, and fall from the tree.
Move a leaf aside, and you might see an acorn or two. Live oak acorns are about 3/4 of an inch long, and dark brown in color when they are ripe. Water oak acorns are a bit smaller, at 1/2 an inch long, with distinctive bands of black and brown wrapping around the nut. Both live and water acorns are topped with little caps covered with what looks like fish scales, but water oak acorns have shorter, wider caps.
Step even closer to the tree and take a look at the bark. Young live oaks have trunks striated with reddish-brown bark covered with small scales. As live oaks mature, the bark darkens almost to black and becomes more blocky. Water oak bark has fewer furrows than live oaks when young, and the furrows are smooth and dark. As the oaks mature, the bark develops rough patches here and there, eventually merging into large, rough, scaly patches.