Identify different types of acorns that denote different species of oak trees by looking at the size and appearance of the cupule, the length and diameter of the nut, and the color of the acorn. Also, look at the length of the stems upon which the acorns are growing and how many acorns are growing per stem.
The cupule or woody cup in which the nut grows can be smooth, spiny, or hairy and small or large. The nut itself varies in length and diameter with rounded or pointed ends. The acorn colors are different among different species or vary between the cupule and the nut. Some species grow in clusters, while others grow one per stem.
Blue oak acorns, native to California, are long with short, dark and smooth cupules and light-colored nuts with rounded ends. Also native to California, black oak acorns are round with pointed ends and large, hairy cupules. Laurel oak acorns grow two per stem with short reddish cupules and a ligh-colored nut.
The large 1.5-inch bur oak acorn has a fuzzy fringe on the deep cupule. The valley oat acorn grows up to 4 inches long and has a short cupule. Live oak acorns grow in large clusters. The 1-inch oblong nut is topped with a light brown cupule.