All oaks native to North America and Europe have a spiral of overlapping scales on the cup. Some (but not all) East Asian oaks have scales that form concentric rings instead. These are called ring-cupped oaks, and belong to the Cyclobalanopsis subgenus.
The acorns of some species have parallel ridges (striations) running between the two points. This can vary within one species, so a smooth acorn isn’t always diagnostic.
The size and shape of the cup is also useful, and you can easily eyeball it by comparing it to the nut. For example, the Northern red oak has a cup that sits flat at the top of the nut, while the overcup oak and bur oak wrap almost the entire nut in a cup. The length of the stalk the acorn grows on can also help identification.
Woolly: long, matted hairs. Some species only have hair near the tip of the acorn, so check there. Pubescent: short, fine hairs. Glabrous: smooth.
Here, we’re talking about the white and red oak categories. These include the specific species “white oak” and “red oak,” but there are many more possibilities. Most acorns lose their cup before sprouting. The Asian ring-cupped species are the main exceptions. [10] X Research source
Central and Eastern United States: East of Minnesota in the north and Texas in the South. Florida is included, but many species do not grow there. Western United States: This covers the entire Midwest and Pacific coast.
White oak (Quercus alba): short, light grey cup with warty scales; cup covers about ¼ of nut Chinkapin oak (Quercus muehlenbergii): thin cup with fine grey hairs and moderately warty scales; cup covers ¼ to ½ of nut Scarlet oak (Quercus coccinea): glossy, dark reddish-brown cup; nut has a blunt tip Willow oak (Quercus phellos): shallow, flattened cup with hairs inside and out; nut is less than ½" (13 mm) long Northern red oak (Quercus rubra): cup scales are reddish-brown, hairy, often with dark margins; inside of cup is smooth or has ring of hair around the scar; nut may have grey stripes Shumard oak (Quercus shumardii): Similar to Northern red oak, but scales often have pale margins; some have a deeper, bowl-shaped cup, but not all
Bur oak (Quercus macrocarpa): The largest acorns on the continent, about 1. 5 inches (4 cm) long with a very deep cupule that covers at least half of the nut Water oak (Quercus nigra): shallow cup with fine hairs; round, blackish nut Southern red oak (Quercus falcata): thin, reddish-brown cup with fine hairs; nut sometimes have lines and hairs near tip. Pin oak (Quercus palustris): thin, smooth, reddish-brown cup; light brown nut is often striped, and can be either round or ovoid Post oak (Quercus stellata): thin cup with grey, hairy scales; light brown nut ¾" (19 mm) long or less, sometimes with faint dark brown stripes; nut can be round or ovoid Black oak (Quercus velutina): reddish-brown, hairy cup with a knob on top and a fringed edge; hair inside cup as well; light reddish-brown, ovoid nut with faint stripes
California black oak (Quercus kelloggii): found throughout the state. Nut usually at least 1 inch (2. 5 cm) long, cup varies but scales at margin are usually loose and may be warty[15] X Trustworthy Source University of California Division of Agriculture and Natural Resources Extension program of the University of California system devoted to educating and improving local communities Go to source Coast live oak (Quercus agrifolia): found along the coast. Variation makes identification by acorn tough, but cup scales have loose tips and are never warty. [16] X Research source [17] X Trustworthy Source University of California Division of Agriculture and Natural Resources Extension program of the University of California system devoted to educating and improving local communities Go to source Blue oak (Quercus douglasii): found mostly in north and central California. Cup is extremely shallow with thin scales. [18] X Trustworthy Source University of California Division of Agriculture and Natural Resources Extension program of the University of California system devoted to educating and improving local communities Go to source Engelmann oak (Quercus engelmannii): found in the extreme south. Light brown, oblong nut. Cup has grey hairy scales, warty at the fringe
Bur oak (Quercus macrocarpa): Found in the northeast Great Plains. 1. 5 inch (4 cm) acorns with a cup that covers more than half the acorn. Post oak (Quercus stellata): Found in the Cross Timbers region. No more than ¾" (19 mm) long, light brown; grey, thin, hairy cup Blackjack oak (Quercus marilandica): Also in the Cross Timbers. Narrow nuts about ½" (13 mm) wide, half covered by the cup.
Oregon white oak / Garry oak (Quercus garryana): The only oak in Washington state, and the most common one in Oregon. Nut is large (1 inch / 2. 5cm+) and cup is shallow with yellowish or reddish brown scales. California black oak (Quercus kelloggii): found in southern Oregon. See California oaks above. Canyon live oak (Quercus chrysolepis): found in southern Oregon. Its acorns are extremely variable, so treat it as a “catch all” for this region. (Also present in CA and NM, where it is easily confused with the numerous other species. )
