There are approximately 400 species of oak trees worldwide, almost all of them in the Northern Hemisphere. Oak can be deciduous in winter or evergreen (live oak), which retains its leaves all year round. While there is a great deal of variation in the appearance of the leaves, bark, and various other features, all types of oak produce a type of nut called an acorn. Because acorns and their scales vary widely, often the acorn alone is sufficient to identify the species.
Part 1 of 2: Characteristics of the Acorn
Step 1. Check the acorn's cup scales
Acorns grow out of wood chips, which may remind you of a hat. The tiny scales that make up the cup may be thin and flat, or thick and form growing warts (tubercles). These variations can be an excellent start for narrowing down the species possibilities.
All oaks native to North America and Europe have scales that spiral in shape and intersect at the cup. Some (but not all) East Asian oaks have scales that form concentric rings. This kind of oak is called ringed oak, and belongs to the subgenus Cyclobalanopsis
Step 2. Look at the shape of the acorn
Acorns come in many forms, but can generally be divided into two groups. There are round (globose), or almost round with a blunt tip. The other group is elongated ("oval" or "longitudinal") and usually tapering to a point ("taper").
Acorns of some species have parallel serrations (striations) running between the two ends. One species can have various forms. So, smooth acorns can't always be diagnosed
Step 3. Check the color
Ripe acorns can be light brown, dark brown, black, or red. If it's still green or greenish-grey, chances are the acorn will fall off the tree first.
Step 4. Measure the nuts
Acorns range in size from tiny 1.25 cm long, to as big as the palm of your hand. Most acorns within a single species and region are approximately 1.25 cm long. One notable exception is the cork oak from the Mediterranean, which drops large acorns in the fall and smaller clusters during the winter.
- The shape and size of the cup is also useful, you can easily notice by comparing it to the beans. For example, North American red oak has a cup that sticks flat on top of the nut, whereas in overcup oak and bur oak almost all of the nut is covered with the cup.
- The length of the stalk where the acorn grows can also help with identification.
Step 5. Look at the hair
The beaks of some acorns have inner and/or outer surface hairs. You can also look for the inner surface hairs of the shell, after breaking them open. Botanists describe this hair-like material as follows:
- Wool-like hair, long, and tangled. Some species only have hairs near the tip of the acorn, so check there.
- Pubescent: short and fine hair.
- Glabrous: smooth.
Step 6. Look for germinated acorns
If an acorn in the ground shoots through its shell, it is certain that it is from a species that germinated at that time of year. In North America, oaks are divided into white oaks, which germinate in the fall just after the acorns have fallen, and red oaks, whose acorns have dormant and germinated in spring.
- Here, we will talk about the categories of white oak and red oak. This includes certain species "white oak" and "red oak," but there are more possibilities.
- Most acorns lose their cup before germination. The main exception is the Asian ringed Betta species.
Part 2 of 2: Identifying United States Common Oak Species
Step 1. If possible, obtain a local field guide
There are about 400 species of oak worldwide, and more than 200 of them are in North America. This guide covers only a few of the most common species in the United States. A tree identification guide for a region or state in the United States will help to identify species that are less common or confined to a narrower area.
Step 2. Select your region
This guide is broken down by region where oak is grown. Note that many species only grow in a narrow area of the region.
- Central and Eastern United States: Eastern Minnesota to the north and Texas to the south. Florida is also included, but many species don't grow there.
- Western United States: Covers the entire Midwest and Pacific coasts.
Central and Eastern United States
Step 1. Narrow the brown and longitudinal oak
The acorn has the classic oblong or ovoid shape. The chocolate is light brown or dark brown in color, with little or no red tinge. Here are the most common species that fit the description in this region and the characteristics that make them identifiable:
- White oak (Quercus alba): short, light gray beak with warty scales, cupola covering to of the nut
- Chinkapin oak (Quercus muehlenbergii): thin beak with fine gray hair and moderately warty scales. Cupcakes cover to of the nuts
- Scarlet oak (Quercus coccinea): glossy, dark reddish-brown cup, nut with blunt tip
- Willow oak (Quercus phellos): flat, shallow cup with hairs inside, nut less than 13 mm long
- Northern red oak (Quercus rubra): caudal scales are reddish brown, hairy, and often have dark margins, the inside of the lobe is smooth or has a ring of hairs surrounding some sort of streak, the nut may have a gray stripe
- Shumard oak (Quercus shumardii): Similar to Northern red oak, but cup scales have paler margins, some of which have deeper cupolas and are bowl-shaped, but not all
Step 2. Identify other acorns in the area
These various species produce acorns that are round or of different colors and shapes:
- Bur oak (Quercus macrocarpa): The largest oak in the Americas, about 4 cm long with a very deep cup covering at least half of the nut
- Water oak (Quercus nigra): shallow cup with fine hairs, round black oak
- Southern red oak (Quercus falcata): thin reddish-brown beak with fine hairs, sometimes the nut has stripes and hairs near the tip.
- Pin oak (Quercus palustris): thin, smooth, light brown cup and often striped, may be round or ovoid
- Post oak (Quercus stellate): thin beak with gray-haired scales, light brown nut 19 mm or less long, ovate
- Black oak (Quercus velutina): cupola is reddish-brown and hairy with knobs at the ends and tufted edges, hairs inside the cup as well, oval-shaped nut is light reddish-brown and has pale stripes
Western United States
Step 1. Learn about California oak
California is dominated by many species, and hybrids that occur naturally. Here are some of the most common in different parts of California:
- California black oak (Quercus kelloggii): found throughout the state. Pea length is usually at least 2.5 cm, cupola varies but scales at margin are usually loose and may be warty
- Coast live oak (Quercus agrifolia): found around coastal areas. The variety is quite difficult to identify by acorn, but the betta scales have loose ends and are never warty.
- Blue oak (Quercus douglasii): mostly found in northern and central California. The cup is very shallow with thin scales.
- Engelmann's oak (Quercus engelmannii): found in the southernmost regions. Peanuts are oblong and light. Bettas have gray hairy scales, warts on the edges
Step 2. Know the oaks of the Great Plains
Oaks are scattered throughout the region, but of course they are abundant in the forested subregion:
- Bur oak (Quercus macrocarpa): Found in the northeast of the Great Plains. The oak is 4 cm long with a cup that covers more than half of the acorn.
- Oak Post (Quercus stellate): Found in the Cross Timbers area. Not more than 19 mm in length, has a cup that is gray, light brown, thin, and hairy.
- Blackjack oak (Quercus marilandica): Also in Cross Timbers. The beans are about 13 mm wide, covering half the cup.
Step 3. Identify the oak in the Pacific Northwest
Much of the oak habitat has disappeared in this region. There are three species remaining:
- Oregon white oak / Garry's oak (Quercus garryana): The only oak in the state of Washington, and the most commonly found in Oregon. The beans are quite large (over 2.5 cm) and the cup is shallow with yellowish-brown or reddish-brown scales.
- California black oak (Quercus kelloggii): found in southern Oregon. See various California oaks above.
- Live Canyon oak (Quercus chrysolepis): found in southern Oregon. The acorns are so varied that they are treated as "all possible" for the region. (Also in California and New Mexico, where it is easy to confuse it with other species)
- When identifying oaks, you may encounter several trees and shrubs that are called oaks, even though they are not part of the same genus as oaks. Some of them have nuts that resemble acorns. Poison oak is not a variety of oak, but a variety of sumac.
- If you see an odd oak with lots of folds and knobs, it may have a swarm of wasps laying around in it.