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Oak is part of the common name of about 400 species of trees and shrubs in the genus Quercus, from the Latin for "oak tree." The genus is native to the northern hemisphere and includes deciduous and some evergreen species extending from cold latitudes to tropical Asia and the Americas. Oaks can be long-lived (hundreds of years) and large (70 to 100 feet high) and are excellent wildlife feeders because of their production of acorns.
Oaks have spirally arranged leaves with lobed margins in many species. Other oak species have serrated (toothed) leaves or smooth leaf margins, which are called entire leaves.
Oak flowers, or catkins, fall in late spring. Acorns produced from these flowers are borne in cup-like structures known as cupules. Every acorn contains at least one seed (rarely two or three) and takes six to 18 months to mature, depending on the species.
Live oaks, which have evergreen or extremely persistent leaves, aren't necessarily a distinct group, as their members are scattered among the species below. Oaks can, however, be divided into red and white oaks, distinguished by the hue of the tight-grained wood when cut.
In summer, look for alternate, short-stalked, often lobed leaves, though they vary in shape. The bark is gray and scaly or blackish and furrowed. Twigs are slender with a star-shaped pith. Acorns, not all of which have caps, drop on the nearby ground over a month each fall. If a tree is stressed, it drops some acorns while still green during summer; if conditions aren't right for the tree to support all the fruit on its branches, it discards what it won't have enough energy to ripen.
You can identify oaks in the winter by the five-sided pith of the twigs; clustered buds at the tip of a twig; slightly raised, semicircular leaf scars where the leaves were attached to the branches; and individual bundle scars. In the South, live oaks and water oaks retain most of their leaves over the winter.
Red oaks commonly have generally symmetrical leaves at least 4 inches long with points to their lobes and veins that extend all the way to the edges. Indentations run the gamut, from dramatic to none at all. White oaks often have rounded lobes on their leaves and indentations that vary widely.
Here's more information on 17 common oaks:
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Black oaks inhabit the Eastern half of the United States except Florida and grow 50 to 110 feet tall, depending on location. They tolerate poor soils. Leaves are shiny or glossy with five to nine lobes that terminate in one to four teeth. Bark is dark gray to near black. Habitat is from Ontario, Canada, to the panhandle of Florida.
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Bur oaks extend from Saskatchewan, Canada, and Montana to Texas and grow up to 80 feet tall. They have wide crowns, though they're more shrubby at the northernmost and easternmost reaches of their habitat. They're one of the most drought-resistant oaks. Leaves are elliptical with five to seven rounded lobes. Scales where the acorn cap meets the nut form a fuzzy fringe. The cap covers half to most of the nut.
miguelvieira / Wikimedia Commons / CC SA 2.0
Fast-growing cherrybark oaks often reach 100 feet. Shiny, dark green leaves have five to seven lobes that spread at right angles from the center and end in one to three teeth. The acorn cap covers a third to half of the round nut. The tree grows from Maryland to Texas and from Illinois to the panhandle of Florida.
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Chestnut oaks easily reach 65 to 145 feet tall. Leaves hardly have indentations, looking almost serrated with 10 to 14 teeth instead of lobes. The acorn cap has gray scales with red tips, enclosing a third to half of an oval nut. The tree is found in rocky, upland forests and dry soil from Ontario and Louisiana to Georgia and Maine.
Laurel OakVeraOsco / Getty Images
Laurel oaks don't have typical "oak-looking" leaves; theirs are narrow blades, resembling those of its namesake, the laurel. Acorns on this large tree, reaching 100 feet high, are dark brown to black and only 1/2 inch long, with a cap that covers up to a third of the nut.
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Live oaks are evergreen, as their habitat is the South. If you've seen iconic images of huge trees in sandy soils draped in Spanish moss, you've likely seen live oaks. They can live hundreds of years and grow quickly when young, to 40 to 80 feet with a spread of 60 to 100 feet. They have short, skinny leaves and dark brown to almost black oblong acorns.
Northern Red Oak
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Northern red oaks grow from 70 to 150 feet tall and have red-orange, straight-grained wood. They're fast-growing, hearty, and tolerant of compacted soil. Leaves have seven to 11 lobes with one to three teeth and indentations less than halfway to the center. The acorn cap covers about half the oblong or oval nut. They grow from Maine and Michigan to Mississippi.
Overcup oaks are slow-growing and reach up to 80 feet. Dark green leaves are deeply indented and feature rounded lobes with one to three teeth and may be shiny. The underside is grayish-green with a whiteish bloom that comes off when rubbed. Acorns are light brown and oblong with a cap that covers most of the nut. The trees reside in poorly draining lowlands on the Southern coast and along rivers in the South and West.
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Pin oaks have downward sloping lower branches and grow 60 to 130 feet tall. Their inner bark is pink. Leaves have deep indentations and five to seven toothed lobes with one to three teeth. The acorn cap covers only a quarter of the round nut and has smooth scales.
Post OakDesign Pics / John Short / Getty Images
The slow-growing post oak can reach 50 to 100 feet. Its leaves have five to seven smooth lobes and indentations on roughly half. Round acorns have wart-like marks and caps that cover one quarter to two-thirds of the nut. The trees are found throughout the Deep South and beyond, extending from Texas to New Jersey.
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Scarlet oaks tolerate drought and grow best in sandy soil. Look for C-shaped indentations between the lobes, which vary in depth even on the same tree. Narrowest lobes will have teeth. They grow 40 to 50 feet tall and have hairless, glossy acorn caps and medium gray to dark, furrowed bark.
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Shumard oaks are among the largest Southern red oaks. They reach up to 150 feet and reside in well-draining soils near streams and rivers, Ontario to Florida to Nebraska and Texas. Leaves have five to nine lobes with two to five teeth and deep indentations more than halfway in. Caps cover up to a third of the oblong nuts.
Southern Red Oak/Spanish OakTheBigMK / Getty Images
Southern red oaks, sometimes called Spanish oaks, grow from New Jersey to Florida and west to Oklahoma and Texas, reaching 70 to 100 feet high. Leaves have only three lobes, not evenly spaced. The species prefers sandy soil. The rounded, brown acorn has a downy cap that covers up to a third of the nut.
Swamp Chestnut Oakcturtletrax / Getty Images
Swamp chestnut oaks grow from 48 to 155 feet high and prefer moist soils and well-draining floodplains in the central and Southern forests, from Illinois to New Jersey, Florida to Texas. Leaves are wide and wavy and look more like serrated leaves, featuring nine to 14 rounded teeth and a pointed tip. Acorns are brown and egg-shaped, with caps looking like bowls.
Water OakBriBar / Getty Images
Water oak trees mostly retain their leaves through the winter, as their habitat is in the Deep South, from Texas to Maryland. They're rapidly growing shade trees that can reach 100 feet high. Leaves are shaped more like neckties than the leaves of many other species that have indented, lobed leaves. Acorn caps cover up to only a quarter of the round nut.
White oaks are long-lived shade trees that grow to 60 to 150 feet tall. Leaves have rounded lobes, sometimes deeply indented, and are grayish-green and widest near the end. Acorn caps are light gray and enclose only a quarter of the light brown oblong nut. They're found from Quebec, Ontario, Minnesota, and Maine to Texas and Florida.
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Willow oaks' leaves don't look like what you might imagine "typical" oak leaves to be. They're thin and straight and only an inch wide, with no lobes. The trees grow up to 140 feet tall and are found by rivers, primarily in the Deep South. Dark-colored acorns have faint stripes.