Oak is any
of a large variety of trees or shrubs that bear acorns. There are more
than 600 species of oaks, all of which grow naturally only in the
Northern Hemisphere. Most species of oaks are found in tropical and warm
regions. Only a few species grow in Canada and other regions that have
short summers and long winters. Oaks thrive in forests throughout the
continental United States except for Alaska. They also are important in
forests of China, Japan, and central and southern Europe.
widely in size and in the way they grow. Some oaks never become taller
than small shrubs. Others reach heights of more than 100 feet (30
meters). Many oaks that grow in warm climates do not lose their leaves
in the autumn. They are called evergreens. But most oaks found in
regions with cold winters are deciduous--that is, they shed their leaves
each autumn. The leaves of many deciduous oaks turn beautiful colors,
such as deep red or golden brown, in early autumn.
spring, oaks produce small, yellowish-green flowers. The male flowers,
which form in dangling clusters called catkins, produce large amounts of
pollen. The wind carries the pollen to the female flowers and fertilizes
them. Once fertilized, a female flower will become an acorn. The acorn
is the fruit of the oak. Acorns vary in length from less than 1/2 inch
(13 millimeters) to more than 2 inches (51 millimeters).
slowly and usually do not bear acorns until they are about 20 years old.
But these trees live a long time. Most oaks live for 200 to 400 years.
Oaks are an
important source of lumber. Oak wood is heavy, hard, and strong, and it
has a beautiful grain. Manufacturers use it for furniture, barrels, and
railroad ties. The wood and bark of some oaks contain tannin, a bitter
substance used in the preparation of leather. Cork comes from the bark
of certain oaks. Acorns are an important source of food for wildlife. In
the past, many Indians of North America used acorns for food. Today,
oaks are grown as ornamental trees worldwide.
300 species of oaks are native to North America. Only about 60 of these
grow north of Mexico. Botanists divide oaks into two groups: (1) white
oaks and (2) red oaks, sometimes called black oaks. The trees in these
two groups differ chiefly in the shape of their leaves and in how long
it takes their acorns to mature.
have lobed leaves, and the tips of the lobes are rounded. Many red oaks
also have lobed leaves, but these leaves have pointed tips that end in a
hair like bristle. Some red oaks have unlobed, oval leaves with a sharp
of white oaks form in the spring, mature during the summer, and are shed
in the autumn. The acorns of red oaks need two growing seasons to
mature. In general, white oak acorns taste less bitter than do red oak
thrive throughout deciduous forests of eastern North America. A species
that is named the white oak is especially common. It is easily
recognized by its grayish bark, which forms shingle like plates about 20
to 30 feet (6 to 9 meters) up the trunk. Its leaves usually have seven
or nine lobes. The wood of the white oak makes excellent furniture.
other white oaks are important members of eastern deciduous forests. The
post oak grows in dry soils. It has gray bark and leaves with five
lobes. The overcup oak, the swamp chestnut oak, and the swamp white oak
are common in swampy areas. The leaves of these three species usually
have more than seven lobes. The bur oak ranges from the eastern part of
the Great Plains east to New Brunswick and Nova Scotia. This species
produces large acorns.
Only a few
species of white oaks grow in western North America. The California
white oak is common along streams and in moist valleys of California,
where it provides shade for livestock. This oak has leaves with 7, 9, or
11 lobes. It bears long, slender acorns. The Oregon white oak is found
in river valleys and warm, dry areas from central California to
southwestern British Columbia. The leaves of the Oregon white oak
resemble those of the California white oak, but it has much plumper
acorns. The Oregon white oak also grows as a low shrub along the Oregon
coast and in mountainous regions. This oak may be found as high as 4,000
feet (1,200 meters) above sea level.
Red oaks are
common in forests of eastern North America. The northern red oak grows
from the southern Appalachians north to Ontario and Nova Scotia, and
west into Arkansas and Missouri. It is often found along with two other
red oak species, the black oak and the scarlet oak. These three species
look similar and even experts sometimes confuse them. All three species
usually have leaves with seven or nine lobes. But the leaves of black
oaks have yellow hairs on the underside. Most scarlet oak leaves have
sharply pointed tips and deep notches between the lobes. These oaks are
an important source of timber and tannin.
oaks grow only in certain soils or locations. For example, the blackjack
oak grows in soils that contain much clay. This oak has pear-shaped
leaves with golden hairs on the underside. The turkey oak is found in
sandy soils of the Atlantic coastal plain. Its leaves have deeply
in the red oak group are often called live oaks. The live oak of the
Southern States grows in the Gulf and Atlantic coastal plains. Its
large, spreading branches are often draped with Spanish moss. In coastal
dune areas, these trees grow as shrubs. Live oaks are also common in dry
regions of Texas. The California live oak grows on hillsides along the
southern California coast, where little rain falls during the summer.
A number of
shrubby, evergreen oaks grow in dry areas of California and the
Southwest. The California scrub oak is found in regions that have
frequent wildfires. Some reach a height of only 3 to 4 feet (0.9 to 1.2
meters). The roots of this oak have many buds. Following a fire, these
buds sprout and grow into new shrubs. The shrub live oak is one of
several species of shrubby oaks found in the deserts of the Southwestern
United States and of Mexico. The shrub live oak is common on mountain
and northern Europe, two common deciduous trees are the English oak and
the sessile oak, also called the durmast oak. These two species of oaks
are easy to tell apart by the way their acorns grow. The acorns of
English oaks grow on long stalks. The acorns of sessile oaks attach
directly to twigs. Both of these oaks may grow to a height of more than
110 feet (34 meters). Some have lived more than 800 years. The wood of
these oaks is used for beams and shipbuilding.
deciduous species of oaks also grow in the mountain forests of southern
Europe. These include the Hungarian oak and the Pyrenean oak. Both are
used for ornament and shade in gardens worldwide.
grow in regions of southern Europe that have mild winters. The holm oak
looks much like the live oaks of the Southwestern United States. It is
an important timber tree and an excellent source of charcoal. The holm
oak is grown in parks and along roads throughout the world. The cork oak
is found chiefly in Spain and Portugal. Cork comes from the bark of this
species grow in the forests of central and eastern Asia. The Lebanon oak
is common in mountainous regions of the Middle East. The Mongolian oak
is an important timber tree in eastern Asia, including Japan. Both of
these species can be found in parks and gardens in the United States.
Classification. Oaks are in the beech family, Fagaceae, and make up the
genus Quercus. The scientific name for the white oak is Quercus alba,
the northern red oak is Q. rubra, the live oak of the Southern States is
Q. virginiana, and the English oak is Q. robur.
Norman L. Christensen, Jr., Ph.D., Dean, Duke Univ. School of the
See also ACORN