Quercus marilandica (Blackjack oak) is a small oak, one of the red oak group Quercus sect. Lobatae, but fairly isolated from the others. It is native to the southern and central United States, with populations following the eastern seaboard north to Long Island.
It is a small deciduous tree growing to 15 m tall, with bark cracked into rectangular black plates with narrow orange fissures. The leaves are 7-20 cm long and broad, and typically flare from a tapered base to a broad three-lobed bell shape with only shallow indentations. They are dark green and glossy above, pubescent underneath, and often remain attached to the twigs through the winter after turning brown in the fall. The acorn is small, 12-20 mm long and 10-18 mm broad; like other red oaks, it takes 18 months to mature.
The Blackjack oak grows in poor, thin, dry, rocky or sandy soils where few other woody plants can thrive, usually on low ground, from sea level up to 900 m altitude. It does not have the beautiful form of many oaks, but is nonetheless a valuable tree for growing in problem sites. It is sometimes an understory tree in pine stands on sandy knolls in the southeastern US. Along the coastal plain of New Jersey the probability of finding this species is increased in relatively sunny, open areas such as those near coastal salt marshes. It often occurs near Scarlet and Post Oaks as well as Pitch Pine; understory companions include winged sumac, bracken, sweetfern, and bayberry.
A variety, Quercus marilandica Münchhausen var. ashei Sudworth (D. M. Hunt 1989), grows in the western portions of its range—northern Texas, Oklahoma, and into southern Kansas. In this area, Blackjack, along with Post oak, forms a semi-savanna area composed of forested strips intermixed with prairie grass glades along the eastern edge of the southern Great Plains. This semi-savanna is known as the Cross Timbers. Blackjacks in the Cross Timbers can grow from 50 to 60 feet (15-18 m) high with a trunk diameter of 16 inches (40 cm), but seldom reach more than 40 feet (12 m). The leaves are from 4 to 10 inches (10-25 cm) in length and about the same width. Blackjack acorns provide food for both whitetail deer and wild turkey. Blackjacks may, however, cause tannic acid poisoning in cattle.