Hickories have alternate compound leaves with 5 to 13 leaflets. They are monoecious; that is, the stamens and pistil are borne in separate flowers on the same plant. Flowers generally develop after the leaves are three-fourths to full grown. Staminate flowers are in catkins, which occur on last year’s wood and usually have three branches. The stalkless pistillate flowers are in several-flowered spikes at the apex of the current season’s growth. Catkins last from 10 days to 3 weeks and are usually not useful in distinguishing the species. The fruit, however, is widely used for identification. The husk may be very thin (about 1 mm), moderately thick (3 to 5 mm), or very thick (more than 5 mm). Similarly, the shell may be thin, moderately thick, or very thick. The meat varies from sweet to bitter. Upon drying, the husk usually separates along sutures and frees the nut. In some species the husk partly splits at the sutures but still encloses the nut when it falls. Such nuts are usually sterile, because the meat is seldom filled out.
For identification purposes use nuts from the ground with caution; if more than one species grows in the vicinity, the fruits could be mixed. Also some trees shed defective or sterile fruits early, and these fruits are often different in shape and size from typical fruit.
Hickories are generally divided into true hickories, which have overlapping (imbricate) bud scales, and pecan hickories, which have valvate bud scales. True hickories are divided into those with tight bark and those with loose bark. Mockernut Hickory, Carya tomentosa Nutt., a tight bark species, has very pubescent leaves, large buds, and a thick husk over a large nut. Pignut Hickory, C. glabra (Mill.) Sweet, also a tight bark hickory, has three to nine glabrous leaflets and a tardily dehiscing husk about 3 mm thick. Shagbark Hickory, C. ovata (Mill.) K. Koch, is one of the few species with bark in thin, loose plates. It has small buds and usually has five leaflets. Bitternut Hickory, C. cordiformis (Wangenh.) K. Koch, belongs to the pecan hickory group and has yellow buds with valvate scales. It also has tight bark, thin four-winged husks, thin shells, and bitter meat.
Oaks are also monoecious, and staminate flowers are in drooping catkins, which consist of a central, flexible axis with sessile, apetalous and pubescent flowers. They are most abundant on the developing new twigs. Although catkins vary among oaks, they are usually not used as distinguishing characteristics because they last only 2 to 3 weeks. Pistillate flowers occur on wood of the previous season and in leaf axils of twigs. In red oaks, however, pistillate flowers on current twigs do not mature until the second fall.
The fruit, a nut or acorn, consists of the seed enclosed in a shell. The nut is seated in a cup consisting of many scales. At maturity nuts are shed from the tree by shrinkage of the cup (dehydration). Considerable variation in mature acorn size occurs among trees of a species, notably in water oak, willow oak, and white oak.
Differences in foliage within a species also complicate identification. Apparently because lower leaves receive less sunlight they differ in size and appearance from those in the middle or upper crown. Most taxonomists prefer a specimen from the upper half of the crown because they consider leaves from this area typical for the species. To get such specimens easily, use a tree pruner and take leaves from at least 20 to 30 feet off the ground or collect them from freshly felled trees. Moreover, foliage of seedlings and small saplings often differs from that of mature trees, and spring and summer growth flushes on the same branch often appear markedly different. This seasonal variation has been noted for many oaks, including laurel, post, blackjack, and especially water oak. For example, by late March or April, water oak leaves reach their normal size and shape—obovate with a wavy apex; the second flush of growth, which starts in late April or May, produces lobed leaves.
Identification is further complicated because hybrids commonly occur among the oaks.
The oaks are divided into two main groups: the white oaks and the red (or black) oaks. The white oaks have leaves with rounded lobes and no bristles; acorns mature the first fall, and the inner surface of the shell is glabrous. White oak, chestnut oak, and post oak belong to this group. The red oak group is characterized by bristle-tipped leaf lobes in such species as Shumard, southern red, northern red, cherrybark, and black oak. The red oak group also includes non-lobed species such as water, laurel, and willow oak, which usually have entire margins. Blackjack oak leaves usually lack lobes and bristles, but leaves with terminal lobes occasionally occur and may have one bristle per lobe. Acorns mature the second fall, and the inner surface of the shell is pubescent.
Southern Red Oak, Quercus falcata Michx., and its variety Cherrybark Oak, Q. falcata var. pagodaefolia Ell., have almost identical acorns, but they can be distinguished by other characteristics. The leaf base of southern red oak is somewhat bell-shaped, but that of cherrybark is not. Southern red oak forms heavy ridges of bark; cherrybark does not.
The distinctive feature of Black Oak, Q. velutina Lam., is its yellow-orange inner bark. Its leaves take three forms. Seedlings to small saplings have a “bear-paw” leaf with only a suggestion of lobing. Lower leaves on large trees also have little lobing, but leaves from the middle and upper crown are clearly lobed. The uppermost leaves, which have seven lobes and deep sinuses, are usually illustrated as typical.