Trees in the genus Carya (from Ancient Greek κάρυον "nut") are commonly known as Hickory. The genus includes 17–19 species of deciduous trees with pinnately compound leaves and large nuts. A dozen or so species are native to North America (11–12 in the United States, 1 in Mexico), and 5–6 species from China and Indochina.
Another Asian species, Beaked Hickory, previously listed as Carya sinensis, is now treated in a separate genus Annamocarya, as Annamocarya sinensis.
Hickory flowers are small yellow-green catkins produced in spring. They are wind-pollinated and self-incompatible. The fruit is a globose or oval nut, 2–5 cm long and 1.5–3 cm diameter, enclosed in a four-valved husk which splits open at maturity. The nut shell is thick and bony in most species, thin in a few, notably C. illinoinensis; it is divided into two halves which split apart when the seed germinates.
Another insect that uses the hickory tree as a food source is the hickory leaf stem gall phylloxera (Phylloxera caryaecaulis). Phylloxeridae are related to aphids and have a similarly complex life cycle. Eggs hatch in early spring and the galls quickly form around the developing insects. Phylloxera galls may damage weakened or stressed hickories, but are generally harmless. Deformed leaves and twigs can rain down from the tree in the spring as squirrels break off infected tissue and eat the galls, possibly for the protein content of the phylloxera, or possibly because the galls are fleshy and tasty to the squirrels.
The nuts of some species are palatable, while others are bitter and only suitable for animal feed. Shagbark and Shellbark Hickories, along with the Pecan, are regarded by some as the finest nut trees.
When cultivated for their nuts, note that because of their self-incompatibility, clonal (grafted) trees of the same cultivar cannot pollinate each other. Two or more cultivars must be planted together for successful pollination. Seedlings (grown from hickory nuts) will usually have sufficient genetic variation.