The walnut family contains about 60 species of trees in the family Juglandaceae, divided among seven genera. North American representatives are the
walnuts (Juglans spp.) and hickories (Carya spp.). All of these species produce edible nuts and useful wood, and some are cultivated in orchards for the production of these crops.
Species in the walnut family are woody plants that develop as trees, and mostly grow in angiosperm-dominated forests in temperate and subtropical climates. The range of most species is the Northern Hemisphere, although a few species penetrate to the Andes of South America and the southwest Pacific.
The wood of trees in the walnut family is strong and resilient and is highly prized as lumber. The twigs have a chambered pith which is visible in a longitudinal cross-section.
Species in the walnut family have prominent, hairy buds and seasonally deciduous, compound leaves which are shed in the autumn. The flowers are small and greenish. The staminate flowers occur in catkins, while pistillate flowers occur individually. The ripe fruit of the hickories and walnuts is properly termed a drupe in which their large, hard-coated seed is encased in a leathery case. The fruits of walnuts and hickories are edible.
An interesting characteristic of the black walnut and some related species is their ability to apparently poison the soil in their immediate vicinity. Few plants are able to grow beneath a large walnut tree, an observation attributed to the presence of a toxic alkaloid known as juglone. Even walnut seedlings cannot normally grow beneath a parent tree. The production of phytotoxic chemicals by plants for use in a type of chemical warfare with other competing plants is a form of allelopathy. Black walnut is often cited by ecologists as an archetypal, allelopathic species.
Six species of walnuts occur in North America. The black walnut (Juglans nigra) and butternut (J. cinerea) are widespread in eastern North America. The little walnut (J. microcarpa) and Arizona walnut (J. major) range into Texas and Arizona and south into Mexico. The California walnut (J. californica) and Hinds walnut (J. hindsii) have relatively localized distributions in southern California. The English walnut (J. regia) is native to Europe and Asia but has been widely planted in North America.
Twelve native species of hickories occur in North America. The most famous species is the pecan (Carya illinoensis). This species occurs naturally throughout the central United States and south through eastern Mexico, but is now cultivated more widely throughout the eastern United States. Other species that are widespread in southeastern North America are the shagbark hickory (C. ovata), mockernut hickory (C. tomentosa), pignut hickory (C. glabra), and bitternut hickory (C. cordiformis).
Various species of walnuts and hickories are economically important trees for both their wood and their edible fruits which may be gathered in the wild but are now mostly grown in plantations.
The wood of black, English, and other walnuts is close-grained, dark-brown colored, and very strong. Walnut wood is used to manufacture lumber and veneers for fine furnitures and cabinets, and it is sometimes carved into components for artisanal furniture. A well-formed tree of black walnut with a good grain
Compound leaf —A leaf in which the blade is separated into several or many smaller units, called leaflets, arranged along a central petiole or stalk known as a rachis.
Drupe —A fruit which has a fleshy outer layer, and a hard inner layer which encloses a single seed. A cherry is a typical example, but the fruit of a walnut is also a drupe.
Nut —A generic term for a dry, one-seeded fruit with a hard coat which is usually quite difficult to open.
Pith —A large-celled tissue that is found inside of the roots or stems of certain species. Members of the walnut family have a chambered pith in which the tissue is separated into discrete zones of solid tissue and air chambers.
and solid core can be worth more than $12,000 as raw material for fine lumber or veneer. Because of this enormous per-tree value, walnut trees are sometimes illegally “rustled” from private or public property to be sold in a black market.
Hickories also provide an excellent hard wood, used to manufacture fine furniture and wooden baseball bats.
The best-known edible fruits harvested from species in the walnut family are those of the European or English walnut (Juglans regia), the black walnut (J. nigra), the pecan (Carya illinoensis), and the hickory (Carya ovata). The first three of these species are commonly grown in plantations established for the production of their fruits. When they reach a large size, the walnuts may be harvested from the plantations for their extremely valuable wood. However, this is not done for pecans because their wood does not have qualities that are as desirable as those of large walnut trees.
The most important use of the fruits of walnuts and hickories is directly for eating. However, fresh walnut seeds contain about 50% of their weight as oil, which can be expressed from these fruits and used as an edible oil or to manufacture soap, perfume, cosmetics, or paint.
Walnuts have sometimes been used as minor folk medicines. The inner bark of the black walnut can be used as a laxative, while the fruit rind has been used to treat intestinal parasites, ulcers, and syphilis. An infusion of boiled leaves has been used to get rid of bedbugs.
The doctrine of signatures was a medicinal theory that developed in Europe during the Middle Ages (about 500 to 1,500 years ago), but also occurred independently in some other cultures. This theory held that the potential usefulness of plants for medical purposes was revealed through the growth form of the plant or its parts. For example, a similarity between the form of the plant or its parts and some component of the human anatomy was commonly thought to reveal a signature of usefulness. When the hard, outer shell of a walnut is removed, the seed looks superficially like a human brain, viewed from above with the top of the skull removed. Consequently, it was believed that walnuts were somehow useful for the treatments of insanity and headaches.
Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.