Walnut trees (Juglans spp.) and hickory trees (Carya spp.) are similar in many respects. Walnuts grow in U.S. Department of Agriculture plant hardiness zones 4 through 9, and hickories can be grown in USDA zones 4 to 8, though individual species may have slightly different tolerances. Botanically, both trees are in the Juglandaceae, or walnut, plant family.
Several native species of both walnuts and hickories grow in forests throughout the eastern United States, in a range from Georgia west to Texas and from Michigan east to Maine. The black walnut (Juglans nigra) and the pecan (Carya ovata), a type of hickory tree, are the two most widely known and useful of these species. The English walnut (Juglans regia), which is actually native to the Middle East, is commonly grown in California. It is usually grafted onto the California black walnut (Juglans californica), the only form of walnut native to the west coast.
Hickories and walnuts are used as shade trees, but both have long, sensitive taproots that can make transplanting difficult. Black walnuts are notoriously difficult to grow other plants around, and their leaves and nuts are messy when they drop, making a black stain on sidewalks and cars. Pecans are the only type of hickory nut commonly planted in home landscapes and are a little easier to manage, though they pose similar challenges.
All walnuts and hickories are large forest trees that prefer to grow in deep, fertile soils of undisturbed forests. Both types are long-lived and can eventually reach several feet in diameter and over 100 feet in height in their native habitats. All hickories and walnuts produce a compound called "juglone," which inhibits the growth of many other plants. The effect is most pronounced under black walnut trees, where few other plants can survive. Numerous forms of wildlife are attracted to the nuts of all species in this family.
Black walnuts have high-quality edible nuts, but the walnuts found in grocery stores are the "English" variety, which have thinner shells that are much easier to crack. Several types of hickory have edible nuts, though only pecans are grown specifically for this purpose. Another commonality is the use of the wood, which is naturally hard and rot-resistant. Species of both hickories and walnuts are used in making furniture, tool handles and other specialty wood products.
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