U.S. Department of Agriculture: Forest Service -- National Agroforestry Center



Black Walnut

Date of this Version



Published in Genome Mapping and Molecular Breeding in Plants, Volume 7.


Black walnut (Juglans nigra L.), also known as eastern black walnut or American walnut, is a fine hardwood species in the family Juglandaceae, section Rhysocaryon (Manning 1978). In general, J. nigra will not cross with species in the sections Cardiocaryon or Trachycaryonv, but J. nigra will cross with J. ailantifolia (Cardiocaryon) (Williams 1990). Juglans nigra will also hybridize to some extent with other Juglans species (Dioscaryon and Rhysocaryon), and one hybrid is recognized: J. nigra × J. regia = J. x intermedia Carr. (USDA-NRCS 2004). Native to the deciduous forests of the eastern United States (USA), from Massachusetts to Florida and west to Minnesota and Texas, and occurring naturally in southern Ontario, Canada, black walnut is seldom found in pure stands, but rather in association with five mixed mesophytic forest cover types: sugar maple, yellow poplar, yellow poplar – white oak – northern red oak, beech – sugar maple, and silver maple – American elm (Williams 1990). Black walnut is a large tree and on good sites may attain a height of 30 to 38 m and diameter of 76 to 120 cm and can exceed 100 years of age (Williams 1990; Dirr 1998; USDA-NRCS 2004). Black walnut is shade intolerant, and control of competing vegetation is especially important in new plantations for the first 3 to 4 years. Black walnut grows best on moist, deep, fertile, well-drained, loamy soils, although it also grows quite well in silty clay loam soils or in good agricultural soils without a fragipan (Williams 1990; Cogliastro et al. 1997). These sites include coves, bottomlands, abandoned agricultural fields, and rich woodlands. Black walnut forms a deep taproot, wide-spreading lateral roots, and has been cultivated since 1686. A toxic chemical ‘juglone’ (5- hydroxy-1, 4-naphthoquinone), naturally occurring in the leaves, buds, bark, nut husks, and roots of black walnut, is a highly selective, cell-permeable, irreversible inhibitor of the parvulin family of peptidylprolyl cis/trans isomerases (PPIases) and functions by covalently modifying sullfhydryl groups in the target enzymes (Henning et al. 1998; Chao et al. 2001). Certain plants, especially tomato, apple, and several conifer species, are adversely affected (allelopathy; foliar yellowing, wilting, and even death) by being grown near the roots of black walnut trees (Goodell 1984; Dana and Lerner 1994). Horses can contract acute laminitis, an inflammation of the foot, when black walnut wood chips or sawdust is used for stall bedding or stables and paddocks are located too close to walnut trees (Galey et al. 1991). Historically, the bark of black walnut was used by several Native Americans, including the Cherokee, Delaware, Iroquois, and Meskwaki, in tea as a cathartic, emetic, or disease remedy agent, and chewed or applied for toothaches, snake bites, and headaches (Moerman 1998, 2003). Caution: the bark should be used cautiously inmedicine because it is poisonous. TheCherokee, Chippewa, and Meskwaki also used the bark to make a dark brown or black dye (Moerman 1998, 2003). The Comanche pulverized the leaves of black walnut for treatment of ringworm, the Cherokee used leaves to make a green dye, and the Delaware used the leaves as an insecticide to dispel fleas (Moerman 1998, 2003). The nut meats were also a food source for Native Americans, and the nuts are still consumed today by people and are an important food source for wildlife.