Probably best known for their edible nut, there are approximately 15 to 20 species of walnut throughout the world, and representatives can be found in the Americas, Europe, and southeastern Asia. Conservatively, there are only four species native to North America, although some specialists treat other varieties as distinct species in some regions. The Walnuts of North America are usually found in mixed forests with other hardwoods preferring moist, rich soil although they are adaptable. Generally fast growing, the Walnut ranges in size depending on species and growing conditions from small trees to rather large.
Walnuts have generally well-developed deep root systems, which makes them useful in planting to stabilize areas prone to erosion. However they produce a chemical that inhibits the growth of many plants within the reach of their roots, including other walnuts. They are not particularly shade-tolerant and live anywhere from 50 to 400 years depending on the species. The twigs have a chambered pith, which is a distinct characteristic of the walnut.
Juglans nigra, Eastern Black Walnut
This species of walnut is native to eastern North America and grows mostly in areas along river margins where the alluvial soil is rich and most, although still well drained, especially loams that are neither acidic nor alkaline. They can be found from Ontario to Florida and as far west as central Texas, and may also occupy slopes and dry ridges. This species of walnut is more resistant to frost than the English or Persian Walnut, but thrives best in warmer regions with fertile, lowland soils and high water tables. Seldom found in pure stands, Black Walnut is often found in association with five mixed forest cover types that are adapted for neither particularly dry nor particularly wet environments: Sugar Maple, Yellow Poplar, Yellow Poplar - White Oak - Northern Red Oak, Beech - Sugar Maple, and Silver Maple - American Elm. During this tree’s average lifespan of 130 years, on good sites it may attain heights of 100-130 ft (30-40 m) with a trunk diameter of 2-4 ft (60-120 cm). Forming a deep taproot with wide-spreading lateral roots, this species of walnut has been cultivated since 1686. In forest settings the trunk supports a small, high rounded crown, while in the open its short trunk develops a low spreading crown of stout branches. 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 sites. These include coves, bottomlands, abandoned fields, and rich woodlands.
A toxic chemical naturally occurring in the leaves, buds, bark, nut husks, and roots of Black Walnut is highly selective, cell permeable, and an irreversible inhibitor in the functions of certain plants when exposed to it. When Black Walnut woodchips or sawdust is used for stall bedding or stables, or when paddocks are too close to a group of trees, horses can contract an acute inflammation of the foot from exposure to this toxin. Historically, both the indigenous people and settlers of the Americas have used various parts of the Black Walnut for a variety of purposes. Several tribes where Black Walnut is present depended upon it, using the bark in tea to remedy disease, or chewed to relieve toothaches or then applied to snake bites. Others also used the bark to make dark brown or black dye; some used the leaves in a similar process for green. Settlers used the husks in the same way to make dark dyes for their clothes. Today, the ground shell is a hard, durable, non-toxic, and biodegradable abrasive product used for blast cleaning, polishing, and some beauty products.
The leaves on this deciduous tree are alternate, along stems forming a long compound leaf between 1 and 2 ft (30-60 cm). With 9 to 23 leaflets and somewhat lustrous dark green, the petioles are covered with glandular hairs. Also typical of the walnut species the Black Walnut is monoecious with male and female flowers maturing at different times. The male flowers are cone like catkins 8-10 cm long developing from auxiliary buds on the previous years wood. The female flowers occur in two to eight-flowered spikes born at the shoots of the current year’s growth. The female flowers more commonly appear first and occur with or shortly after the development of leaves in spring. These flowers, once pollinated, ripen into fruit during autumn and fall in October. Black Walnut trees produce seeds at about 12 years of age, with good seed crops occurring every 2 to 3 years. Enclosed in a solid, non-splitting husk, the fruit of the walnut is born singly or in pairs, or occasionally in threes. Seeds, like most species of walnut, have a dormant embryo. This can be broken by fall sowing or by moist pre-chilling for a few months in agricultural practices. These kernels are high in unsaturated fat and protein and are commercially harvested for consumption as well as providing an important fall mast for woodland foragers. The majority of Black Walnut trees occur in natural stands, with walnut plantations accounting for approximately one percent of all the Black Walnut volume in the US. Since the last (1997) comprehensive inventory and summary of the Black Walnut resource in the eastern US, the number and volume of Black Walnut trees has increased in nearly every state except three.
Juglans nigra is the largest and most valuable timber tree of the Juglans species, and is hardy to USDA hardiness zone range of four to nine. Black Walnut is highly prized for its dark-coloured, true heartwood, which often ranges from a rich dark brown to purplish-black. Heavy and strong, yet easily split and worked, this versatile wood was historically used for gunstocks, furniture, flooring, paddles, coffins, and a variety of other products. It shrinks and swells less than any other wood, and is of medium density with moderate bending and crushing strengths. Durable and highly shock resistant it is generally straight grained, although sometimes wavy or curly patterning is present. The heartwood of this coarse textured tree is very resistant to biodegradation.