Black walnut

The black walnut is a large deciduous tree of eastern North America with long, compound leaves. Its fruit is a round, green-husked drupe the size of a tennis ball, which blackens and stains as it matures. Inside the husk is a dark, deeply furrowed, extremely hard shell enclosing a lobed kernel.

The kernel is smaller than that of the English walnut and is hard to extract, but its flavour is rich, bold, slightly bitter and almost wine-like, and it holds up well in baking and ice cream.

Black walnuts are sold shelled in pieces, as flour, oil and as a powdered hull used in traditional remedies and dye.

Juglans nigra is native to the eastern and central United States, from the Great Lakes region to the Gulf states, and into southern Ontario. It grows best in rich, deep bottomland soils.

It was harvested as a wild nut by Native American peoples such as the Cherokee and Iroquois long before European settlement, and it has never been fully domesticated, though selected cultivars exist.

The genus name Juglans derives from the Latin Jovis glans, the acorn of Jupiter. Walnut itself comes from Old English wealhhnutu, meaning foreign nut, a reference to the Roman walnut which arrived in Britain via continental Europe.

Native peoples used black walnuts for food, oil, dye and medicine, and the archaeological record shows that they were widely eaten in the Eastern Woodlands. Settlers gathered them in autumn and cracked them by hand during winter evenings, and the nuts were used in pioneer baking, candies and cakes.

The wood is among the most valued American hardwoods, used for gunstocks, furniture and veneer, which has been a bigger economic driver than the nuts. The hulls were used for brown dye, including by Civil War soldiers on both sides.

In the 20th century, the nut-processing industry in Missouri and neighbouring states grew around buying nuts from rural gatherers, who hulled them with machines and their own vehicles, and the state continues to supply most American black walnut kernels today.

The tree releases a compound called juglone that suppresses the growth of some nearby plants, a reason why gardeners avoid planting vegetables or tomatoes close to it.

Black walnut kernels are added to ice cream, brownies, fudge, cookies, quick breads and cakes, where their strong flavour stands out, and chopped over salads and baked goods. They are also made into nut butters and pressed for oil.

Hull extract has traditionally been used as a natural brown dye, and ground shell is used as an abrasive in cleaning and polishing. The hulls have also been used in folk remedies, including for parasites, though without good evidence.

The sap can be tapped like maple sap and boiled into syrup in some areas.

Black walnuts are energy-dense nuts that provide protein, unsaturated fat, including plant omega-3 alpha-linolenic acid, fibre and minerals such as magnesium and manganese. Studies of walnuts in general, mostly of English walnuts, link regular consumption with improvements in blood cholesterol and possibly heart health.

Evidence on black walnut hull extracts for infections or parasites is not established, and such products are not a substitute for medical treatment.