Juglans nigra Seeds | Black Walnut

Black Walnut (Juglans nigra) seed — Black Walnut (*Juglans nigra*): Juglandaceae; genus Juglans , section Rhysocaryon , the American black walnuts, native to Eastern North America, from southern Ontario and New England south to northern Florida and west to eastern Texas; most abundant on deep, moist bottomland loams of the Ohio and Mississippi valleys.

LATIN NAME: Juglans nigra

COMMON NAME: Black Walnut

Plant Profile

The active substance is 5-hydroxy-1,4-naphthoquinone, a yellow-brown pigment held in roots, bark, leaves, hulls and wood. The tree does not store it as such: it banks the compound as hydrojuglone glucosides, colourless and water-soluble, and releases them into the soil, where oxidation forms the quinone. Juglone disrupts electron transport in sensitive roots, so the damage looks less like poisoning than like slow starvation.

The leaves are pinnately compound, 30 to 60 cm long, with 11 to 23 serrated leaflets that smell resinous when crushed. The twigs carry a chambered pith, a pale centre divided into horizontal air spaces, and that separates black walnut from butternut, Juglans cinerea, whose pith is dark and whose husks are sticky with glandular hairs. Male catkins shed pollen before the female flowers open, so outcrossing rules. The chromosome number is 2n = 32.

Allelopathy here is a gradient rather than a wall. Juglone is most concentrated in the root zone and in hulls left to rot on the ground, and it fades beyond the canopy edge; a tree that has been felled keeps suppressing sensitive plants for several seasons, because the roots outlast the trunk. Tomato, apple, birch, white pine and blueberry suffer; Kentucky bluegrass, maize, cherry, oak and juniper do not, which is why an old walnut grove looks like a lawn with trees in it.

An outdoor seedbed suits the species better than a windowsill. Work the row 30 cm deep, space the nuts 15 cm apart and cover them with grit to deter rodents, then lay wire mesh flat over the bed, because squirrels and mice take an enormous share of any walnut crop.

Science & Biotechnology

Walnut is awkward to micropropagate, and juglone is part of the reason: an explant cut from mature wood bleeds phenolics that oxidise and brown the medium. Improvement work therefore runs through somatic embryogenesis from immature cotyledons, which yields clonal plantlets without the recalcitrance of adult tissue.

Callus and cell-suspension cultures of Juglans have been used to trace the route from the glucoside to the active quinone. The same chemistry underpins applied work: juglone has documented insecticidal, nematicidal and antimicrobial activity, and hull extracts have been examined as a botanical wood stain and as a dye needing little beyond alum.

Named cultivars seldom come true from seed and seldom strike from cuttings, so the trade grafts them onto seedling rootstock. Marker-assisted work in the genus now targets timber quality, nut traits and resistance to thousand cankers disease, the complaint carried by the walnut twig beetle, and the black walnut genome has been assembled to speed those searches.

Uses & Benefits

What falls in September is a drupe, not a nut: the green husk is the outer fruit wall, the ridged shell is the endocarp, and the kernel is the seed. The heartwood is the other harvest, a deep chocolate brown that darkens with light, works cleanly under a plane and holds a chequered figure at the wrist of a gunstock better than any other North American timber.

The kernel is oilier and more strongly flavoured than the Persian walnut, with a tannic bite from the seed coat. Its fat is largely polyunsaturated, linoleic acid with a useful share of alpha-linolenic acid, and it carries tocopherols that slow the oil’s decline.

Hull the nuts as soon as the husks soften, then cure them in a dry, airy place until the shells rattle. Cracking takes a vise or a hammer and a good deal of patience. Because of the oil content, shelled kernels keep far longer in the freezer than in a jar on a warm shelf.

Before the timber was worth anything, the hulls were the crop. Boiled with alum they give browns from fawn to near black, and the same material is sold today as an abrasive blasting grit for cleaning soft metals and turbine parts, because it is hard, angular and free of free silica.

Growing from Seed

Sow in autumn, covering the nut with about twice its own thickness of soil, roughly 2.5 to 5 cm, and let the winter do the rest. Nuts kept dry and warm usually disappoint. Where winters are mild, hold the nuts in damp sand in a cold frame and sow them at the end of the cold season.

The seedling makes a long taproot at once, so raise it in a deep pot or lift it with a spade pushed well under the root. Move it out once the second set of true compound leaves has hardened, spacing lawn specimens 8 to 10 m apart and timber trees 12 to 18 m. Water deeply and infrequently, mulch the root zone for two seasons, and keep nitrogen modest. Prune only in late-winter dormancy, because walnut bleeds from cuts made while the sap runs.

Our Take

Plant a black walnut for someone you will never meet. It is slow, it is demanding about its neighbours, and it will outlive every arrangement made around it, which is why the siting decision deserves an afternoon rather than ten minutes. If the nut is wanted without the allelopathy, look at the Persian walnut, whose husks are kinder to a garden, or at the butternut, a smaller and faster relative now scarce in the wild. For a nut crop with none of the juglone problem, the pecan is the better long bet.

Tree Seeds