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The Black Walnut or American Walnut (Juglans nigra L.) is a native of eastern North America, where it grows, mostly alongside rivers, from southern Ontario to southeast South Dakota, and from to Georgia to central Texas. It is seen both as a valuable tree and a serious landscape problem because it produces an allelopathic substance called juglone that interferes with the healthy development of other plants, causing wilting and yellowing of the foliage or in many cases death.
The Black Walnut is a large deciduous tree attaining heights of 30–40 metres (100–130 feet). Under forest competition it develops a tall, clear bole, while the open-grown form has a short bole and broad crown with large, wide-spreading branches. The bark is grey-black and deeply furrowed. The pith of the twigs contains air spaces. The leaves are alternate, 30–60 cm long, odd-pinnate with 15–23 leaflets, the largest leaflets located in the centre, 7–10 cm long and 2–3 cm broad. The male flowers are in drooping catkins 8–10 cm long, the female flowers terminal, in clusters of two to five, ripening in the autumn into a fruit with a brownish-green, semi-fleshy husk and a brown corrugated nut. The whole fruit, including the husk, falls from early to late autumn. The nut itself is dense and very hard.
Black walnuts grow in both mature forests and open areas, though more common in the latter because the seedlings are not shade tolerant. They are frequently found in groves, and are often found in association with Black Raspberries, which seem to thrive under the canopy of black walnuts.
The ellelopathic chemical juglone is a very serious issue in the garden, as it suppresses or kills many garden plants, including most lawn grasses (though bluegrass is unaffected). See allelopathy on Wikiversity for materials related to juglone in the landscape.
While rarely planted due to its various problems, black walnuts do provide a nice dappled shade during the summer, have a good branch structure, and of course provide nuts. However, the tree has serious liabilities such as a very late leaf-out in spring, early leaf drop in autumn with little or no fall color, and the large, heavy nuts are actually quite hazardous to anyone underneath them both when they are ripe and on windy days in summer.
Black walnuts are tough trees that usually have few needs, though some trees can be prone to wind damage. The most serious maintenance concern is the nuts, which not only are difficult to pick up, but also pose a hazard to anyone below the trees when they are ripe, to the point that helmets should be worn when working under them from early autumn until the last nuts come down in early winter. The branches are quite strong and heavy, making them easy to climb in, however the rough bark easily snags ropes and throw lines.
- Mowing: Effective.
- Cultivation: Does not work well, due to the deep roots.
- Mulching (for prevention): Mulches actually encourage walnuts, as squirrels will be more prone to dig in mulched beds.
- Girdling: Black walnut will regrow from the base after girdling.
- Coppicing: Can be a good management approach to prevent seed production, as the regrowth stems do not flower for some years afterwards.
- Grinding: Effectively kills the tree, but the juglone will persist in the soil for many years afterwards as the outer roots break down.
- Biocontrols (plants): Goldenrod is allelopathic to Black Walnut
- Grazing: Goats will strip and kill young plants, but older trees are protected by the thick bark
- Disposal: Leaves can be composted as normal, but wood chips, roots, and nuts require a longer period in order to ensure that the juglone is decomposed. Chips should not be used for garden pathways, and are toxic to horses so should never be used as stall bedding.
The extraction of the kernel from the fruit of the Black Walnut is difficult. The shell is covered by a thick, indehiscent husk that exudes a dark, staining, strong-smelling juice. Once the husk is removed, the nut itself poses an additional challenge because it is quite literally a hard nut to crack.
The husk is best removed when green, as the nuts taste better if it is removed then. Several methods are used:
- Rolling the nut underfoot on a hard surface such as a driveway is a common method, or on gravel driveways running them over with a vehicle is also a good option.
- Commercial huskers use a car tire rotating against a metal mesh.
- Some take a thick plywood board and drill a nut sized hole in it (from one to two inches in diameter) and smash the nut through using a hammer. The nut goes through and the husk remains behind. To keep the husk juices from splattering, a board or canvas scrap may be used to cover the nut before hammering.
- There are also grinders specifically designed for de-husking black walnuts, but these will likely be difficult to find.
After the husks are removed, the nuts should be stored in a dry place for at least two weeks to cure. Traditionally they are hung up in bags or baskets to provide better air circulation and prevent molds.
After curing, the nuts can be cracked with a hammer, axe, or a heavy nut-cracker (screw types work better than squeeze types due to the strength of the shell). The meat can then either be roasted or salted for long-term storage.
Black Walnut is highly prized for its dark-colored true heartwood. It is heavy and strong, yet easily split and worked. Walnut wood has historically been used for gunstocks, furniture, flooring, coffins, and a variety of other woodworking products. It is so valuable that so-called "Walnut Rustlers" have been known to harvest it illegally by posing as forestry officials, cutting trees during the night, and even using helicopters to take them away quickly; such overharvesting has greatly reduced its numbers and range since colonial times.
Landscape trees can sometimes be sold for the lumber, as long as the branches begin fairly high off the ground and the trunk is quite thick, since the outer, light-colored wood is not valuable as lumber (but makes very good firewood).
Pests and diseases
Black walnuts rarely succumb to pests and diseases unless otherwise weakened, but otherwise can be affected by the same problems as the English walnut (Juglans regia). The most common aesthetic problem is the fall webworm (Hyphantria cunea).
|The Cookbook contains recipes for Walnut|
- Monday Garden: Black Walnut
- Harvesting Black Walnuts
- Growing Black Walnut
- Black Walnut Diagnostic photos: tree, leaves, bark and fruit
- Hoadley, Bruce (1990). Identifying Wood: Accurate Results with Simple Tools. Taunton Press. pp. 240 pages. ISBN 0-942391-04-7.
- Dirr, Dr. Michael A. Manual of Woody Landscape Plants. Stipes Publishing. ISBN 0-87563-795-7
- Petrides, George A. and Wehr, Janet. Eastern Trees. Houghton Mifflin Company. ISBN 0-395-90455-2
- Flora of North America, Profile and map: J. nigra
- Juglans nigra images at bioimages.vanderbilt.edu