The not-so-familiar black walnut

Black walnuts are familiar yet very unusual and valuable trees, and quite rare growing wild around here. At Jordan Lake State Recreation Area’s Seaforth access there are several exceptionally large trees of various kinds, including a black walnut. It’s spreading boughs are shaggy with resurrection ferns, verdant green after rain, contrasting with the yellowish-green of the walnut leaves, and appear dead at other times. The walnut is partially hollow and has dead branches, so it seems precarious and I wonder if the raised water table and/or heavy summer foot traffic is harming it. There are several much younger walnuts nearby. Near UNC’s Friday Center there is a black walnut about 2′ across, with a plaque saying it sprouted circa 1880. Large walnuts line paths at Durham’s West Point on the Eno. There are a few walnuts in the Northeast Creek basin. A tall tree more than a foot across grows at the base of a moist northwest facing slope near tributary Burdens Creek. It is a mystery how it got there. A few miles away there are spindly but nut-producing walnuts growing wild by a road in Chatham County connected only to Durham and Wake.

Their large nuts, classified botanically as drupes, fall around October, later than acorns and hickory nuts. An unusual feature is their smooth, relatively soft, fragrant, yellowish-green changing to black, staining husks, resembling a lemon more than a nut such as a pecan. Last fall there were unusually few walnuts under the Burdens Creek tree, whether because of the drought or because production varies yearly. On the other hand smallish nuts covered the ground under the Chapel Hill tree last October, and squirrels were gathered when I visited.

Black walnuts have dark, regularly furrowed bark and are tall and narrow in forests but short and very broad without competition. Giants can grow to 150′ tall and 6′ across, but walnut wood is valuable, so valuable that trickery and poaching occurs, and many were cut. They have large pinnately compound leaves up to 2′ long and 6” wide, with 9 to 23 long-pointed and serrated leaflets in pairs, smaller at the ends of the leaf, and usually there isn’t a leaflet at the tip. The leaflets have very short stems (petiolules) and the compound leaves alternate along the twigs. There is some hairiness on the underside of the leaflets and the leaf stem, called a rachis. Broken leaves and husks have a distinctive smell, lemony to me. Their leaves turn yellow early, contrasting with the dark trunks.

Several trees have similar leaves. White walnuts or butternuts, native in the Smokies, and introduced here, have oblong nuts, fewer leaflets (but usually with a terminal leaflet), and while both walnuts have chambered pith inside their twigs, it is darker brown in butternuts. Both species have grayish buds and the leaves fall to reveal three-lobed leaf scars, but there is a hairy spot only in butternuts and their buds are elongated. Hickories, such as bitternuts and pecans, are in the walnut family, but typically have fewer leaflets per leaf, a terminal leaflet, buds covered in scales, pith without chambers, and their nuts are usually dehiscent, opening along sutures. Locusts and Ailanthus also have pinnate compound leaves but are often small trees with multiple trunks growing in open areas. Ailanthus have very large leaves and leaf scars, a distinctive smell, and are most common in built-up areas while black locusts are thorny and produce fragrant white pealike flowers in late April.

Black walnuts seem to like well-drained soil and usually grow far from waterways. They grow fast, but are long-lived and don’t like shade, and they have a way to preserve their sunlight.

Black walnuts produce wind-pollinated catkins before leafing, each tree having both sexes. The dangling male catkins are 3 – 5” long. The female flowers are in clusters of 2-5 near the twig tips. The male and female flowers open at different times, but self-pollination is possible. A sapling can produce nuts after only five years, but few until age 10 – 15. The huge nuts are covered in a smooth husk, surrounding a seed with very thick, irregular wooden walls. ‘Everyone’ eats the nuts, yet I often find many left under wild trees and it takes a lot of effort to get inside. Apparently walnuts need a long cold period to sprout, and squirrels can steal planted nuts.

Black walnuts are known botanically as Juglans nigra, the generic name being an abbreviation for Latin Jovis glans, the acorn or nut of Jupiter (it was said people once lived on acorns while the gods ate walnuts). The word walnut originally referred to Persian walnuts, and is supposed to be a combination of the Anglo-Saxon for Welsh or Celtic, signifying foreign, and nut, wealh hnutu, as opposed to hazelnuts. Walnuts are honored in many place names, such as Cary’s Walnut Creek.

An unusual feature of black walnuts is their allelopathy. The walnut near Burdens Creek grows in a mature deciduous forest, but is in a sunnier glade. Around now toothworts and other early wildflowers bloom around it, joined by pink redbuds, an understory tree in the pea family, and in summer other peas cover the ground. Legumes often carpet the ground under walnuts in summer. Black walnuts produce hydrojuglone, which oxidizes into juglone and washes into the soil, to poison competitors, though ash, legumes, etc. seem immune. Some soil bacteria seem to be able to live on juglone.

Despite poisoning other plants, walnuts are edible to many animals. Squirrels, white footed-mice, and chipmunks disperse the nuts. In winter rabbits and deer nibble saplings. On the other hand walnut is harmful to horses and fish. Many insects feed on walnuts, including a curculio weevil, woodboring beetles, the walnut lace bug, stinkbugs, and aphids. Walnut fruitflies are supposed to develop in the husks, but I haven’t seen any. Many lepidopterans consume walnut, including one small butterfly, the banded hairstreak. Moths include gigantic hickory horned devils (caterpillars of the royal or regal walnut moth), related imperial moths, luna moths, walnut caterpillars and other Datanas, monkey slugs, walnut sphinx moths, curved-tooth geometers (a relatively large dark brown moth common at lights around April or May), American dagger moths and relatives, salt marsh caterpillars, fall webworms, walnut shoot moths, and several more.

Walnuts have a lot of versatility for humanity. Nuts are gathered from the ground and the nutmeat can be eaten raw, ground into flour, or boiled for oil used for cooking, fuel, and as polish (butternuts are more famous for oil). The trees can be taped like sugar maples. The husks produce a dye used to color cloth and even hair, and as ink. Walnut is supposed to repeal bedbugs and other insects, and I have wondered if the husks could be used to treat wood. Pulverized nuts are used in abrasives, filters, tires, composted, etc. The beautiful smooth wood was also used in furniture, paneling, veneer, cabinets, carriages, sewing machines, appliances, musical instruments, and for both cradles and caskets. During wars it went for gunstocks and airplane propellers. Walnut resists decay, so it was used for rail fences and railroad ties.

The leaves and husks have been known to cause dermatitis, but there are many herbal uses. The bark was used for toothaches; husks were used for ringworm, external inflammation, and to cause sleep; leaves were used for insect repellant and reportedly for sunscreen; and various parts were used to treat gastrointestinal problems. Juglone could be useful as a sedative and cancer inhibitor.

Thousand canker disease is an emerging threat for walnuts, though still far from here. Unlike other new catastrophic forest maladies like emerald ash borer and laurel wilt, thousand canker disease is native to North America, but was apparently unknown in the East until it reached Knoxville around 2010, so black walnuts still lack resistance. Thousand canker disease is caused by a recently discovered fungus spread by the walnut twig beetle, native in the Southwest and Mexico. In fall 2012 the disease was found in Haywood County, bordering Tennessee, but it is not known to have spread elsewhere in North Carolina since. The disease is present in several Western states, but only four Eastern states. Butternuts are susceptible and also face a canker noticed in 1967, probably from Asia. So walnut is another wood to be careful about moving over distances.

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