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Black Walnut Tree Trivia

Black Walnut LeavesBlack Walnuts, Juglans nigra, grow mainly in the Eastern and Central United States but they occur in moderate numbers nearly everywhere in the U.S. east of the Great Plains, except for New England and south Florida.

They grow up to 150 feet with long, straight trunks up to a yard in diameter. Their black or dark gray bark is deeply marked with furrows and ridges. Their compound leaves, smooth on top and fuzzy underneath, can be up to 18" in length.
Black Walnut LeavesEarly colonists carried seeds of English walnuts to the New World, planting them diligently where they settled in Massachusetts and Virginia. The trees didn't even survive long enough to bear fruit so colonists learned to rely on the plentiful Black walnuts for cooking confections.

Native American Indians enjoyed black walnuts long before Europeans arrived, using walnut tree sap in their food preparation and making dye from the nut husks. Archeological evidence in the upper Great Lakes region indicates walnut consumption dating back to 2000 BC.

The shells and wood of black walnut trees are prized as much as the nutmeat. Woodworkers appreciate the extremely hardy characteristics and straight grain of the timber. Other industries use the hard shells of black walnuts in plastics, glues, sand-blast cleaners and metal polishers. During World War I, the hardy wood of the black walnut was used for making airplane propellers. NASA has put powdered walnut shells to use as thermal insulation in rocket nose cones; the powder can withstand extreme temperatures without carbonizing

Black Walnut TreeBlack Walnut trees often stand alone in the forest because their roots and dead leaves produce juglone, a toxic chemical that can kill other vegetation. The tree's roots secrete the poisonous substance in a toxic zone that can reach a 60-foot radius from the trunk. The affected area extends outward each year as a tree enlarges. Horticulturists recommend not planting tomatoes, rhododendrons and azaleas within 80 feet of any walnut tree.

Not all plants are sensitive to juglone. Some trees, vines, shrubs, groundcovers, annuals and perennials will grow in close proximity to a walnut tree. But, tomato and potato plants, blackberry, blueberry, azalea, mountain laurel, rhododendron, red pine and apple may be injured or killed within one to two months of growth within the root zone of these trees.

Black walnut is still recommended for planting in pastures and on hillsides in the Ohio Valley and Appalachian mountain regions to hold the soil, prevent erosion and provide shade for cattle. Black walnuts seem to have the beneficial effect of encouraging the growth of Kentucky bluegrass and other grasses.

Black Walnut CatkinsAll walnut trees are deciduous, growing well in temperate zones if sheltered from extreme cold and strong wind. They thrive in deep, fertile soil free of alkali and should be planted 60 to 70 feet apart.

In the spring they sprout catkins for pollination (see photo at left). Pollen quantity, not attractiveness to pollinators, is paramount for wind-pollinators.

Black Walnut in husk
Black walnut trees produce nuts in six, seven, and eight year cycles. It's impossible to know which cycle the trees are in, but it can be observed that about 7-8 years following a peak year a given tree will have relatively poor nut production, well below its normal average.

Eastern black walnut (Juglans nigra L.) is said to be an excellent choice for use in agro-forestry systems for several reasons. It is commercially valuable due to the market both for its nuts (on a continual, short-term basis) and high-quality wood (normally in a 50 to 100 year rotation). It has good growth and shade characteristics, and a relatively short growth period. It loses its leaves early in the fall, so that intercrop species benefit from sunlight. The root structure tends to be compact and deep, reducing competition with intercrop species. The market for black walnuts is said to be far from saturated, and the prospects are good for developing species producing higher nut yields.

Black walnut bark

Black Walnut Tree BarkBlack walnuts have tight bark with criss-crossing furrows forming a lattice pattern.

The Walnut family

The walnut family includes butternuts, buartnuts and black walnuts. Soils best suited to these species are rich loam and deep silt/clay bottomland. They are more tolerant than fruit trees to occasional wetness and will become established in the rich, alluvial soils along riverbanks, in flood-plain soils, and, to a lesser degree, on most other soils. Because they are less fussy about soils than fruit trees, nut trees often get "second best" when it comes to site selection, since the prime spots on the farm are reserved for the fruit trees.

But, before you run off to plant your walnuts on the less desirable sites on your property, let us point out that there is a direct correlation between soil quality and the growth rate of these trees. If you plant your nut trees in the richest soils, mulch them regularly and supplement with foliar feeding, they will grow like the dickens, whereas in poor soils with little care they will make little or no growth. We've seen black walnuts 15 years old that were only 2-3 feet tall! Conversely, we've seen them grow 3 to 4 feet in a year. One famous stand of black walnuts reached 100 feet in 26 years.

Young black walnuts are very "carrot" rooted; their entire root system is comprised mainly of one large tap root. There is no root ball, no large fibrous mass of roots. Large woody roots like these are good at storing sugars, but poor at gathering water. This is why, when planting members of the walnut family, watering until fully established is mandatory. Plant your trees in deep holes that generously accommodate the tap root, reversing soil layers as described earlier. A good mulch is important, as this will prevent evaporation from the surface, helping to hold water in the soil around the tree. Soil that is allowed to dry out will often crack along a line with the taproot, shrinking back from the root as it dries. If the soil should shrink back even 1-2 millimeters from the taproot, your tree will die. This often happens in mid and late summer.

Black Walnut TreeThe Black Walnut is one of the first trees to lose its leaves in autumn. See the bare tree in the forefront of the picture? The surrounding trees have not even begun to change colors but the walnut tree has already shed its leaves. Do not worry about the health of the tree, they all shed early.

Black Walnut Tree without leavesLoosing their leaves early makes it easier to spot walnut trees in timber when you are searching for newly-fallen nuts. Many of the nuts will still be hanging on the branches after most of the leaves have fallen.

American Black Walnut is probably the most famous and unique species of all our hardwoods. Because large defect-free trees of exceptionally good form were once common, the wood became prized for furniture, paneling, novelties, and many other items. The wood was abundant and had a natural resistance to decay, so it was also commonly used for construction purposes such as barn timbers. Even as late as the 1960's, I saw walnut 1 x 12's sold for hay-rack boards because it wouldn't rot.

Black Walnut Tree BarkBy the 1970's the wood became relatively expensive, forcing the furniture and cabinet industry to promote other species. The mid-1990's trend toward light-colored hardwoods also lessened walnut's popularity. Today, walnut is preferred in office furniture, architectural millwork, high-end gunstocks, specialty and custom items. While some suppliers feel walnut is once again gaining popularity, walnut lumber currently constitutes less than two percent of all hardwood lumber produced.

Growth and Range

Black Walnut LeavesThe natural range of black walnut is from the East Coast to the Great Plains, and from Texas and Georgia north to central Minnesota, Wisconsin, Michigan and southern Ontario. Most of the best quality trees are found in the Central States region from Ohio to Iowa. The tree has also been widely planted in the Northwest. This species prefers deep, rich, moist soils of alluvial origin. The best trees are frequently found on flood-free ledges, just above stream banks. Walnut will live on poorer soils, but growth is slow and the wood quality is generally poor. Many walnut plantations have failed to perform well due to the trees being planted on the wrong sites.

Walnut is seldom found in pure stands unless hand-planted as shown in the photo above right. The best trees usually occur sporadically and in association with yellow poplar, white ash, cherry, basswood, beech, and hickory. A walnut tree can easily reach 70 to 90 feet in height and two to three feet in diameter. A maximum height of 150 feet and eight feet in diameter has been reported, although large trees are now rare. Black walnut, butternut, and the hickories are in the same botanical family and closely related. Four other minor walnut species occur in California, Texas, and Arizona; none of these are important for commercial lumber production

Quality and Color

Black Walnut LeavesWalnut's quality can vary greatly by site and geographic location, with several factors affecting a single tree's quality. First, look to the walnut's bark for clues to the characteristics of the potential wood quality inside. Deeply furrowed outer bark with an orangish inner bark color indicates a healthy, vigorous tree, but may also signal the presence of undesired excessive sapwood. A blocky or patchy bark indicates a slow-growing tree, often located on a poor site, containing soft textured wood with a reddish color.

Pine knots can be common in walnut and are often difficult to detect in standing trees. As they are most easily found as sharp spikes of wood on the trunk surface after the bark is stripped off, pine knots are particularly troublesome to buyers of trees and logs for veneer.

Worm holes and their associated stain are another difficult characteristic to detect in standing trees. Since log buyers have learned that worms are more prevalent in certain parts of the walnut range, they tend not to concentrate their purchasing efforts in these regions where warmer climates provide for a longer `insect' season. Although a number of insects or larvae feed on walnut, only a few cause damage, with most of this damage occurring during the sapling stage.

Bird peck is an additional problem that may occur in walnut, although it's more prevalent in other hardwoods. Yellow-bellied sapsuckers probably cause most of the bird peck in walnut. It is generally believed that the bird pecks a hole to cause the flow of sap. Insects are attracted to the sap, which the bird then feeds on. Bird peck in walnut may also indicate the presence of worms. If the bird peck holes are open, the peck did not reach the cam-bium layer, and no damage to the wood is likely. However, if callus tissue is located in the peck hole, the hole is said to be "occluded" and a peck mark will show up in the wood beneath the surface.

Raw Black Walnut WoodWalnut's heartwood color and uniformity of color (especially in veneer-quality trees) is an important factor that varies greatly, especially by location. When first cut, the best colored walnut is light greenish or mint color. As the wood is exposed to the air it turns a gray-brown color, considered ideal on the market. Unfortunately, the color can vary, or it can lack uniformity, resulting in dark or splotchy walnut, known as `muddy walnut'.

The color of walnut can also be affected by manufacturing variables such as cooking schedules for veneer flitches and processing time before drying. Some walnut has `flash' or `flare' in the grain pattern. Although small quantities of this material are generally not preferred by stock furniture manufacturers or veneer producers because it doesn't mix well with the straight-grained wood, both lumber and veneer of this kind can be prized for custom-type jobs. Large figured logs capable of paneling large rooms can be valuable as veneer.

The term `texture' describes a walnut tree's growth rate. A fast growth rate results in a coarse-textured and somewhat harder wood, while a slow growth rate gives a finer texture and softer wood. Texture is particularly important in veneer quality trees where a growth rate of eight to nine rings per inch is usually preferred.

Sapwood, the light-colored band of wood on the outside of a walnut cross section, is considered a defect in veneer because most of it must be clipped off and discarded. Fast-growth trees tend to have a wider sapwood zone than slow-growth trees. In lumber, the wood is steamed to darken the sapwood. If the lumber is not steamed, sapwood is also considered a lumber grading defect by NHLA rules.

Physical Characteristics

Black Walnut WoodWalnut is moderately heavy and hard with good strength characteristics. It machines well and is reasonably easy to dry. The wood has moderate shrinkage. When finished, the wood has a lustrous rich appearance.

Its rich, brown, lustrous heartwood has a grain pattern and pore size between that of the grainy hardwoods, such as oak, and the uniform textured woods, such as maple and yellow poplar.

In comparison to the trees available today, large old growth trees frequently formed burls and also had large swellings at the butts of the trees. Both the lumber and the butts, as well as large crotches, were cut for highly decorative veneer. Some production of burls from grafted English walnut orchards still exists today.


Buyers and sellers should be aware that the National Hardwood Lumber Grades for walnut and butternut are distinctly different than the standard grades that generally apply with slight modifications to other species. Short lengths are accepted in the top grades of walnut. Some short boards are graded by counting defects (called standard defects) rather than measuring clear cutting areas. Minimum board widths for the top grade are narrower and the sizes of cuttings are smaller. As a result, any one grade in walnut will appear to be of lower quality when compared to lumber graded by the "standard rules".

Because walnut was such a valuable and in-demand species for so long, it's probably the most studied and researched hardwood species in North America. Scores of articles have been published and genetically superior stock has been offered. Substantial efforts at planting walnut and caring for plantations have also been put forth.

Black Walnut is sought after for its great beauty and toughness. It is fairly straight grained but can be wavy with a course texture and a dark brown to purplish black color. Its workability is good and it glues well while holding its bending properties. It accepts natural wood finishes extremely well and can be polished to a fine finish. By hot steaming the boards at the kiln, the lighter sapwood will darken to the desired color of black walnut. Perhaps one day, when you're walking through the forest, pick up a few walnut husks from under a tree, then take them home. Use the pigment inside them with household ammonia to make a rich walnut stain that will allow other woods to darken up and continue to darken with time.

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Updated 3/10/06   •   Validated 2/25/06