Black Walnut Trees

As a child, I loved swaying in the wind on a tire swing suspended from a sturdy branch of the black walnut tree on our property. That huge tree was also good for climbing because of its strong, upward-spreading limbs, although the rough, deeply furrowed bark could be a bit rough on the hands!

Gray, furrowed bark of black walnut tree with diamond patterns; photo: Melissa King

Description

The eastern black walnut tree (Juglans nigra) is a majestic deciduous tree native to eastern and central regions of the United States. A member of the Juglandaceae family, this large species boasts dense leaf cover with a beautiful oval crown. Classified as a perennial stone fruit tree, it is well-known for generous clusters of edible “drupes” (skin-covered, fleshy fruit with one central stone and seed), which we refer to as black walnuts.

Round stone fruit on black walnut tree; photo: Melissa King

The round, tough, green drupes, which turn brown as they ripen, are roughly 2 ½ inches in diameter. Beneath their bumpy outer covering, each drupe has a black husk (or hull) containing a substance that will stain clothing and skin, so be careful when handling drupes. Deep inside each drupe is a dark brown fruit (or kernel) commonly known as a black walnut. These drupes ripen in September to October, but no fruit will appear until the tree is at least 4 – 6 years of age, so don’t lose hope if your tree seems barren at first.

Pinnately compound leaflets droop downward on black walnut tree; photo: Melissa King

Black walnut trees grow up to 100+ feet tall, with 70-foot-wide canopies and trunks up to 6 feet in diameter. The dark gray bark is quite distinctive with a diamond-shaped pattern running vertically up the hefty trunk. The tree’s lance-shaped leaflets appear in pinnately-compound arrangements that tend to droop downward. These leaves turn golden yellow in autumn, dropping to the ground early in the fall and not re-appearing again until late spring. In fact, it takes so long for flowers and foliage to leaf out (late May) that one might easily think the tree is dead. On the bright side, many black walnut trees will live a long, productive life and often yield their greatest harvests after 30 years of age.

Reproduction

A black walnut tree is monoecious, with both male and female flowers appearing on the same plant in late May to early June. Male flowers are thin yellowish-green catkins, while female flowers develop as short, green, spiky clusters. Wind pollination is the primary method for reproduction, and regeneration frequently occurs when squirrels bury black walnuts in the ground around the tree.

Climate and Soil Requirements

Black walnut trees grow in zones 4 – 9. They prefer neutral pH and rich, well-drained soil (moist loam is best), but they tolerate drought and moderate, intermittent flooding. Their sizable tap root is deep and widely spread, which enables effective water collection in drier conditions. These trees are commonly found in riparian zones – transition spaces between forests, creeks, and rivers – or scattered on the edges of wooded areas. They thrive in full sun and will not do well in partial shade. Another noticeable characteristic is that a black walnut tree often appears as a “solo” in the landscape, without many other plants nearby. Why? Because it is allelopathic, or capable of suppressing the growth of other plants.

Toxicity

A chemical called juglone is found in the leaves, bark, roots, and nut husks of a black walnut tree. This toxic substance is a respiration inhibitor that can deprive other organisms of the energy needed for their own metabolic activity. Plants that are sensitive to juglone may exhibit negative symptoms rather quickly, turning yellow or brown, looking stunted, wilting, and eventually dying. Depending on the drainage and soil type of the affected area, juglone can remain present in the soil for extended periods of time.

Drupes from black walnut tree in different stages. photo: Melissa King

Juglone is strongest beneath the canopy of a black walnut tree, so it is best not to grow other plants or fruit trees within the drip line. The recommendation is to keep other plantings at least 70 feet away from this type of tree. Due to possible deleterious effects on other plants, avoid using wood chips, bark, or leaves from a black walnut tree as mulch on any garden beds. However, some plants, such as hickory, oak, and sycamore trees, can tolerate juglone and are less susceptible to the ill effects of that chemical. For a comprehensive list of plants that are either sensitive or tolerant of that toxic substance, visit this webpage.

Juglone may cause allergic reactions in some humans, but others may have negligible sensitivity to this chemical. Mild symptoms include hives or itchiness, and more severe reactions include trouble breathing, vomiting, fainting, or swelling of the face or throat. Dogs and other animals who consume moldy walnuts are subject to convulsions, so be sure to clean up fallen drupes under your own black walnut trees if you have pets. Fresh wood shavings from black walnut trees may lead to laminitis in horses, although well-aged shavings do not cause that problem.

Black walnuts fall from the tree when ripe, covering the ground under the canopy; photo: Mellissa King

One more cautionary note about planting location: A profuse crop of black walnuts can be hazardous. Drupes can fall onto pedestrians, who might also trip on them, so avoid planting this tree near sidewalks or residential driveways. Plus, it can be especially difficult to remove those dark stains from shoes!

Diseases

Black walnut trees are susceptible to some diseases. The most serious is walnut anthracnose, a fungal disease which leads to darkened leaf spots, premature leaf fall from the tree, and potentially reduced nut crop. An integrated pest management approach is recommended to control this disease before it becomes widespread in a black walnut tree. Another problem is thousand cankers disease (TCD), which begins when walnut twig beetles carrying fungal spores burrow into the bark to feed on healthy phloem tissue. This results in canker formation, or dead tissue beneath the bark, which impedes the flow of important nutrients throughout the tree. Unfortunately, there is no currently-known method to save a black walnut tree infected with TCD.

Value and Uses

In terms of wildlife support, the black walnut tree is a true winner! Black walnuts are a favorite treat for squirrels and birds, and deer will eat the young buds. The tree is a host plant for more than 100 different species of butterflies and moths, and eastern screech owls will make their homes in black walnut trees.

Full view of black walnut tree in autumn; photo: Melissa King

Black walnuts are known for their bold, earthy flavor and aroma, which many people like. We consume them raw or roasted, cook with them, and use them in a wide variety of baked goods, such as breads, muffins, and cookies. Black walnuts, which are low in carbs and high in fiber, are considered a “superfood” associated with certain nutritional benefits. Per serving, they offer a high level of protein content and omega-3 fatty acids, which support metabolism and cardiovascular health. The husks have antimicrobial and antifungal properties.

Split open shells reveal the dark husks of black walnuts; photo: Melissa King

Lumber from black walnut trees is highly prized for its strength and unique appearance. This hard wood is often used to make furniture, flooring, and veneer. Hull extracts are used to make dyes and in natural hair coloring treatments. In addition, ground up shells from the fruit have interesting applications: as a non-skid component of car tires, filler for dynamite and smokestack scrubbers, and for cleaning jet engines.

I’m happy to say that better understanding of this unusual tree has boosted my fascination with Juglans nigra.