Juglans Nigra, commonly known as the Black Walnut tree, is a species of deciduous tree native to eastern North America. The tree produces edible nuts that are highly valued for their flavor and nutritional value. In addition to its practical uses, the Black Walnut is also a popular shade tree and ornamental plant, admired for its large size, attractive foliage, and interesting bark.
I. Appearance and Characteristics
Juglans nigra, the eastern American black walnut, is a species of deciduous tree in the walnut family, Juglandaceae, native to North America. It grows mostly in riparian zones, from southern Ontario, west to southeast South Dakota, south to Georgia, northern Florida and southwest to central Texas. Wild trees in the upper Ottawa Valley may be an isolated native population or may have derived from planted trees.
- Odor: Most parts of the tree including leaves, stems, and fruit husks have a very characteristic pungent or spicy odor. This odor is lacking in the nut itself.
- Trunk: Height 30–40 m (100–130 ft). Under forest competition, it develops a tall and straight trunk. When grown in an open area it has a short trunk and broad crown.
- Bark: The bark is typically grey-black and deeply furrowed into thin ridges that gives the bark a diamond shaped pattern.
- Pith: The pith of the twigs is chambered and light brown.
- Buds: The buds are pale silky and covered in downy hairs. The terminal buds are ovate, and 8 mm (5⁄16 in) long, and slightly longer than broad, the lateral buds are smaller and superposed.
- Leaves: The leaves are pinnately compound and alternately arranged on the stem. They are 30–60 cm (1–2 ft) long, typically even-pinnate but there is heavy variation among leaves. The stems have 15–23 leaflets, when terminal leaf is included, with the largest leaflets located in the center, 7–10 cm (2+3⁄4–4 in) long and 2–3 cm (3⁄4–1+1⁄4 in) broad. The leaflets have a rounded base and a long pointed (acuminate) tip as well as having a serrated edge. The leaves are overall dark green in color and are typically hairy on the underside.
- Leaf scar: The leaf scar has three prominent bundle scars and has a notch on the side that points toward the tip of the branch (distal side)
- Flowers: Black walnut flowers are monoecious. The male (staminate) flowers are in drooping catkins 8–10 cm (3+1⁄4–4 in) long. These are borne from axillary buds on the previous year’s growth. The female (pistillate) flowers are terminal, in clusters of two to five on the current year’s growth.
- Fruit: Ripens during the summer/autumn into a spherical fruit (nut) with a brownish-green, semi fleshy husk and a brown, corrugated nut. The whole fruit, including the husk, falls in October; the seed is relatively small and very hard.
The fruit production tends to occur irregularly with some years producing larger crops than others (see mast year). Fruiting may begin when the tree is 4–6 years old, but large crops take 20 years. Total lifespan of J. nigra is about 130 years. Like other trees of the order Fagales, such as oaks, hickories, chestnuts, and birches, it is monoecious, with wind-pollinated catkins. Male and female flowers are in separate spikes, and the female flowers typically appear before the male on a single tree (dichogamy). As a consequence, self-pollination is unlikely. However, individual trees are commonly self-compatible; if they are not pollinated by neighboring trees, they may set self-fertilized seeds.
For maximum seed germination, the seeds should be cold-moist stratified for 3–4 months, although the exact time depends on the seed source. The seedlings emerge in April or May. While most trees with taproots have a reputation for slow growth, black walnut is an exception and can achieve very rapid growth in the seedling stage, typically 90 cm (35 in) their first year and even more in the second year. Black walnut will not leaf out until temperatures have warmed sufficiently. Leafout in spring is initiated when daytime highs reach approximately 70 °F (21 °C) and leaf drop in fall when daytime highs fall below 65 °F (15 °C).
As such, the exact timing will vary in different regions of the US and depending on the weather conditions from year to year, leafout is typically early April in the southern part of its range and sometimes not until the end of May or beginning of June in cooler areas. Leaf drop in fall may begin in late September in cooler regions and not until November in southern areas.
Black walnut has a strong taproot, which makes the seedlings resilient, but difficult to transplant.
Black walnut is more resistant to frost than the English or Persian walnut, but thrives best in the warmer regions of fertile, lowland soils with high water tables, although it will also grow in drier soils, but much more slowly. Some specimens have been found to survive frosts down to −43 °C. Some soils preferred by black walnut include alfisol and entisol soil types. Black walnut grows best on sandy loam, loam, or silt loam type soils but will also grow well on silty clay loam soils. It prefers these soils because they hold large quantities of water, which the tree draws from during rainless periods.
Visually, black walnut is similar to the butternut (Juglans cinerea) in leaf shape, and the range also overlaps significantly. The fruits are quite different, and their presence makes an identification easy, as black walnut fruits are round (spherical) and butternuts are more oval-oblong shaped. When a fruit is not available, two species can be differentiated based on the leaf scars, or the place where the leaf meets the stem: butternut has a leaf scar with a flat upper edge and with a velvety ridge above that flat part, but black walnut has an indented leaf scar with no hairy ridge.
II. How to Grow and Care
Sunlight
Black walnut can grow in full sun, part shade, or dappled shade. Full sun is considered best, so south and west are considered to be the best aspects for its growth (or north and east, if in the southern hemisphere). It is important to note that black walnut secretes a chemical into the soil that makes it difficult for other plants to grow underneath its canopy.
Watering
Regular watering is needed for black walnut, especially during the rainless periods, during the first summer after planting. Container-grown plants should never be dry out; this may mean watering daily is suitable.
Temperature
Black walnut is considered hardy and can survive in a minimum temperature range of -20 ℃ to -15 ℃. It likes moist but well-drained soil and requires little if any, ongoing maintenance.
Soil
Black walnut grows well in any type of soil. Drainage of the soil is the only important thing.
Fertilizing
Ground-grown black walnut does not need extra fertilizer, while container-grown plants need a slow-release fertilizer applied every spring (unless recently repotted).
Planting Instructions
When choosing the best place to plant a black walnut, it is important to consider two things: the amount of sunlight, and any surrounding planting. When it comes to light, black walnut can cope with part or dappled shade but will grow and fruit best when planted in full sun. East-facing aspects (or west, if in the southern hemisphere) are best avoided in areas that are prone to late frosts. In the spring, young shoots can be damaged if exposed to morning sunlight after a frosty night.
Black walnut is classed as allelopathic trees, as it releases chemicals that have some type of effect on another plant. The fallen leaves and husks contain a chemical called juglone, which acts as a natural herbicide and prevents competing vegetation from growing. Juglone levels are highest within the drip line of the tree (the outermost circumference of the tree’s canopy, from which water drips onto the ground), so anything growing in this area is likely to yellow and wilt. Apples, tomatoes, pines, and birches are most susceptible to juglone poisoning.
Black walnut can be planted at any time from winter to spring into any rich, well-drained soil type. Dig a hole three times wider and to the same depth as the pot. Loosen the soil around the sides and base of the hole with a fork. Remove the tree from its pot and slightly loosen the root ball with your fingers to tease out the root system and encourage it to grow out from the planting hole. Place the tree into the hole so that the top of the root ball is at the same level as the soil surface. Turn it until you are happy with the appearance and direction the branches are going to grow in.
Backfill the hole, firming the soil in gently as you go. Water well and add a layer of mulch 5 to 8 cm deep. Do not mulch too close to the stem, as this can lead to rotting. Larger, top-heavy trees can be staked to prevent wind rock. Hammer in a stake at a 45-degree angle to avoid damage to the root ball, and attach the tree to the stake with a tree tie.
Although not recommended for good nut crops, black walnuts can be grown in containers. It is planted in the same way as a ground-grown tree: into a tree or shrub compost with sharp sand incorporated to assist with drainage. As before, firm soil gently and water well. Mulch can be added to the container if required. Container-grown plants should always be raised slightly from the ground to allow water to drain freely from the holes in the base of the pot.
Pruning
Black walnut naturally develops an attractive, branching framework, and therefore needs very little pruning. Only dead, diseased, damaged, or crossing branches need to be cut off in spring. Simply cut just above a bud or branch with sharp and sterilized tools.
Propagation
Black walnut can be grown from seeds. Collect and shell the nuts when they are fresh in the fall after they have fallen naturally from the tree. Seeds of black walnut need to go through a period of winter cold to trigger germination. This process is known as stratification. In areas that do not experience long periods of winter cold, the stratification process can be mimicked by placing fresh seeds into the refrigerator for 3-4 months.
After that, sow individually into a gritty seed and cutting compost. Water well and place outside in a sheltered position out of direct sunlight. Seeds will usually emerge in late spring when the weather begins to warm again. Seedlings of black walnut can be difficult to transplant due to the long taproot, so pot into deep pots at an early stage and take care not to disturb the root system.
Pests and Diseases
Maggots (larvae of Rhagoletis completa and Rhagoletis suavis) in the husk are common, though more a nuisance than a serious problem for amateurs, who may simply remove the affected husk as soon as infestation is noticed. The maggots develop entirely within the husk, thus the quality of the nutmeat is not affected. However, infestations of maggots are undesirable because they make the husk difficult to remove and are unsightly. Maggots can be serious for commercial walnut growers, who tend to use chemical treatments to prevent damage to the crop. Some non-chemical controls also exist, such as removing and disposing of infested nuts.
The walnut weevil (Conotrachelus retentus) grows to 5 millimetres (3⁄16 in) long as an adult. The adult sucks plant juices through a snout. The eggs are laid in fruits in the spring and summer. Many nuts are lost due to damage from the larvae, which burrow through the nut shell.
Black walnut is affected by European canker (Neonectria galligena). The infection spreads slowly but infected trees eventually die.
The walnut caterpillar (Datana integerrima) and fall webworm (Hyphantria cunea) are two of the most serious pests, they commonly eat the foliage in midsummer and continue into autumn.
Codling moth (Cydia pomonella) larvae eat walnut kernels, as well as apple and pear seeds.
Important leaf sucking insects include species of aphids and plant lice including (Monellia spp. and Monelliopsis spp.), which suck the juices from leaves and often deposit a sticky substance called “honey-dew” on the leaf surface that may turn black and prevent photosynthesis; and the walnut lace bug (Corythucha juglandis), which causes damage when the adults and nymphs suck the sap from the lower surfaces of walnut leaflets.
A disease complex known as thousand cankers disease has been threatening black walnut in several western states. This disease has recently been discovered in Tennessee, and could potentially have devastating effects on the species in the eastern United States. Vectored by the walnut twig beetle (Pityophthorus juglandis), a fungus, Geosmithia morbida, spreads into the wood around the galleries carved by the small beetles. The fungus causes cankers that inhibit the movement of nutrients in black walnut, leading to crown and branch dieback, and ultimately death.
III. Uses and Benefits
- Ornamental uses
Black walnut is a beautiful enough tree to have earned an Award of Garden Merit. It is commonly grown commercially, in large public parks or very big yards (it usually grows to around 30 m tall), as it provides excellent shade; but is too big for urban areas. Black walnut is usually planted as a standalone specimen due to the toxicity of falling fruits that can harm other plants, but trees like the sugar maple and flowering dogwood work well with it.
- Culinary uses
Black walnut nuts are edible and shelled commercially in the United States. About 65% of the annual wild harvest comes from the U.S. state of Missouri, and the largest processing plant is operated by Hammons Products in Stockton, Missouri. NPR affiliate KCUR stated in an article that “Ralph Hammons began the company in 1946 with a nut cracking machine acquired from Tennessee.” The Stockton Black Walnut Festival, which has been held annually since 1961, “brings the community together for a 3-day event jam-packed with activities including a carnival, tractor pull, nut roll and 2-hour parade,” stated Alexa Hodges in a VOX article.
The nutmeats provide a robust, distinctive, natural flavor and crunch as a food ingredient. Popular uses include ice cream, bakery goods and confections. Consumers include black walnuts in traditional treats, such as cakes, cookies, fudge, and pies, during the fall holiday season. The nuts’ nutritional profile leads to uses in other foods, such as salads, fish, pork, chicken, vegetables and pasta dishes.
Tapped in spring, the tree yields a sweet sap that can be drunk or concentrated into syrup or sugar that is not unlike the sap of sugar maple.
- Dye
Black walnut drupes contain juglone (5-hydroxy-1,4-naphthoquinone), plumbagin (yellow quinone pigments), and tannin. These compounds cause walnuts to stain cars, sidewalks, porches, and patios, in addition to the hands of anyone attempting to shell them. The brownish-black dye was used by early American settlers to dye hair. According to Eastern Trees in the Petersen Guide series, black walnuts make a yellowish-brown dye, not brownish-black. The apparent confusion is easily explained by the fact that the liquid (dye) obtained from the inner husk becomes increasingly darker over time, as the outer skin darkens from light green to black. Extracts of the outer, soft part of the drupe are still used as a natural dye for handicrafts. The tannins present in walnuts act as a mordant, aiding in the dyeing process, and are usable as a dark ink or wood stain.
- Industrial
Walnut shells are often used as an abrasive in sand blasting or other circumstances where a medium hardness grit is required. The hard black walnut shell is also used commercially in abrasive cleaning, a filtering agent in scrubbers in smoke stacks, cleaning jet engines, cosmetics, and oil well drilling and water filtration.
- Wood
Black walnut wood showing the color and grain
Juglans nigra wood in cross section
Black walnut is highly prized for its dark-colored, straight grained, true heartwood. It is heavy, strong, shock resistant and yet can be easily split and worked. Along with cedars (Thuja spp.), chestnut (Castanea spp.), and black locust (Robinia pseudoacacia) black walnut is one of the most durable hardwoods in the US. The wood can be kiln dried and holds its shape well after seasoning, which makes this wood even more attractive for wood working.
Walnut wood has historically been used for gun stocks, furniture, flooring, paddles, coffins, and a variety of other wood products. Black walnut has a density of 660 kg per cubic meter (41.2 lb/cubic foot), which makes it less dense than oak.
