Black Walnut is a native deciduous large tree with an open rounded crown growing from 70 to 90 feet high and up to 4 feet in diameter. Bark is a medium to dark gray (or dark brown) with a darker tone under with, on older trunks, much furrowing and ridging into a rough diamond pattern. There is some toxicity associated with the tree as explained at the bottom section of this page.
Twigs are stout, light brown with leaf scars forming 3 lobes of a shield shape - resembling a so called "monkey face"; the scar does not have any hair on the upper portion of the scar (unlike Butternut). The pith of the twig is light brown separated into a series of chambers. First year twigs have fine hair. Buds are tan with some scales that have fine hair.
Leaves are alternate, and compound with 9 to 21 (13 to 23) leaflets but the odd terminal leaflet is either missing or mal-formed. These compound leaves are dark green on top and can be up to 24 inches long with a stout stalk that has fine hair. Leaflets are broadly lance-shaped, unequal symmetrically, with pointed tips, almost stalkless and have one primary mid-vein with forked laterals. Edges are fine toothed and there may be soft hair on the underside. The central leaf stem ( a rachis) is lighter color - green to yellow-green with fine hair. Fall color is yellow.
Flowers: The tree is monoecious, that is, the male and female flowers are separate. Male flowers appear as long catkins, hanging from the leaf scars of the prior season. Each floret on the catkin is about 1/8 inch wide, has 15 to 40 (17 - 50) stamens and a short calyx of 5 to 6 scales. The female flowers are grouped 2 to 5 on short spikes near the end of the twig on new growth. They are also about 1/8 inch long, have a 4-lobe calyx with fine hair, pistil and a 2-lobed style. Flowering occurs as the leaves begin to develop. Pollination is by wind or self-pollination of the female flowers.
Fruit: Flowers mature to a hard nut enclosed in a furrowed hard husk that is itself contained within a round husk, 2 to 2-1/2 inches in diameter, with an outer shell that is green initially, turning very dark at maturity. Fruits are either single or 2 together. The outer shell does not open by itself. The edible nut inside is 4-celled, oily, but sometimes bitter.
Habitat: Black Walnut grows from a taproot and wide spreading laterals. Transplanting is therefore difficult. The tree will do best in loamy soil in full sun, but tolerates some shade. Best specimens will be free standing but in close quarters the lower branches will wither. Black Walnut has been planted as a boulevard tree and specimens seem to do well in confined quarters as long as moisture can reach the roots, but falling nuts are a hazard in a street planting. Trees need 12 years to produce any quantity of nuts.
Names: For the genus, Juglans, Mrs Grieve (Ref.#7) has the best derivation: "It is said that in the 'golden age' when men lived upon acorns the gods lived upon Walnuts, and hence the name of Juglans, Jovis glans, or Jupiter's nuts." The species, nigra, means black, which refers to the brownish-black bark and wood. The author name for the plant classification- 'L.' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. The name Walnut is thought by many to be derived from the old Roman word for a Walnut tree - nux - and an old German word for it - Wallnuss - this to the English - Walnut. However, Pliny the Elder in Natural History puts it back to the Greek kara, meaning 'head' and karyon for 'nut'. See below for his explanation.
Comparisons: The closest confusing species is J. cinerea, Butternut, usually a smaller tree where the leaflets are fewer in number, the nut husk if more ovoid in shape and forms in clusters of 3 to 5, and the leaf scar bears a row of hair on the upper ridge.
Above: Black Walnut in Eloise Butler. Illustration from North American Sylva by Andre Michaux & Thomas Nuttall.
Below: 1st photo - A tree of medium age showing the open rounded crown. 2nd photo - Walnut bark is a medium to dark gray (or dark brown) with a darker tone under, thin on younger trunks, becoming ridged on medium age trunks and on older trunks (3rd photo), with much furrowing and ridging into a rough diamond pattern.
Below: 1st photo - The characteristic leaf scar having 3 lobes of a shield shape - resembling a so called "monkey face" 2nd & 3rd photos - The male catkins occur near the ends of branches. Each small floret is only 1/8 inch long but can have over 40 stamens in a short green calyx.
Below: The female flowers are grouped 2 to 5 on short spikes near the end of the twig on the new growth. They are also about 1/8 inch long, with fine hair and have a 4-lobe calyx, pistil and a 2-lobed style.
Below: Fruits of Black Walnut form as singles or a cluster of two. The outer shell that is green initially, turning very dark at maturity and does not open spontaneously.
Below: 1st photo - First year twigs have fine hair, retained overwinter as shown in the spring on the prior years twig. Buds are tan with some scales that have fine hair. It is from the old leaf scars that the male catkins grow. 2nd photo = Leaves are compound with 9 to 21 (13 to 23) leaflets but the odd terminal leaflet is either missing or mal-formed.
Below: The underside of a leaflet. Note the unequal symmetry from side to side.
Below: During the Great Depression artists Jackson Lee Nesbitt and Thomas Hart Benton took a sketching trip into the Ozarks and at Jasper Arkansas they met this blacksmith-farmer. Nesbitt writes: "He and his wife were practically living on the walnuts that grew on their farm. In the picture, walnut shells are strewn about the floor as he sits on a seat made of an old explosives crate." Nesbitt turned the sketch into this lithograph in 1988. Click the image for a larger version.
Notes: Black Walnut is not indigenous to the Garden. Eloise Butler first planted it in May 1909 with plants obtained from the Park Board Nursery; 3 more in 1914. Martha Crone also planted it in 1937, '48 and '49. Garden Curator Susan Wilkins added additional plants in 2008. In North America the plant ranges from the central plains eastward, excepting only the Canadian maritime provinces. Within Minnesota it is somewhat restricted in the wild, being found mostly in the counties along the southern border of the state and along the Mississippi River north to Washington County, and Ramsay, Scott and Hennepin in the metro area.
Species: There are two species of Juglans growing in Minnesota: J. nigra and J. cinerea, Butternut. In addition there is one other species in the eastern U.S., four others in the western U.S.; 17 in all in North America and over 50 worldwide.
Uses: Black walnut wood is heavy, strong and highly resistant to shock, fairly straight grained, works easily with tools, machines nicely, and makes a fine surface for furniture products and veneers. Because large trees must be 100+ years of age to produce large pieces of lumber, Walnut trees are quite valuable. Most people, and all squirrels, are familiar with the walnut as a food source. A black dye can be made from the husks. The downside is that the nut husks will put a highly resistant stain on any object or sidewalk they rest on for any length of time. In England, the green nuts of the English Walnut were frequently used for pickling. Fernald (Ref. #6) gives a recipe from 1706 for that and when done correctly, they are delicious delicacy that should not be missed if you have the opportunity. The dried leaves and bark were used for a long time in folk medicine.
Toxicity: Black Walnut produces a chemical, juglone, that is toxic to certain plants growing in or close to the root zone. These include tomato, potato, apple, lilacs, rhododendrons and peppers.
Comments of Pliny the Elder in Natural History, ca 79CE: "Walnuts have acquired their name in Greek (kara = head; karyon = nut) from the heaviness of head that they bring about. The trees themselves and their leaves give out a poison that affects the brain. If the kernels are eaten they have the same effect, although the pain is less severe. Walnuts are more pleasant when fresh; when dried, they are oily, harmful to the stomach and difficult to digest; they cause headaches and are also bad for coughs. The more walnuts one eats, the easier it is to expel tapeworms. Very old walnuts cure gangrene, carbuncles and bruises. The bark of walnut trees cures ringworm and dysentery. The pounded leaves mixed with vinegar cure earache."
Comments of Henry Thoreau in The Dispersion of Seeds: "We have no approach to a pure walnut wood of any size – or any wood at all. They seem to require more than oaks or other hardwoods, light and air and room to expand, before they can become even moderate-sized trees in great numbers. A dense thicket of a thousand or two little trees apparently becomes at last – a few scattered dozens of distinct trees in a pasture. Probably fires do often kill a great number of them. There were a great many in the open land about me when I first came to Walden, but on account of fires or frost or some other reason, there are very few there now. It is remarkable how walnuts love a hillside. Four of the five localities I have chanced to refer to are hillsides. Is it because of the light and air they get there? They spring up almost unaccountably in such places, as if they loved the prospect, or as if they had been ordered to occupy these posts."
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"