South Dakota Trees: Black Walnut

Black Walnut with spring leaves—its "yearly trick of looking new" (Philip Larkin, The Trees). Union Grove State Park, South Dakota.

Walnuts from a previous year, still unopened. Is extreme toughness adaptive?

Continuing my quest to get to know South Dakota's trees, I chose Black Walnut (Juglans nigra) for this month's tree-following. It's native to the state but barely, probably just in the far east. This is another deciduous hardwood that's common in the midwestern and eastern USA, but rare in South Dakota. Humans have long contributed to the spread of these species, so it can be difficult to pin down their "native" range.

Black Walnut trees can reach 150 ft in height. On open sites the crown is large and rounded. In forested habitat it's smaller, atop a tall mostly unbranched trunk. 

Black Walnut is popular in landscaping; Victoria, British Columbia (TreeLib).

Juglans nigra was once "a dominant and majestic canopy species of primeval midwestern and southeastern forests." But with clearing for agriculture, and harvest for railroad ties, gunstocks, log cabins, furniture, ship-building and more, it became much less common (GoBotany). The venerable giants—to 6 or 7 ft in diameter—are gone. Even so, Black Walnut wood is widely available and quite popular—prized for its dark grain (more information at The Wood Database).

Black Walnut in cross section; photo by Roger Culos.

Black Walnut bowl and photo by Joe Nestlerode.

Juglans nigra leaves are alternate, but often cluster at branch tips and appear whorled (MWI).

Black Walnut usually can be recognized by its leaves, which are long (to 6 dm) and pinnately compound with 8–23 leaflets (terminal one often reduced or lacking). Leaflets are lanceolate, to 15 cm in length, and have serrate margins.

Carl Linnaeus, who named and described the species in 1753, included leaf features in the polynomial name: Juglans foliolis lanceolatis argute serratis—a Juglans with lanceolate leaves with serrate margins. Did Linnaeus mistake leaflets for leaves? Perhaps there was no such distinction in his day.

Black Walnut in Linnaeus's Species Plantarum (1753). Note "nigra" in fine print on right.

In the image above, "nigra", "alba" and "regia" on the page margin hint at a revolution underway. The long descriptive polynomial names were being replaced with binomials (still in use). Species Plantarum is considered the first major botanical work to use binomials consistently. This is where Juglans foliolis lanceolatis argute serratis; exterioribus minoribus became Juglans nigra L. (L. refers to Linnaeus, the authority).

Juglans nigra L. from The Linnaean Herbarium, with permission. Linnaeus did not cite a type specimen in his description, so this one was selected as the lectotype (Reveal et al. 1987).

Like all walnut species, Juglans nigra has unisexual flowers and trees bear flowers of both sexes (i.e., are monoecious). Flowers are inconspicuous—small, yellowish to greenish, with no petals and only tiny sepals. Males form catkins (elongate pendulous clusters) to 10 cm long, with numerous flowers. Female flowers occur in little clusters of 1–4 in leaf axils (where the leaf stem attaches to the branch).

Catkins of male flowers, each with numerous stamens (MWI).

Three female flowers in a leaf axil, each with 2 feathery stigmas (TreeLib).

A female flower has a single pistil. With fertilization, it matures to become ... well ... that depends on whom you ask.

 Juglans nigra fruit, fibrous covering partly removed. Is this a nut? drupe? pseudodrupe? (Plant Image Library)

In their treatments of the Black Walnut, early botanists just described the fruit. The first to do so may have been Mark Catesby, an English naturalist who studied the flora and fauna of the American colonies. In his Natural History of Carolina, Florida and the Bahama Islands (1731) he wrote:

"The thickness of the inner shell requires a hammer to break it. The outer shell is very thick and rough on the outside. The kernels are very oily and rank tasted; yet, when laid by some months are eat by Indians, squirrels, etc." [Kernels are seeds, also called nutmeats.]

Illustrations in Catesby's book "were etched by the author"; plants and animals often were paired, e.g., Black Walnut and American Redstart (relative sizes of fruits and bird are correct) (BHL).

In his 1819 American Sylva, François-André Michaux (featured in this post) provided a bit more detail:

"The husk is thick, and ... when ripe it softens and gradually decays. The nut is hard, somewhat compressed at the sides, and sulcated [with narrow grooves]. The kernel, which is divided by firm ligneous partitions, is of a sweet and agreeable taste, though inferior to that of the European Walnut." 

Black Walnut, by the great botanical illustrator Pierre-Joseph Redouté, in Michaux 1819 (BHL). Male flowers upper left; fruit with green husk lower right next to brown nut.

However it wasn't long before botanists were attempting to assign fruits to defined types. Asa Gray, the eminent Harvard professor of botany, included a system of fruit types in his 1868 Lessons in Botany and Vegetable Physiology. Now 156 years later, it's still in use because botanists haven't come up with anything better. This is not for lack of trying (see Judd et al. 2002, for example).

So what did Professor Gray call the walnut? Well, actually ... he hedged!! In his New Manual of Botany. A handbook of the flowering plants (1908) he called it a "a kind of dry drupe". Dry drupe? But Professor, you defined a drupe as having a fleshy outer part and hard inner part, both derived from the pistil. You provided the cherry, plum, and peach as familiar examples.

As Professor Gray demonstrated, the walnut is not easily categorized. And it's not alone—more than a few fruits confound us in this way.

Parts of a walnut: husk, shell (nut), nutmeat (seed). Wild Harvests.

Some botanists take a strict approach: a fruit must develop from a pistil. This is where the fruit of the walnut—specifically the husk—causes problems. "[It] superficially resembles a drupe, with a hard 'stone' surrounded by a soft, often fleshy husk. The husk, however, is not part of the fruit wall (it develops from the involucre and calyx), and the fruit is actually a nut." (Juglandaceae in Flora of North America).

The Wikipedia Walnuts article mostly agrees, but calls them "accessory fruit because the outer covering [husk] of the fruit is technically an involucre and thus not morphologically part of the carpel [pistil]; this means it cannot be a drupe but is instead a drupe-like nut."

Other botanists are more broad-minded. A fruit with a (relatively) soft outer part and hard inner part can be a drupe, no matter the origins of the parts. Some avoid the controversy altogether by calling the walnut fruit a "drupe or nut" or explaining that it's a nut "but some experts call it a drupe."

That's more than enough discussion of disputed terminology. Let's turn now to something for which there is widespread and probably unanimous agreement. The Black Walnut is a tough nut to crack, in fact one of the toughest!

Cracking a Black Walnut Appalachian style (Blind Pig & the Acorn).

Black Walnut nutmeats are available commercially thanks to "high-tech" processing (video here). But there are alternatives. For those who aspire to collect, clean and crack Black Walnuts themselves, a wealth of helpful information is available online. The most common approach is a hammer, as Mark Catesby recommended nearly three centuries ago. Iowa State University Extension and Outreach explains:

"The hammer method involves placing the nut, pointed end up, on a hard surface and striking the point with the hammer until it weakens and splits into sections along its axis ... shattering of the kernels is often a problem. Shattering can be reduced by soaking the nuts in water for 1 or 2 hours before cracking. The soaking process allows the kernels to absorb enough moisture to become somewhat flexible, resulting in larger kernel pieces."

"Once split, use a pick or plier to remove the kernels inside." (ISU Extension & Outreach)

And for those of us with less patience, John Sankey includes a variety of tools on his Black Walnut Crackers webpage, with tips on use.

The Duke Black Walnut Nutcracker is highly-recommended ($78 on Amazon).

Sources, in addition to links in post

Catesby, M. 1731. Natural history of Carolina, Florida and the Bahama Islands. Volume 1 (Black Walnut p 67). "The illustrations were etched by the author from his own drawings and hand colored under his direction." Catesby also paid for printing. BHL

Gray, A. 1868. Gray's lessons in botany and vegetable physiology. NY: Ivison, Blakeman, Taylor & Co. (BHL).

Gray, A. Circa 1908. Gray's new manual of botany. A handbook of the flowering plants and ferns of the central and northeastern United States and adjacent Canada. NY: American Book Co. BHL

Judd, WS, et al. 2002. Plant Systematics, 2nd ed. Sinauer Associates, Inc.

Linnaeus, Carl. 1753. Species Plantarum 2, p. 997. BHL.

Michaux, F-A. 1819. The North American sylva, or A description of the forest trees of the United States, Canada and Nova Scotia. BHL

Nelson, G, et al. 2014. Trees of eastern North America. Princeton University Press.

Reveal, JL, et al. 1987. On the identities of Maryland plants mentioned in the first two editions of Linnaeus' Species plantarum. Huntia 7:209–246. PDF