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

South Dakota Trees—why a mulberry is not a blackberry even though it looks like one

Geek alert! (ʘ‿ʘ)╯(photo modified from Flickr).

Continuing my exploration of the South Dakota sylva, this month's featured tree is the mulberry. Supposedly there are two species in the state, more on this later. The fruit is much more interesting.

A mulberry looks a lot like a blackberry—enough so that the Wikipedia article on blackberries mentions mulberries. But blackberries are in the genus Rubus in the Rose Family, while mulberries are in the genus Morus in the Mulberry or Fig Family. And though the "berries" look alike, botanists who know them (as I now do) will tell you they are very different.

Wild blackberry harvest, by Gandydancer.

Fallen mulberries in peak season, by Awinch1001.

Botanically speaking (vs. grocery store produce classification) fruits develop from flowers, specifically the female sexual part(s). Pistil(s) containing ovules mature to become fruit(s) containing seeds. If we run this film backwards—i.e., reverse development—it becomes obvious how different mulberries and blackberries are.

A blackberry consists of tiny fleshy seed-containing units called drupelets, making it an "aggregate fruit". If we run the film backward, we see that each drupelet started as one of many pistils in a single flower.

Blackberries in a range of ripeness, each one a cluster of drupelets; by Ragesoss.

Blackberry flower with many yellow-tipped pistils in the center. Each pistil will become a drupelet; together they will be a blackberry. By I, Luc Viatour.

But if we run the mulberry film backward, we're in for a surprise (I was anyway). A mulberry also is a cluster of little fleshy units, but each of these units started as the single pistil of one flower. In other words, a mulberry develops from a cluster of several to many flowers—an inflorescence! 

Back to the first photo. It's a mulberry—a cluster of fleshy modified flowers.

Female mulberry flowers, each with a single pistil (two styles at tip); the four inflorescences (clusters) will become four mulberries (Minnesota Wildflowers).

Morus flowers are unisexual (often trees are as well). A female flower contains a single pistil that develops into a small dry fruit (achene), while the outer whorl of flower parts (the calyx) becomes fleshy. The result looks like a drupe but is a modified flower. Fruits such as these—formed from multiple flowers—are categorized following a long and venerable tradition.

The great Asa Gray of Harvard University, whose 150-year old system of fruit classification is still in use (because botanists haven't come up with anything better) covered fruits in Lesson 20 of Lessons in Botany and Vegetable Physiology. He started with a brief definition—"The ripened ovary with its contents, becomes the Fruit"—and then immediately addressed an especially problematic situation.

"Some fruits, as they are commonly called, are not fruits at all in the strict botanical sense. ... mulberries, figs, and pineapples are masses of many fruits ... resulting from several or many blossoms, aggregated into one body" (italics mine).

Gray assigned them to the category Multiple or Collective Fruits, as many of us do today (more here).

Asa Gray, "the most important American botanist of the 19th century"; photo from 1870s, source.

Both White and Red Mulberries have been reported for South Dakota. The former, M. alba, is non-native, used for landscaping, and occasionally escapes and persists. Red Mulberry, R. rubra, is said to be native to southeast South Dakota, but so far I've seen no reliable specimens, partly because the two are tough to tell apart.

Their names don't help. The fruit of both can range from white to red and is usually black at maturity but sometimes remains white in White Mulberry. Both have edible fruit, but while red mulberries are sweet and delicious, white mulberries are said to be bland.

Leaves are highly variable in shape, ranging from deeply lobed to entire in both species.

Leaves of White Mulberry, Morus alba, c. 3–4 inches long; Minnesota Wildflowers.

Red Mulberry, Morus rubra, entire and lobed leaves on the same branch! TreeLib.

Flora of North America offers hairiness of leaves as a way to distinguish between the two species (see key to species), but then warns that hairiness is variable in both, perhaps due to hybridization.

During my recent trip to southeast South Dakota, I met a mulberry. Whether it was white or red I can't say. If you-the-reader have mulberry identification tips, please leave a Comment below.

Mulberry tree (Morus sp.) front and center; Union Grove State Park, South Dakota.

Three young mulberries (clusters of flowers).

This is my monthly contribution to the gathering of Tree Followers kindly hosted by The Squirrelbasket. It also contributes to a web-based Guide to South Dakota Trees and Shrubs currently under construction.