Black Walnuts
The black walnut is a polarizing plant, depending on who you ask. While it produces large nuts with complex flavors, those same notes of butter can be described as old feet. For non-foragers, the black walnut is a menace to suburban living, dropping hundreds, if not thousands of tennis ball-sized drupes, finding their way beneath the feet of the unsuspecting passerby. The black walnut, however, has been a staple of North American communities for thousands of years, originating on the continent's southeast and working its way across the continent.1
Evidence suggests the black walnut has existed since at least the late cretaceous period, spanning a longer existence than even the ubiquitous oak tree.2 The delicious— or feety— black walnut is one of the few foods we share today with the dinosaurs that inhabited our planet millions of years ago. Despite walnuts existing in every hardwood forest type, they have never played a central role in forest ecology. That doesn’t diminish their important role in human history as food, timber, dye, and more. The process of selecting trees with large, sweet nuts with sufficiently thin shells for easy cracking is thought to have begun in or around Persia over 1000 years ago, and this selection has significantly impacted how it exists in the landscape today.3 While pushed to the margins of conventional agriculture, it was only in the early 20th century that the black walnut was researched in earnest. This was part of the permanent agriculture movement, which focused heavily on tree crops, spearheaded by folks like J. Russell Smith and others. Outside of this movement, black walnuts have been considered historically (post-colonially) as a pest in farm fields, sprouting up quickly in deep, fertile soils.
This doesn’t mean farmers weren’t casually selectively breeding black walnuts prior— in the 1800s, many farmers were selectively choosing the large black walnuts with the thinnest shells.4 However, despite this, little work was being done to replenish the trees lost to clearing, and interest focused on propagation and identification of the species that existed. Organizations like the Northern Nut Growers Association sprung up to accelerate the process of sharing knowledge and cultivars through contests and journals. The reality is that much of the breeding work had likely been done by indigenous tribes and farmers enjoyed the fruit of these hundreds of generations prior.
Alongside organizations like the NNGA, the Tennessee Valley Authority spent many years, from 1934-1960, identifying trees and cultivars that were well adapted to the valley, under the watchful eyes of folks like Arthur E. Morgan and John W. Hershey (under J. Russell Smith’s watchful eye).
As the 1960s rolled in, efforts shifted from the identification of existing black walnuts and instead centered on walnut breeding for improved genetics. As shortages became apparent in the walnut timber industry, the focus was placed on breeding faster, straighter black walnuts. However, a poor understanding of walnut genetics inhibited these breeding projects; stands of trees for breeding would show significant variation in every trait, with southern walnuts growing taller and wider, faster.5
By the end of the 1960s, there were 18 nut cultivars that were widely used for nut production in the Central Hardwood Region. Researchers were working to find a way to produce straighter trees, thinner-shelled nuts, and trees that showed stronger resistance to Gnomonia lepostyla, a fungus from Iran that spread across the globe, causing leaf loss and fruit damage.6
Newer breeders focused on nut production, like the Missouri Center for Agroforestry, have utilized the successes in Persian Walnut breeding to highlight the goals and methods of a black walnut breeding program. In Persian walnuts, wide-spreading tree crowns have proven to be the most productive.7 Similar to the Persian walnut, spur-bearing in eastern black walnut greatly increases productivity.8 Spur-bearing habit refers to a crown architecture with multiple, short, compact branches that are at least 2 years old that arise along primary limbs and annually produce a fruiting cluster— the way you might think of apple trees in an orchard with large amounts of fruit on each branch.
The counterweight to increasing annual production is working to even out year-to-year yield stability, as high production within a given year can impact the following year’s crop because of reduced availability of nutrients, ultimately leading to low yields. Therefore, year-to-year yield stability is equally relevant to high production within a given year. Historically, these high production years have been called mast years, and while historically these have driven economies based on surplus, as described in our oak piece, reducing these variations in productivity is crucial in making black walnut a meaningful part of our diet.
Historical use
On a longer timeline, black walnut has existed in North America as a fixture of indigenous diets on a much shorter timeline than one might expect. For example, while there’s evidence of chestnut and butternut pollen in the northeastern United States around 6,000 BP (before Present), black walnut only appeared marginally until roughly 500 BP.9 While chestnuts, oaks, and hickories thrive in rocky and acidic soils, black walnuts prefer loamy, neutral pH soils, so it’s not entirely surprising that a continent that is primarily made up of the former soil type would support little black walnuts without specific management, whereas the black walnut was utilized more heavily in the rich, loamy soils west of the Appalachians.
The idea of the late arrival of black walnuts on much of the East Coast is explored in detail in M.W. Wykoff’s research in 1991, where they suggest that the distribution of black walnut trees in central New York specifically lines up with the locations of Five Nations settlements. While alone, this is an interesting finding, paired with the fact that these black walnuts are isolated from any other black walnut populations at the time suggests that Haudenosaunee people may have deliberately planted these trees, along with species like hackberry and hickory.10 Black walnuts likely thrived after moving into these spaces, as they are well adapted to fire with thick, protective bark.11 These trees were also planted appropriately, for example, early 19th century records highlight black walnut trees “nine feet in diameter” around Walnut River in eastern New York.12
Historically, the bark of black walnut was used by several Native American communities including the Cherokee, Delaware, Iroquois, and Meskwaki, in tea as a cathartic, emetic, or disease remedy agent, and chewed or applied for toothaches, snake bites, and headaches.13 The Cherokee, Chippewa, and Meskwaki also used the bark to make a dark brown or black dye. The Comanche pulverized the leaves of black walnut for the treatment of ringworm, the Cherokee used leaves to make a green dye, and the Delaware used the leaves as an insecticide.
While many American Indian tribes ate the walnuts, the Apaches were particularly fond of the nuts. They ate nut meats fresh and raw or added them to a variety of culinary dishes.14 Nuts were often ground and added to pemmican, soups, and baked goods. Oily mashed nuts were also used to flavor beverages, particularly in the southwest where they were mixed with agave pulp.15
More recently, after the black walnut had been transported to Europe in the 17th century, the French developed complex silvicultural systems around these trees. Their managed forests were designed around a coppicing-with-standards model.16
Coppicing with standards means cutting a tree down to its roots, allow one of the new shoots to return as the ‘main’ trunk, while the other shoots are cut on a shorter cycle for other uses, giving the landscape the benefits of the canopy tree as well as the young shoots which can be used for a number of things (we’ll be writing about this shortly).
Check out these Jug(lone)s
A cursory interest in black walnut is typically met with the dangers of a chemical compound in black walnuts, juglone. What is juglone? Juglone, also called 5-hydroxy-1,4-naphthalenedione, is an organic compound with the molecular formula C10H6O3. In 1921, M.T. Cook at the Virginia Agricultural Experiment Station discovered that tomato plants near black walnut were adversely affected, showing wilted leaves. A big break in the case against black walnut came in 1928 when Everett Davis of the Virginia Agricultural Experiment Station presented the results of a study describing a newly isolated chemical from black walnut. Now, these two studies were based on anecdotal evidence, going back to Pliny the Elder, who wrote in his Naturalis Historia:
The shade of the walnut even caused headaches in man and injury to anything planted in the vicinity.
Black walnut became (and in some cases remains) the textbook example of allelopathy. But is it valid?
To examine this claim, we should circle back to the original experiment by Everett Davis. His experiment involved direct stem injection of unknown concentrations of juglone, without mention of control groups. All that remains today of this experiment is the published abstract of a presentation— nothing that could be recreated.17 The tests that followed applied juglone at far higher concentrations than would ever be found in nature, and the field tests often relied on anecdotal evidence from crop failures at the edges of agricultural fields where failure would be more common.
Fortunately, other scientists noted how this testing was done, and in a 3-year study, researchers assessed the same plants with exposure to black walnut and found no significant plant growth differences based on exposure to black walnut.18 This was, obviously, not the final study, and while evidence has shown juglone to have negative effects, its impacts have not been consistently proved— for every test that has shown it inhibits growth, another has shown otherwise.
The fact that there’s inconsistent results should highlight that there’s something more to this ‘walnut wilt’ caused by juglone than what is being tested, right? And the fact that researchers have documented juglone in other common plants, including fabacaeae (beans), the penultimate companion crop, should also give someone pause.19 What that should mean is that it’s clearly not as simple as black walnuts equal juglone equal poor companion plants. In fact, research is beginning to explain some of this, although there is still much work left to be done.
One thing that has become clear is that juglone is not distributed evenly across the tree, and that distribution changes based on a number of factors, including time of year.20 Those concentrations are further complicated, being unique on a tree-by-tree basis. While it’s produced in the leaves, evidence seems to be pointing to the roots carrying most of the juglone within the plant.21
Only in the last few decades have researchers begun exploring this process in further detail. New studies are suggesting that there’s a number of factors that dictate how much juglone is made available in the soil for other plants, including the soil composition itself, the microbial community, the time of year, soil organic matter, and even the clay content specifically within the soil.22,23 While seasonal differences were measurable, they were not significant, and soils showed that they were often capable of breaking down small amounts of juglone before it would ever reach another plant.
The question of where the juglone is coming from is also an important one— in Gillespie’s study (cited above), they trialed using root barriers to assess the impact of roots and juglone concentration, which showed that the areas where the roots had been blocked had 80% lower amounts of juglone in the soil. Further, the areas where the roots had been barricaded from expanding further showed significantly higher amounts of juglone than any other parts of the root system.
Now this raises the obvious question— how does this juglone eventually break down? A bacterium (Pseudomonas putida) isolated from soil beneath black walnut trees has been found to metabolize juglone.24 When in contact with this bacterium, juglone is converted to 3-hydroxyjuglone, then more slowly to 2,3-dihydroxybenzoate, and further to 2-hydroxymuconic acid semialdehyde. The allelopathic potential of these juglone degradation products still remains unknown. The evolutionary development of this bacterium to specialize would suggest that the juglone is, in fact, a consistent chemical around walnuts (and, as we’ve covered, other species), and the question is whether or not the chemical is metabolized quickly enough, and further— what are the impacts of those untested chemical compositions?
One area that’s been insightful and might help explain the inconsistencies of testing comes from a rather boringly titled paper, “Bioassays for allelopathy in terrestrial plants”. In this paper, they argue that the factor that is consistently inconsistent in much of the research is plant density. Reduced growth at low but not at high plant densities is very difficult to explain on the basis of resource competition. One implication of this experiment is that even when juglone is present in toxic amounts, toxicity may not be manifested if the target plant density is high enough.25 In other words, by planting densely, the test subjects (i.e. tomato plants), are able to redistribute the amount of juglone, reducing the juglone-in-soil-per-plant. While the evidence of this is slim, it does help close some of these conflicting pieces of research.
While testing has been applied around juglone and plant toxicity, it is surprisingly limited. There is substantial circumstantial evidence to suggest that juglone contributes to instances of “walnut wilt”, or plants suffering in proximity to black walnut, but the lack of actual, quantifiable data between laboratory and field studies leaves even this basic hypothesis surprisingly poorly supported.26 Now this doesn’t mean that juglone is not an issue, but rather that there’s a significant gap in data-backed field evidence to account for the diversity of players in a forest than, say, a laboratory.
Processing Walnuts— building new food infrastructures
One of the biggest challenges of black walnuts as a staple in our diet is simply that the nuts are hard to process. Historically, for cracking nuts, cuplike depressions the size of the nut were picked into small boulders or slabs of shale. The nut was placed in the depression and cracked or crushed with a suitable stone.27
The first major attempt at scalable black walnut processing came as a secondary effort of pecan processing with E.A. Smalley’s Smalley Manufacturing Company. Until this point, and even for decades longer, most black walnut processing was done by employees using hammers to crack the nuts and separating the shell from the nutmeats by hand— As the Gravette Shelling Company did well into the 1960s. Almost all of these companies focused on making black walnuts a viable crop stemmed from the same region— the western half of Appalachia.
Companies came and went, all trying to crack the nut (hah) of making a viable black walnut industry. Many lasted only a few years, offering new techniques, many of which were ineffective or not scalable. Gravette Shelling Company, the folks still using hammers to crack open nuts into the 60s, was later purchased by Hammons Products Company, which is the largest producer of black walnuts today.28 While they’ve refined the process of separating the nut meat from the shell, their equipment is proprietary.
However, what is of particular interest is their business model. They own no trees, they produce zero walnuts. Their model is based on individuals collecting the fallen nuts from their own trees and bringing them to weigh stations, where they are paid for the nuts by the pound. This model offers insight into how many similar ‘wild’ foods could potentially be processed, such as acorns, hickories, and more. Centralizing production and giving a reason for individuals to have a stake in healthier ecosystems through nut trees in particular is a win-win; the ideal situation would be to set up production under a cooperative model. Even if an individual didn’t care to harvest the nuts themselves, teams of folks could travel through neighborhoods, increasing efficiency and building the early stages of a community resource collection team.
Think of it this way; there’s a team of folks who would harvest wild foods from a community— acorns, then hickories, black walnuts, persimmons, and through the spring they could harvest maple and then birch sap. The same way we have libraries and even tool libraries, we could have processing centers for communities to process their own foods.
Currently, according to nuts.com, there’s still a substantial price difference between black walnuts and English walnuts, $18.99/lb versus $10.99/lb, respectively, so there’s still work to be done. At the very least, this should give folks pause to consider how different ways of processing and relating to food can offer new insights into what it means to create place-based food systems.
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