This post concerns eastern black walnut (Juglans nigra). Why? Being aware of their edibility and never having tasted a black walnut, I decided to take advantage of their abundance while living in Belgrade Lakes, Maine. Black walnuts have highly valued wood and interesting ecological significance in addition to their tasty nuts.
Two species closely related to black walnuts are the English walnut and the butternut. The English walnut (Juglans regia) is common to grocery store mixed nut selections and native to Europe. Like black walnuts, butternuts (J. cinerea) are native to Eastern North America, have edible nuts and are common ornamental and forest trees. I vividly remember collecting neighbors' butternuts in a bucket for squirrels when I was a child: Pericarps adhered uncomfortably to my bare feet; grime stuck to my hands and stained my clothes.
The pericarp, the outermost layer of the black walnut, differs from the butternut's. The butternut's is thin and sticky; the black walnut's isn’t and can be quite thick. The outer pericarp is often called the "husk," it is green at maturity, gradually turns yellow, softens and will eventually turn black. Having an almost citrus-like scent when fresh, the husk will stain skin black. Inside the husk lies the hard, rough inner pericarp. Cracking this will reveal the "nut meat", which is, in fact the seed's cotyledons. This website has a good explanation on nut anatomy: http://waynesword.palomar.edu/ecoph8.htm.
I collected roughly two hundred black walnuts while in Maine. The first batch of 75 I neglected, their outer pericarp turned black and started to grow mold. After removing the rotting husk with a hammer and knife (to scrape the gunk off), I placed the nuts back into the 5-gallon bucket I collected them in-bad idea. It was too moist, mold continued to grow on them. So I tried to heat them in an oven to kill the mold, but wound up desiccating the nut meat.
The second batch I collected also started to rot, but this time, after removing the outer pericarp, I placed them in a cardboard box. Because they were only layered 2-nuts deep moisture wasn't trapped and no mold grew. I then placed this cardboard box on a radiator, which helped reduce moisture and "cured" the nut meat. Curing is essential if you are going to consume the nuts. Usually, it takes a couple months. I believe it only took a couple of weeks on the radiator because of the constant heat.
Once the nuts cured, I set to work cracking the inner pericarp with a metal vice. Of course, I was wearing safety glasses and gloves to protect myself from any shell shrapnel. Despite reading a couple informal methods on cracking the nuts, none seemed to prevent damage to the cotyledons. The result: hours of picking through shell debris and meat extraction with a cake tester. Eventually, I would up with about two and a half cups of meat. Obviously, I was (and remain) frustrated that I wasted at least four hours "husking" and six hours "shelling", not to mention the black fingers and fingernails that resulted from both processes. And by the way, this stain can last for weeks of your skin. Those F***ING nuts better taste like ambrosia, I thought. Honestly though, the nuts taste pretty good and I will likely them in a fudge. If I had to describe their flavor, it would be similar to English walnuts, a little sweeter with a slight hint of rum. Evidently, the black walnuts can be for dyes and medicines (5).
I recently learned that there are a few different cultivars of J. nigra, some of which are better for nut production, others for timber. Cultivars best suited for nut production are grown in orchards and can be trellised for easy access. The nut cultivars apparently have thinner shells, making them easier to crack, avoiding my problem of nut meat fragmentation (9). One acre of trees is capable of producing 2000 pounds of nut meat, which can sell for 15 dollars per pound-that’s about $30,000 per acre if you don’t subtract costs of harvesting and husking-a considerable amount of money (7). Other sources indicate a range in price from $443 to $2,844 per acre, when sold for 0.07 cents per pound (3). Cultivars grown for their timber can fetch $2,472 per acre. However, the USDA and Illinois Department of Forestry published a flier with prices from $130 to & $1500 per board foot (board foot=1ft x 1ft x 1in) and up to $2200 if delivered directly to a mill. Black walnut and other timber prices can be found here: http://web.extension.illinois.edu/forestry/il_timber_prices/index.html.
A professor of mine once mentioned that mature trees can be worth up to $100,000, and I suppose that is true, since mature black walnut can be 150 feet tall with diameters of 8 feet (2). Let’s say you have a 2-foot diameter tree that is 150 feet tall (with these dimensions, the tree would likely be top-heavy; the trunk probably wouldn’t support the canopy). Keeping in mind the diminishing size of the trunk and branching, let’s say the trunk reaches 6o feet before branching or decreasing in diameter (also unlikely). Using these measurements, you can have twenty 60 foot x 1 foot x 1 inch boards. This comes out to be 1.8 million dollars for that 60-foot trunk. That’s a lot of money. Obviously, this calculation is a best case scenario.
Before you start your black walnut orchard, you should probably know they do best in well drained, nutrient-rich soils (2, 12). They require full sun, which can cause their trunks to branch close to the ground. I assume this low branching would decrease the value of the wood for three reasons: smaller diameter, curvier trunks and knots. It would be possible to train and trim your tree to grow vertically with one trunk, don’t fret. Plant your walnuts now, because it will take about 75 years for them to reach 80 feet with optimal conditions (11). Luckily, the walnuts are easy to propagate.
After husking them all they need is a little stratification. 120 days at 35-45 degrees F should do it (7). Plant the seedlings in fall. There are a number of cultivars to consider as I mentioned earlier. Most cultivars need to be grafted (5). Make sure you select a cultivar or rootstock that is best suited for climate and soil. You probably don’t have to worry about this if you collect black walnuts from a tree close to your planting location. Additionally, the trees require at least 170 growing days and an average yearly temperature of 55 degrees F. Given its natural range, the black walnuts probably tolerate most USDA hardiness zones.
Most of the information I’ve collected seems to be from the Midwest or South, leading me to believe that is where most black walnut orchards exist. Plus, I haven’t heard of any in the Northeast. Walnut species native to western states, J. californica and J. hindsii, are under threat from Pityophthorus juglandis, a beetle and Geosmithia morbida, a fungus (4). The beetle is common to Southwestern states and does little to harm walnut trees as it burrows into twigs and trunks. The fungus, however, transported by the beetle produces cankers that girdles and kills trees in as little as 3 years (13, 4). Multiple beetle wounds can result in multiple cankers, giving the disease its name: thousand cankers disease. Thousand cankers disease has the potential to reach eastern states and devastate J. nigra populations. Butternuts could also be at risk, but they are already experiencing depressing declines resulting from a separate pathogen, Sirococcus clavigignenti-juglandacearum (8). Thankfully, J. nigra is rarely infected by the butternut’s pathogen (7).
This information is a little scary. Perhaps now is the time to start hybridizing resistance Japanese walnuts with susceptible American species (7). It may be difficult to grow other plants within proximity of eastern black walnuts because of the allelopathic chemical juglone (5-hydroxy-l,4-naphthoquinone if anyone was curious) produced by black walnuts (10, 6). Allelopathy is the inhibition of plant growth by a phytochemical. Most allelopathic chemical are secondary metabolites. It is generally accepted that juglone inhibits respiration and photosynthesis (1). Although, I don’t believe the exact inhibition mechanism is known. I didn’t search exhaustively, but there also seems to be lack of research on intraspecific J. nigra allelopathy. Maybe this is because J. nigra exhibits no noticeable growth limitations in orchards, meaning it has juglone tolerance. It makes sense that it would have tolerance, but I know spotted knapweed exhibits inter- and intraspecific competition resulting from its allelopathic chemical catechin. Nevertheless, what are the effects of juglone on J. nigra seedlings? Different cultivars? Rootstocks?
Anyways, some plants are tolerant of juglone, like black raspberries and cherries (1). Mmm… black raspberries, cherries and black walnuts, all within an arm’s reach. Doesn’t that sound amazing? If growing black walnuts it is important to consider how they will interact with nearby plants or gardens. As with any garden, one must do their research. Other thoughts to ruminate are the implications of allelopathic tolerances in plant ecology and defining plant communities.
In conclusion, black walnuts are a valuable tree, worth planting. It can serve as a food source for humans and wildlife or provide income. There are many potential pests, especially in orchard monocultures, but planting some may preserve genetic diversity if a pest or pathogen were to start destroying the population. At the very least, planting a black walnut could sequester carbon and decrease your carbon footprint. Growing an eastern black walnut might just give you the satisfaction of explaining allelopathy to your friends, or simply give you some shade on a summer’s day.
Works Cited:
1 Dana, M.N. & Lerner, B.R. 2001. Black Walnut toxicity. Purdue University Cooperative Extension Service. Department of Horticulture.
2 Dickerson, J. 2002. Plant Fact Sheet: Black Walnut. USDA NRCS New York State Office.
3 Garrett, H.E., Jones, J.E., Kurtz, W.B., & Slusher, J.P. 1991. Black Walnut (Juglans nigra L.) agroforestry-its design and potential as a land-use alternative. The Forestry Chronicle 67 (3): 213-218.
4 Garvey, K.K. “Beetle and Fungus One-Two Punch Threatens Black Walnut Trees, Scientists Warn” UC Davis Department of Entomology, 2 July, 2009. Web. 23 Dec. 2011. <http://entomology.ucdavis.edu/news/walnuttwigbeetle.html>.
5 Hurteau M.D. 2003. Plant Guide: Black Walnut. USDA NRCS National Plant Resource Center.
6 Jose, Shibu & Gillespie, A.R. 1998. Allelopathy in black walnut (Juglan nigra L.) alley cropping. II. Effects of joglone on hydroponically grown (Zea mays L.) and soybean (Glycine max L. Merr.) growth and physiology. Plant and Soil 203: 199-205.
7 McKenna, J., Ostry, M. & Woeste, K. 4th International Workshop on the Genetics of Host‐Parasite Interactions in Forestry Eugene, OR USA 2011. “Screening Butternut & Hybrid
Butternut for Resistance to the Butternut Canker Canker Fungus.” USDA-Forest Service, Northern Research Station–HTIRC at Purdue Web. <http://ucanr.org/sites/tree_resistance_2011conference/files/121573.pdf>.
8 Ostrsy, M.E., Mielke, M.E. &, Anderson, R.I., “How to Identify Butternut Canker and Manage Butternut TreesUSDA Forest service, North Central Forest Experiment Station, Northeastern Area, S&PF Region 8, S&PF. 1996. Web. 23 Dec. 2011. .” < http://na.fs.fed.us/spfo/pubs/howtos/ht_but/ht_but.htm>.
9 Reid, W., Coggeshal, M., & Garrett, H.E. Growing Black Walnut for Nut Production. Agroforestry in Action. U. Missouri Center for Agroforestry.September 2009.
10 Reitveld, W. J. 1983. Allelopathic effects of juglone on germination and growth of several herbaceous and woody species. J. Chem. Ecology 9(2): 295-308.
11 Schlesinger R. & Funk, D.T. Managers handbook for black walnuts. North Central Forest Experiment Station. Forest Service. USDA. General Technical Report NC-38.
12 Schesinger R.C. & Williams, R.D. 1984. Growth response of black walnut to interplanted trees. Forest Ecol. & Management. 9: 235-243.
13 Steven Seybold, Dennis Haugen and Andrew Graves.2011. Thousand Cankers Disease. Pest Alert. USDA Forest Service.
