Tree of the Month: Black Walnut

​You’ve probably heard of Juglans nigra in the context of black walnut wood, which is a luxury material used in everything from china cabinets to hope chests to dining room tables. Black walnut wood is coveted for its colors and striations as much as its heartiness and durability. A black walnut piece of furniture will last for decades to come (if taken care of). 

​But black walnuts aren’t just pretty as furnishings. The trees themselves are beautiful and larger than life. Young trees can grow up to 2 feet per year until they reach maturity after 12 to 15 years. Their crown spread tends to mirror their height, meaning they grow out just as much as they grow up. How far out, and how high? On average, we’re talking 50 to 75 feet when grown out in the open. When left to their own devices in the forest (or lovingly cared for on a plant farm), black walnuts have been known to reach heights eclipsing 150 feet. 

​Black walnuts generally reach about 6 feet in diameter, their trunks covered in medium brown bark with thick, interfacing ridges. Their branches will happily grow low to the ground if there’s room, but in more crowded forests, they’ll self-prune up to about 2/3 their total height (basically, they’ll let lower limbs die off to direct energy upwards, where leaves have the best chance of getting sunlight, and therefore nutrients). If you’ve got one out in the open and you’d rather not have huge branches growing out into where you’d like to walk around, you can prune them yourself. Black walnuts are bleeders (which means sap runs from any wounds), so be sure to prune during the dormant season, not spring.

Did You Know?

​The wood of the black walnut is very easily worked, and is prized in the furniture industry for its attractive color and exceptional durability. So much so, in fact, that thieves (called “walnut rustlers”) will steal and then make off with with whole black walnut trees in the dead of night. They’ve even used helicopters in their operations!

​Though they can tolerate less, black walnuts prefer full sun, and need around 6 hours of unfiltered sunlight every day. Luckily, this isn’t usually a problem for them, considering how tall they get. They tolerate a range of soils, and can thrive in acidic, alkaline, loamy, moist, rich, sandy, well-drained, wet, and clay soils. They also have some drought tolerance, and are generally disease- and pest-free. 

​The leaves of the black walnut tree, though skinny, are proportionally just as large as the tree they grow on, reaching anywhere from 1 to 2 feet long! Leaves grow in pairs down a single stem, sometimes ending with a singular leaf at the tip. Leaflets, on the other hand, have toothy ridges—not that you’ll ever see them. They’re usually hidden by their larger, older brethren. This fern-like foliage offers mottled shade that filters harsh sunlight, making them a popular addition to parks and along sidewalks.

​As a deciduous tree, black walnuts lose their leaves in the autumn. During the warm seasons, leaves are a vibrant green, but as the winter months approach, those leaves will turn golden yellow before falling to the ground. When spring rolls back around, black walnuts grow inconspicuous flower clusters. Male flower clusters droop down, while female clusters have terminal spikes. With the help of a little wind, a black walnut can self-pollinate, and the female flowers will grow into delicious black walnuts.

​These nuts don’t start out looking like what you buy in the store. As they ripen in early- to mid-autumn, they’ll look like lime green, fleshy baseballs. Black walnuts aren’t usually harvested off the tree, but are instead collected from the ground after they fall. They’re just as heavy as they look, and those with black walnuts near their house are more than familiar with the intermittent thudding on their roof that signals ripe fruit. 

Did You Know?

​With a higher protein content than most nuts (7 grams per serving), high levels of Manganese, Omega-3, and antioxidants, and a range of other nutrients, black walnuts are considered a superfood. They support the metabolism, encourage healthy bone structure, and can even help protect against cardiovascular disease, diabetes, and certain neurodegenerative conditions.

Fallen black walnuts are known for littering sidewalks and bruising innocent passersby, and are even more notorious for staining any surface they come in contact with. ​ ​ Black walnut tree owners may spend hours picking up the fallen fruit (wearing gloves, of course, because those green husks will stain skin just as readily as clothes). Even if they have no plans to eat it, the tripping hazard alone is worth putting in the work. The fallen fruit will also attract animals like squirrels interested in an easy meal, and will eventually rot and mold. 

​After harvesting, the green baseballs are broken to reveal the corrugated, thick black inner shell. This is the part you think of when you think of walnuts. They’re left to dry out, and can be stored for up to a year. When you’re ready to eat them, you may find them tough to break, but the effort ends with a delicious, sweet nut. Be careful at this stage too, though, because these inner shells, when crushed, can also stain pretty much anything.

Unfortunately, these beautiful trees come with a catch much worse than their sidewalk-staining fruit. Black walnuts are allelopathic. Basically, this means they suppress the growth of other plant species around them to eliminate the competition for resources like sunlight, nutrients, and water. Black walnuts do this by producing something called juglone, which is toxic to many species of flora. 

​Juglone is emitted in large quantities from black walnut roots, which may stretch out 50 feet or more from the base. The substance can also be found to a lesser extent in the leaves and fruit husks, but there’s still enough to harm anything growing beneath the crown of the tree.

​Don’t misunderstand, though, because not all plants are threatened by juglone. Some species even seem to thrive on it, and grow happily and healthily beside the black walnut.

Click here to read a comprehensive list of plants sensitive to Juglone

​With careful planning and a watchful eye, you can easily incorporate a black walnut into your landscape without losing any of your existing flora, and enjoy the gorgeous view and delicious nuts every fall.

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Author

Emily Casuccio is sister and sister-in-law to Rebekah and Scott Rushing, and has over half a decade of experience in copywriting, copyediting, proofreading, and developmental storyboarding. She's worked with both published and undiscovered authors on both fiction and nonfiction, and takes pride in supporting local businesses. Her passion lies in the written word and helping authors of all capacities realize their dreams and achieve their fullest potential. To learn more about her, read samples of her work, or contact her, visit her online portfolio. 

Sources:
Arbor Day Foundation: Black Walnut
The Old Farmer’s Almanac: Black Walnut Trees
Plant Spotlight: Black Walnut
The Morton Arboretum: Black Walnut

Images:
Juglans nigra
Planting Under A Walnut
MDC: Black Walnut
Black Walnut Tree
Gardener’s Path: Black Walnut
Black Walnut Wood
Allelopathy