The black walnut tree looming over our backyard is dripping with yellow-green catkins.

_{The black walnut tree astride our backyard is roughly 70-75 years old, according to city maps. (Photos/Teri Berg)}

Cue the theme of “Jaws.”

Dun-dun. 

Ominous. Instinctually menacing. Two notes of terror.

It might be the soundtrack to a litany of headlines that spring from Google about this dire threat.

“Your Black Walnut Tree Is Out to Get You” announces the University of Georgia’s College of Agricultural & Environmental Sciences.

Other sites, often run by outfits that ought to know better, use punny titles spun from low-budget horror flicks to report on black walnut trees and juglone, the mildly toxic orangish chemical it produces. 

“The Curse of the Black Walnut” from ProvenWinners.com, HDTV’s “Killer Plants … Chemical Warfare in the Plant Kingdom,” then “Death by Black Walnut: The Facts on Juglone Toxicity” from GardenersPath.com, and YouTube influencer Gardenguy’s “A Dangerous Tree | Are Black Walnut Trees Harmful to Gardens and Grass?” 

Dun-dun dun-dun.

The gigantic black walnut in our backyard has inspired my anxiety for a couple of years. The tree sits on city property; City Parks superintendent Tyler Baird said the tree first appears on old maps in the 1960s. “This would place it around 70-75 years old.” Most of its large branches hang over our yard. That means the tree’s catkins, spent leaves and twigs, and its plentiful fruit bombard our landscape and provide me with ample opportunities for clean-up throughout the growing season. Little good a clean-up would do, since the roots are reportedly the deadliest part of the tree – and those roots are deep and widespread (up to 75 feet).

I’d learned online what a menace this tree is, with its withering effects on a variety of unsuspecting vegetables and perennials. According to many gardening websites, the battle against juglone is practically hopeless – I shouldn’t compost the tree’s leaves and branches nor toss them into the mulch pile, as the toxins would remain in the soil for years. 

A few summers ago, I transplanted a small lilac within the supposed Death Zone of 60 feet from the black walnut’s trunk, and the shrub’s pitiful state and ultimate demise seemed to bear out juglone’s terrifying reach and power. So did my husband’s nursing of a stunted asparagus patch, which was located even closer to the tree line and eventually ghosted him despite his best efforts. 

Lilac and asparagus are just two of the many plants considered vulnerable to juglone toxicity. Depending on which websites you believe, tomatoes, peppers, rhubarb, cabbage, eggplant, potatoes, blueberries, pine trees, peonies, and scores of other trees, shrubs and plants are reportedly under threat from aggressively competitive black walnut trees and other juglone producers, including English walnut, pecan, hickory, and butternut – go ahead, pick your poison.

Dun-dun dun-dun.

Trouble is, the pines lining our back lot are doing just fine. So are our sizable and robust rhubarb patch and the never-say-die daylilies that proliferate within the span of a first down of the black walnut. And just 20 yards out from the trunk – within the red zone of falling debris and the drip line – the tomato and pepper plants, pumpkins, squashes, cucumbers and marigolds in our raised beds thumb their noses at the black walnut every summer.

So, what gives? 

Well, for starters, my raised beds are situated in the sun, not under the canopy of a mature and healthy photosynthesizing tree. The veggies get a good 6-8 hours a day of direct sunlight, as do many of the native flowers and shrubs populating our backyard. (We don’t have many trees.) The plants that thrive, or struggle, under the shade of the black walnut are exactly those you’d expect to live, or die, under the canopy of any stately landscape tree with a long trunk and open-branching growth. My hostas and ferns, that is, are peppy and abundant, but the turf grass under the shadow of the tree suffers from too little sunlight and drought-baked clay soil.

According to Aaron Steil, horticulture specialist at Iowa State University, “Very little research has been done to show that juglone causes damage to plants near black walnuts or near the root system, stump, leaf litter, fallen fruit, or mulch created from black walnut.” 

Plants said to be juglone-sensitive reportedly turn yellow, look stunted, grow poorly, and eventually die when grown near black walnuts, but Steil says “these symptoms can also be caused by other conditions like dry soil conditions and low light (both common under the shade of any tree, including black walnut).”

Horticulture specialist, certified arborist, and professor emeritus at Washington State University Linda Chalker-Scott says in a definitive literature review on the subject that “despite the lack of any direct, supporting evidence, … the public perception that walnut trees would kill other plants [has] persisted” – and grown.

Little wonder, given the dramatic, reality-show headlines.

Chalker-Scott believes such misinformation has spread because such lore is attractive to gardeners, even scientists in their own gardens, who correlate a nearby tree with the puny growth of their own struggling plants. But such “evidence” is riddled with subjective eyeballing and fun-to-tell anecdotes rather than solid science.

Field studies over the last 100 years have found “little to no negative effects,” Chalker-Scott said, and in the past the USDA has issued press releases stressing the harmlessness of black walnut trees – to no avail. Much like Florida’s fluoride scaremongers and JFK conspiracy theorists, the gardening public holds tight to its myths.

“Juglone is present in very small amounts in leaves, stems, and wood,” says Aaron Steil, and it breaks down quickly in the soil. If one goes so far as to remove a black walnut tree, most of the juglone will dissipate in just a few months. According to studies cited in Chalker-Scott’s article, it’s perfectly OK – even wise – to use walnut wood chips for mulch. “They will not harm plants and work just as well as those from any other woody species.”

For my part, I’ll keep picking up black walnuts and their mustardy husks, and depositing them in our city yard-waste bin. Their messiness is a nuisance and they attract chipmunks, which dig hellscapes full of holes and tunnels all over our yard, even under the garage. But the tree itself I now hold in awe, as it is home to owls and numerous other birds, including hummers and woodpeckers, and it provides the only cooling shade our yard enjoys.

Sources:

“Do black walnut trees have allelopathic effects on other plants?”

“How long does juglone last in the landscape after the removal of a walnut?”

“What plants are sensitive to the juglone produced by black walnuts?”