Sitting in the soft, feathery shade of a black walnut tree, I’m reminded of the paradox that one person’s trash is another person’s treasure (or, perhaps, one person’s love is another’s frustration). And labeling plants “good” or “bad” may say as much about our own judgment and agenda as it does about the plants themselves.
Often, the solution to the negative side of the equation is simply to cut down the tree — or so I hear from gardeners who find themselves challenged by the presence of a black walnut tree in their landscape. And indeed it is a challenge. Because despite the black walnut’s many virtues, it will inevitably mean struggle for the gardener who must cope with it. The problem is getting other plants to survive — much less thrive — in what could be called the “no-grow” zone beneath these wonderful native trees. But before you succumb to the temptation to fire up the chain saw, the black walnut — highly prized for its wood and beauty, but also the bane of many gardeners’ existences — deserves long and careful contemplation. Here’s why.
Black walnut, or Juglans nigra, is a big tree. It reaches a height of 50 to 75 feet with an open, spreading canopy of comparable breadth. In the summer landscape, its fine-textured, dark-green leaves provide a light, airy shade. In winter, the black walnut stands in stately silhouette, showcasing its dark-brown, deeply furrowed bark and coarsely textured branching. Although the tree prefers deep, rich, moist soils, it will tolerate drier soils. Transplanting can be difficult, however, because of this species’s extensive tap root.
The black walnut enjoys an extensive range, thriving in woods and gardens from Massachusetts and Minnesota to Florida and Texas. Flowering from April to mid-June, it sets a nut (1-1/2 to 2-1/2 inches in diameter) that ripens in the fall and is a favorite of squirrels and other hoarding wildlife. If you’ve ever handled a black walnut husk, though, you know that as it softens it turns dark brown to black and can be messy. Removing the husk can be a nasty job, however, so be sure to wear your oldest clothes and gloves: By sundown, all will be black. Cracking a black walnut shell is no mean feat either, though it’s made easier by the prospect of enjoying its distinctive yet delicious flavor embodied in a dessert or bread.
Black walnut heartwood, chocolate brown in color, is a strong and durable material that has long been used to make gunstocks, cabinets and other high-quality furniture items. Walnut rustlers, in fact, are said to roam the woods in search of this tree and to make lucrative sales to timber and veneer buyers. (If you’re considering changing occupations, however, be sure to do your homework first: Only trees greater than 18 inches in diameter are considered valuable.) Local weavers and basket makers, meanwhile, use both the leaves and the nut husks to produce dyes ranging from yellow to gold and tan to brown, enhancing the beauty of hand-hewn oak strips and handwoven fabrics.
But this glorious picture quickly shifts when we consider the black walnut’s potential as a landscape plant. Although it’s a beautiful shade tree, black walnut is also a member of a select group of plants (including tree-of-heaven, sugar maple, American sycamore, black cherry, red oak and sassafras) that are allelopathic (i.e., they have toxic effects on neighboring plants). Allelopathy involves the secretion of biochemical substances (in this case, into the soil) that inhibit the germination and growth of surrounding vegetation. And though this defense mechanism enhances the black walnut’s chances of surviving and reproducing, it can wreak havoc on other plants that get in the way (whose yellowing, wilting and dying will make this pretty obvious).
According to the literature, hydrojuglone — a nontoxic, colorless chemical found in the stems, leaves, nut husks, inner bark and roots of the tree — is the ultimate culprit. When exposed to air or soil, hydrojuglone oxidizes into the highly toxic allelochemical juglone. Related plants (such as English walnut, hickory and pecan) also produce juglone, but in smaller and less toxic quantities. The chemical is exuded through root contact, decaying leaves and wood, or rainwater. Plants directly beneath the tree’s canopy or within the range of far-reaching roots are most at risk. And urban soils that are wet, compacted and low in organic matter increase the accumulation of toxins (and thus the affected plant’s exposure and sensitivity to the black walnut’s harmful effects).
The good news is that juglone is not highly water-soluble and doesn’t move far in the soil. So improving the drainage, aeration and quantity of aerobic microorganisms in the soil helps reduce the toxicity.
Organic matter is key. Besides encouraging healthier microbial populations, it keeps toxins from being absorbed by adjacent plants. (Don’t include black walnut debris or any portion of the plant in your compost or mulch, however.) Bringing in abundant amounts of organic matter and planting in raised beds can definitely help.
Another effective strategy is avoiding plants particularly susceptible to the toxins and placing black walnut-tolerant plants beneath the tree and in areas that might come in contact with its roots or debris. This is particularly important because even removing the visible portion of the tree doesn’t eliminate the belowground problem. Unless the roots are removed, juglone will remain in the soil until they decompose (which could take years).
A Web search yielded lists of plants at both ends of the spectrum: those especially sensitive to and more tolerant of this difficult neighbor. (Be aware, however, that such lists may not jibe, since most of them are compiled from observation and experience.)
That said, here’s a simplified list of black walnut-sensitive plants. Vegetables: cabbage, tomato, potato, eggplant and pepper. Fruit: apple, blackberry, blueberry. Landscape plants: azalea, white birch, lilac, saucer magnolia, mountain laurel, white pine, Norway spruce and rhododendron.
The list of tolerant plants offers many choices. Perennials, for instance, include astilbe, ferns, sweet woodruff, geranium, day lily, coral bell, iris, bee balm, phlox, spiderwort and viola. That’s why, when people say they can’t get anything to grow under their black walnuts, I have a hard time understanding their dilemma.
So before you or your neighbor breaks out the heavy artillery, take a few moments to sit in the black walnut’s cool shade and consider whether there might be a middle ground of peaceful coexistence: One in which you choose to work with a more limited plant palette in order to allow this native beauty its place in the garden. And if that’s your decision, I — along with many weavers, bakers, woodworkers and squirrels — will be grateful.