Ever notice that patch of bare earth beneath your Walnut trees? All trees in the Black Walnut family (Juglans), plus cousins like the Butternut and some Hickory trees, all produce a toxic substance known as juglone through their roots, carried in their leaves and stems and even seeping into the rainwater that washes over the foliage and falls to the ground!
Juglone is highly toxic to many plants, acting as a natural herbicide, it kills almost anything beneath their canopies ‘drip-line’ and beyond! This is a self-preservation trick to reduce competition for water and nutrients, ensuring the Walnut tree gets it all!
For the gardener, that means it also prevents many plants from growing under or near them. Occurring in the highest concentrations in the new buds, shells and hulls, and in their far-reaching roots, it’s also present in the leaves, bark, stems and twigs. Anything that comes in contact with the ground begins to decay and releases into the soil, adding to the build-up of juglone, laying waste to anything even thinking about competing for water and nutrients in the soil!
A mature Black Walnut tree that’s been present in a location for a long time can have a ring of death 50 plus feet out past the drip line! Younger trees obviously have less of an effect on their environment, but eventually, they will win in the long run.
While the shade from your Walnut tree will be lovely, it doesn’t leave much room for a garden. Even gardeners that rigorously rake up fallen leaves, clean up the fallen nuts and keep the area neat and tidy, will see anything, including turfgrass, begin to wither.
Plants affected by Juglone become stunted, discolored and slowly die, all showing the same symptoms of herbicide drift or other chemicals sprayed onto them, or even appearing as if a pest or disease is affecting your plants. Before you reach for the sprayer to treat it, look around to see if you have, or had, a Walnut tree nearby!
The toxicity can remain in the ground for weeks to months, sometimes years, depending on how much of the tree still remains in the ground. Those underground roots will still be decomposing and releasing more juglone.
Once affected, plants cannot be transplanted in hopes of recovery, the juglone effects are permanent.
Unfortunately, there is no spray or treatment; only time and the systematic removal of all remaining parts of an old tree. For currently living trees, luckily there are a few workarounds you can put in place to still enjoy gardening in the shade of your mightly Walnut tree!
Sometimes the easiest way to deal with juglone is to create a mulched area beneath the tree, not only does this give your tree a finished look and a clean line and a polished look, it makes for easy clean up of leaves and debris. And no worry about plants dying if there aren’t any.
Putting a barrier down under the raised bed, or very high mounded planting berm, to prevent juglone from seeping upwards. Using soil that is juglone free you can plant mildly to fully resistant plants without too much worry. Just be rigorous in cleaning up falling debris from your tree that lands in and onto the area.
Create a container garden using varying heights of pots or large planters that will be kept separate from the effects.
You’ll still need to use plants that are resistant because of the eventual saturation due to rainfall that picks up juglone on the way through your tree and into the raised planting areas.
Choose shallow-rooted and resistant plants that don’t grow too deeply into the soil, adding a layer of very well-draining fresh soil (without burying the trunk of your tree, also helps resistant plants cope with the effect.
These plants will of course need a good layer of mulch that will help add a barrier from fallen debris from decaying into the soil. Again, you’ll want to religiously remove fallen matter from the planting area.
Take a look around at neighbors and in parks where you see Black Walnuts and observe what does well and what doesn’t. Nature is the first to break her own rules and you’ll start seeing what plants do well despite the juglone!
Not only are you fighting the chemical compounds in the soil, but also shade! This can be a highly limiting factor on what can grow beneath the dense canopy of your tree. At least the squirrels looking for nuts won’t be much of a problem.
Here are a few plants that are excellent choices for a juglone saturated environment and will do well despite your Walnut tree's best attempts at hogging the area for themselves!
In nature, you’ll see Crabapples, Dogwoods, Silver and other types of Maples, Elm trees and Locust growing side by side with Walnuts. Hemlock and Eastern Red Cedar seem unaffected as well. Choose native forms of these plants since their cultivars aren’t as hardy and tolerant.
Fruit shrubs and plants that you see growing in the wild are usually the best rated for growing near Walnut trees, however for fruit to do well, they need sun, and they won’t get much under the tree's canopy.
Some native Grapevines do well, Quince and native black raspberries. Remember though, fruit needs sun to really give you the best harvest.
If you have the room, go ahead and plant your own nut tree grove and include Butternuts, other Black Walnut varieties and Hickory Nut Trees! Throw in a few Hazelnuts and Chestnuts for a squirrel and bird-friendly forest of snacks!
Plus more good news - after raking up after your Black Walnut, those leaves can be compost just fine and won’t leach further juglone into your garden. The composting process and air oxidize the chemical very quickly so it breaks down safely for use in your vegetable beds and beyond.
If worst comes to worst, just lay down a patch of Arborist’s bark chips and a hammock for you and a swing for the kids, and relax in the shade! You’ll have a yummy harvest of Walnuts for roasting and snacking on while you do!
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