For years we have been told that walnut trees send out a toxic substance (juglone) that is fatal to other plants, making planting around these trees difficult or impossible. But, we may be blaming the wrong problem and all hope is not lost.
If you are planning a new garden, start here: How to Grow Your First Vegetable Garden (Right Now).
Growing Near Walnut Trees
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Don’t plant under a walnut tree! Watch out for juglone! Your garden is doomed!
We’ve heard this for years. When one plant creates a substance that can deter or change the growth of another it’s called allelopathy and walnut trees and their juglone are often cited as a prime offender.
When we bought our current property and I realized the largest tree in the garden was a 60-year-old black walnut tree, I thought my gardening future was doomed.
But, after digging into the research and planting under and near this tree for several years, I no longer regard it as a plant killer.
While it’s true that all walnut trees contain juglone, it’s only some trees, under certain conditions and with particular plants where there seems to be any sort of problem. And often—if ever—the juglone may not be the culprit.
Here’s what we know.
Various parts of walnut trees contain hydrojuglone which can transform into juglone in soil. The roots and husks have the highest concentration, leaves much less, and the wood has very little.
So, on a side note, since there’s so little hydrojuglone in the wood, using walnut leaves and wood chips as mulch shouldn’t be a problem. I certainly do it.
In laboratory studies, concentrated amounts of juglone given to seedlings in Petri dishes and other lab conditions have been shown to do harm. But, in a natural setting and backyard growing conditions, we don’t have good evidence that this happens.
Walnut are not nuts.
In the world of botany they are drupes.
So what are nuts?
Nuts are single seeds inside a hard pericarp.
Most of what we call nuts are fruit—dry fruit—and what we eat as nuts are the seeds of those fruits.
So what’s a drupe?
A drupe (or stone fruit) has an outer fleshy part surrounded by a single shell with a seed inside.
Cashews, almonds, pecans, and pistachios are also drupes.
So perhaps we should be calling walnuts waldrupes.
So why do some plants struggle or die under walnut trees?
Unfortunately, the answer is that we still don’t really know. There are long anecdotal lists of plants that do fine near walnut trees (with adequate growing conditions), so whatever the explanation is, it only seems to be a problem with some plants in some situations. Much of it could just be that walnut trees are very good at competing for resources and any nearby plants lose out.
Plants need good soil, air, and varying amounts of light and water to grow.
And, like many large trees, as a walnut tree ages, it’s going to demand more and more of these resources. The soil may become depleted, the tree is going to need a lot of water, and that massive overhead canopy is going to cast a lot of shade. That doesn’t leave much for other plants.
Click here to learn about tapping walnut trees for syrup making.
So, no wonder people say tomatoes don’t like walnut trees or any other big trees growing overhead! The growing conditions are antithetic to everything the tomato—or any other hungry, thirsty, sun-loving plant needs. And the deeper its roots, the more the smaller plant will struggle.
What Can I Grow Below a Walnut Tree?
There are many different anecdotal plant lists online stating what can and cannot grow near a walnut tree.
Before planting near yours, look over the recommendations and find real life examples in your region.
I have not listed them here simply because there are lots of inconsistencies between lists which again may indicate that it’s growing conditions rather than the walnut tree that matter.
Always make plant selections based on existing growing conditions and what is suited to your region.
What is the soil around the tree like? Is it sandy, silt, or clay, or some combination? Is it rich in organic matter? How much shade is there from spring to fall? Or morning to night? Is water available or do you have droughts? The answers to those questions will help determine what you can or cannot grow.
In my case, I’ve got terrible, sandy soil and our walnut tree casts a huge shadow from mid-day onward once the leaves have opened in late spring. That right there rules out many plant options.
I have installed a few raised beds below the canopy and use them for vegetables that need just a few hours of sun each day. Filled with 12-inches of rich soil, there is no root competition with the tree.
Some naturalized plantings have also found their way around the tree. Hostas, ferns, and some other native plants do nicely in the deepest shade near the tree without any help from me.
I’ve also got phlox, primroses, coneflowers, and several shrubs including a heritage white lilac, each positioned for their light needs.
Rolling Nut Collector | Amazon
While I would never choose a walnut tree due to the stench of the fallen fruit—particularly in mast years when it produces ridiculous amounts of walnuts that take hours to clean up—I can’t say its been the toxic threat to other plants I was warned about it.
So, fear not the walnut tree: all plant growing hope may not be lost. And basic fundamental planting rules are always good to follow.
Read More: Do Black Walnut Trees Have Allelopathic Effects on Other Plants? | Researchgate.net
~Melissa the Empress of Dirt ♛