A few years ago I was chatting with a neighbor and he mentioned that he was having trouble growing tomatoes and peppers on his property. I looked around and immediately pointed out three black walnut trees as the culprit. He adamantly disagreed with me. I walked away (and quickly passed his house anytime I was walking my dog).
When I saw him again in spring, he came out of his house calling out, “I owe you an apology. You were right!” He had cut the trees down.
We discovered a serious issue at my husband’s family farm about 10 years ago when we had stunted growth and leaf curling on tomatoes, peppers and eggplant in his grandmother’s garden. This was the best soil on a clay-heavy property, but I was only able to grow certain plants in the very center of the plot.
Meanwhile, my father-in-law was obsessed for decades with growing black walnut trees everywhere they sprouted up. When they first bought the property, they could not get them to grow, so anytime they did take, they let them flourish. As I diagnosed the leaf issue, it became apparent that the black walnut trees all over the property were the offenders.
It snuck up on us over time as the saplings grew into mature trees and the roots expanded. The first casualty was rhubarb. This was also the first time I experienced pushback on this topic, and as I’ve been talking about it for a decade, it continues to be a hard concept to grasp. This is especially true for those who strongly value the wood at the center of these trees. Basically, they feel something so beautiful cannot be so detrimental to everything growing around it.
Unfortunately, in addition to the valuable wood, black walnut trees produce a toxin called juglone that enters the soil through the trees’ roots. Depending on the age and root system, this can spoil a large area around a tree. The reach of the roots of a young tree can be a 25-foot radius of the trunk; a mature tree can reach 85 feet out. A felled tree can still produce this allelopathy, releasing it into the soil from the roots for up to 13 years.
There is no soil test for juglone. It can be assumed the greatest concentration is below the foliage dripline as leaves and nut hulls also secrete it, especially in autumn.
The symptoms of juglone contamination on other plants include yellow, brown or twisted leaves. Over time, exposure can damage the vascular tissue responsible for drawing water up the stems and eventually kill the plants.
According to Wisconsin Horticulture Extension, the twisted leaves can easily be confused with wilts from bacteria or fungi. Another misdiagnosis is herbicidal injury. We first (incorrectly) assumed the damage was from the neem oil I sprayed on the leaves of the tomatoes.
Identifying a Black Walnut Tree
For those trying to sort this problem out for themselves, there is a great article on the Morton Arboretum site that explains it well and offers lists of plants impacted. (Find it by searching “Morton Arboretum black walnut toxicity.”)
There are other great lists out there, but most are focused on trees, shrubs, vegetables and ornamental bedding plants. None of these addresses cut flowers specifically. In order to make sense of this, we need to understand plant families to determine what is affected and what can safely blossom in this environment.
Members of the nightshade family, known also as Solanaceae, are the main ones affected by the toxin. Tomatoes, peppers, eggplants and potatoes all suffer from juglone in the soil. Some bedding plants including Datura, Petunia and Nicotiana are also sensitive. There are not many cut flowers in this category. Other cut flowers such as lilac, asparagus and hydrangea are on the caution list.
Winter is not the issue with growing in this spot – even though it has the best soil on the farm, it is completely surrounded black walnut trees. This is why we are growing sunflowers there. Photo by Betsy Busche
Those in the carrot family are mostly immune to the toxin including Dara, Ammi, Angelica, Queen Anne’s lace and seaholly. The herbs parsley, fennel, cumin and a form of celery are also safe. This means the biggest bully of them all, giant hogweed, will also flourish in infected soil.
Many members of the aster family are also in the safe zone such as coneflowers, zinnia, sunflowers, marigolds and daisies. Lettuce falls in this category, so some greens can be cultivated for filler. However, chrysanthemums and peonies do react, so staging trials in your own microclimate is necessary before investing in expensive roots.
If you find the leaves of your plants are yellowing and curling, check for black walnut trees. According to the National Park Service, black walnut trees can be identified by deeply ridged gray bark, a spray of leaves that turn yellow in autumn and the green balls of the walnut fruit. The trees thrive in USDA Hardiness Zones 4 – 9, mostly in the Northeast and Midwest.
Looking at existing plants can help you examine your soil. For instance, thyme is one of the perennial herbs that reacts strongly and becomes a litmus test in the field. If a planting of thyme is flourishing, there is no problem with juglone. Establishing these plants within the 50-foot radius helps determine if other high value plants will be affected. Because of these distances, the offending tree may not be on your property, forcing a switch to raised beds or limiting varieties grown.
Plant incompatibilities can be helpful tools as well. Ironically, this year we have taken advantage of the allelopathic nature of sunflowers to limit the growth of weeds in our fields. Grass tends to not react, thus is something we try to kill off with silage tarps before direct planting the seeds. I cultivate between the rows twice. Once the canopy of leaves forms, most of the weeds are suffocated.
There are still great reasons to grow black walnut trees. As already mentioned, the wood is gorgeous. The nuts can be used in many culinary applications both raw and cooked, even making tasty cocktails.
The issue surrounding these trees is why we need to understand the entire ecosystem on our property. Ruling out herbicide drift to focus on the real problem ensures that we are crop planning for success. With all of the challenges of rotation to prevent disease, we are adding one more layer to consider.
This last year we focused on working out the kinks of growing sunflowers; now we are ready to add some dried flowers back in next year as we determine what will and will not thrive on the land that is available to us. My husband called it our “learning year” and I’m sure there are many more of them to come.
