Black walnut trees have a reputation as a valuable member of large-scale landscapes and an important hardwood for the lumber industry. Unfortunately, they can create a lot of messes with their fruits and leaves in small spaces. But that isn’t the most challenging gardening conundrum facing these attractive trees.
The most concerning issue that gardeners come up against is their toxicity to other plants. These trees have an allelopathic relationship with many other plants, producing growth-inhibiting substances that can keep some of your other garden plants from performing their best, if at all.
That’s terrible news, but here, we’ll focus on the species you can plant near black walnut trees. Before we get to that, let’s look more in-depth into why some plants have such an adverse reaction to the tree.
The toxic substance is most potent in its nut hulls, buds, and root system.
Juglone is the substance in the tree that harms other plants. This substance is used as a herbicide, a natural dye, and a food and cosmetic coloring agent. I highly advise against its use for any of these reasons, as juglone is toxic, not only to plants but to humans and animals as well.
Juglone poisoning can involve short-term effects such as throat and lung irritation and long-term poisoning from repeated ingestion. The ripe fruits of the tree are a very nutritious food, but avoid touching or ingesting all other parts of the plant.
The effect of this chemical on other plants is growth inhibition and inhibition of plant respiration. Juglone is present in most parts of the plant, with the lowest concentrations in the stems and leaves and the highest in the nut hulls, buds, and roots. Its presence in the roots and the tree’s canopy makes it difficult for other plants to grow in its shadow.
Juglone can linger in the soil for up to three years after tree removal. Removing the tree won’t necessarily solve the juglone issue. Instead, it is more effective to identify which plants can and cannot tolerate proximity and adjust the garden accordingly.
Juglone Sensitive Plants (Plants to Avoid)
Nightshades don’t tolerate juglone.
Plants have varying degrees of sensitivity to the toxin. Some are highly sensitive and react to even small amounts, while others experience mild effects.
The plants most sensitive to toxicity are those in the nightshade family, such as tomatoes, potatoes, peppers, and eggplants. Apple, some pine, spruce, silver maple, and birch trees are all sensitive to this substance.
Berry plants and shrubs like cotoneaster, hydrangea, privet, lilac, peony, yew, and rhododendron are all very sensitive to the toxin. For these plants, proximity can be fatal.
Other plants that may be intolerant of large amounts of the chemical include asparagus, chrysanthemums, and petunias.
Look for stunted growth, yellowing and twisted leaves, and wilting of all parts of the plant. These are all signals that a plant needs to be moved farther away from the source of the toxin.
How to Fix it
Move Impacted Plants
Carefully transplant any plant that may be in harm’s way.
Moving these plants away from the tree can be arduous. Unfortunately, it is their only real chance at thriving.
Make sure you move them out from under the tree’s canopy, as falling limbs, leaves, and an extensive and expansive root system deposit the chemical into the soil. Generally, anything under the tree’s canopy is within range of the root system!
Remove the Tree
In some cases, it’s best to completely remove these trees to avoid further destruction.
Another option is to have the tree removed and sell the lumber. A mature tree can fetch a reasonable price as the wood is desirable.
However, as I mentioned, the roots contain large amounts of juglone, which can remain in the soil for years as the root system slowly decays. You are likely to have to relocate your sensitive plants just the same.
Remediate the Soil
Aerating the soil will help speed up the decomposition process.
If you have removed the tree from your garden and want to expedite the process of decomposition of juglone, there are some ways to remediate your soil. Since the chemical is only water-soluble once the plant matter containing it fully decomposes, it doesn’t move very far from where it is deposited. Roots and fallen hulls/leaves continue to spread more juglone through the surrounding space.
You can speed up the decomposition process by aerating the soil and making sure that there is a healthy population of microbes present. Soil that remains wet, has poor microbial activity, and lacks organic matter may hold onto the juglone for an extended time. Mushroom compost contributes to faster breakdown of juglone in soil, so amending your soil with this material can expedite the process.
A note before we go on: This chemical negatively impacts humans in multiple ways. I recommend wearing a good pair of gardening gloves when working to remove and plant things near your tree. Not only will this prevent any lingering walnut hulls from staining your hands brown, but it’ll reduce any risks you may have by being exposed to oxidized juglone.
Grow in Raised Beds or Containers
Placing your more vulnerable plants in a raised garden bed is a great solution to this problem.
Another option that can be effective in keeping plants that are only moderately sensitive is using durable and deep raised beds. You can reduce contact with the tree’s root system by elevating the soil. For extra protection, consider using landscape fabric under the bed to prevent upward root shoots from developing.
Container gardening is another solution. By using pots or grow bags, you can take advantage of the tree’s shade while still growing plants that may otherwise be susceptible to the juglone.
Be aware that if using either containers or raised beds, you’ll need to remove the leaves and nut hulls that will drop under the tree’s canopy. These can be composted, although the compost created should be aged for at least a year after the composting process concludes to allow the juglone to break down.
There are a variety of different plants that will be able to thrive when planted near these trees.
It will be obvious which plants are sensitive to juglone by their general health and response to being planted near your tree. But who wants to sacrifice many plants that may or may not work out in space by trial and error? I know I don’t.
The most economical solution is to find out which plants can tolerate juglone without sacrificing their health and plant those instead. Knowing which plants are the most susceptible is helpful, but you still risk other plants being even somewhat sensitive to the chemical and then having to relocate another plant away from the tree.
Let’s talk about what types of plants tolerate juglone and make good neighbors with your black walnut tree. Since there haven’t been many studies about juglone tolerance in plants, much of the data is from trial and error. I wish I could guarantee that these plants will thrive despite the environment, but these are more observations than facts.
These plants have been determined by other gardeners and horticulturists to survive growing near your tree:
|Perennials||Shrubs & Vines||Fruits & Vegetables||Trees|
|Ferns||Grapes and their relatives (inc. Virginia creeper)||Stonefruits||Oak|
|Yarrow||Clematis||Quince||Maple (excluding Silver Maple)|
|Mayapple||St. Johns Wort||Squash||Virginia Pine|
|Leopard’s Bane||Witch Hazel||Melons||Beech|
The tolerance of these plants varies based on environmental factors. The level of contact, soil condition, and plant health all influence whether or not these species can stand up to juglone toxicity.
Black walnut trees are complicated landscape members, even more so if you are an avid gardener and like to maximize your space. Understanding how toxicity works and affects plants is a step toward diagnosing the issue. Knowing which plants tolerate juglone is helpful when landscaping with these valuable trees.