Black Walnut Toxicity: How to Save Your Plants and Protect Your Garden

Black walnut trees are beautiful and provide valuable timber, edible nuts which have great nutritional value, and are also used for traditional herbal medicines. So learning that black walnut trees can kill your others plants is disappointing....but not hopeless. Read on to learn how to continue to grow plants amongst black walnut trees with simple tips and tricks.

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What is Black Walnut Toxicity?

So first of all, it is important to understand how black walnut toxicity works. Black walnut trees produce a substance called "juglone" in a high enough concentration that it impacts plants around it. It causes plants that are sensitive to this chemical to wilt, turn yellow, and die. You can understand that this benefits the tree because it decreases competition around it; however, it is not so great if you want to grow a garden and the trees are nearby.

All parts of the black walnut tree produce this substance, although it is present in greatest quantity in the buds, nut hulls and roots of the tree. This means that at least 50 feet out from the tree trunk (maybe even as far as 80 feet for a mature tree) this toxic substance is present in the soil. The highest concentration will be in the soil under the tree canopy. Another kicker, is that after a black walnut tree is chopped down, the root pieces underground can continue to release the toxin for several years as they slowly decay.

How Does the Toxicity Impact Surrounding Plants?

There are plants that are tolerant to juglone and others that are not. The ones that are not tolerant will likely die in the presence of it, while the tolerant ones will grow unaffected. Some Juglone sensitive plants die immediately, or within a few days, while others will survive for years until enough of the toxin builds up as the plant increases in size and then they will die. Sadly, once a plant is affected, there is no way to reverse the effects of the toxin.

Examples of Plants Sensitive to Juglone (This is not a complete list, visit The Morton Arboretum to learn more)

• Tomato

• Potato

• Eggplant

• Pepper

• Asparagus

• Cabbage

• Blueberry

• Rhubarb

• Hydrangea

• Lilac

• Peony

Examples of Plants Tolerant to Juglone (also not an exhaustive list)

• Onions

• Beets

• Squash

• Melons

• Black Raspberries

• Carrots

• Elderberry

• Hostas

• Daffodils

• Purple Coneflower

• Daylilies

• Bleeding Heart

How to Combat Black Walnut Toxicity in the Garden

Everything described above may sound bad, but there are always solutions to be had if you are willing to search for them and get creative!

Solution 1: Garden Planning

The most obvious solution would be to plant species known to be tolerant to Juglone in the areas closest to black walnut trees. A little garden planning can keep the sensitive plants at a safe distance. This means you would grow the sensitive plants at least 50-80 feet away from the closest walnut tree trunk. I recognize, this solution is only possible if you have enough land to be ABLE to space a garden this far from the walnut tree.

Additionally, if the entirety of your land was cleared of walnut trees to make way for your house and immediate yard area, you may want to learn how long ago those trees were removed to gauge if the toxin may still be in the soil. In the absence of this information, or in the presence of knowing that the trees were rooted there until quite recently, the next option is to avoid planting directly into the affected soil.

Solution 2: Raised Garden Beds

Consider raised garden beds with enough depth that plant roots won't reach the affected soil below. You can also add a bottom to the bed to physically inhibit roots from going too deep. You can customize a garden bed/beds for your space easily if you don't want to be limited by what is pre-fabricated on the market. Annual plants would be a great choice for garden beds because they die each winter anyway. This means they won't have the time to put down roots quite as deep as a perennial. You can also research shallow rooted plants for these spaces (Tomato, Pepper and Cabbage are examples of shallow rooted plants from the "sensitive" list that would work perfectly for the raised bed solution!). Just remember to fill the beds with raised bed mix and not incorporate any of your Juglone contaminated soil.

Solution 3: Containers for Sensitive Plants

Another option is to plant sensitive crops in containers. There are light-weight fabric containers that can be useful as long as you consider drainage so that puddles of ground water don't leach up into the bags. These bags also come in lots of shapes and sizes so that you can really grow a lot in them. The light-weight nature makes them easy to set up...obviously the soil will make them heavier. Placing spacer type, low to the ground stands under the bags or even mulching them up could be viable options to ensure proper drainage and act as a barrier from the affected soil.

Bonus: some crops actually do VERY well in these fabric bags. Potatoes are an example of a Juglone sensitive crop that does great in a grow bag (and harvesting is made super easy with this method. Check out the link to see what I mean!)

Solid containers are also a great option. Metal, ceramic or terracotta are all great for growing sensitive plants. Just remember to elevate anything porous off the ground and away from flood water to avoid contamination during rain.

Solution 4: Clean Up Tree Litter

Finally, make sure to rake or sweep up fallen nuts and leaves consistently so that they don't end up in your raised beds or containers. This step will prevent tree litter from contaminating the soil from the top down in addition to the previous solutions that are considering contamination from the ground up.

Conclusion

Black walnut trees may be native to your region and present a problem for growing certain Juglone sensitive plants in your garden. However, there are easy solutions to work around the tree toxicity and still have a bountiful garden and great harvest! I focused largely on food producing plants in this post, but there are tolerant and non-tolerant flowering plants as well, if an asthetic garden is more to your interest. Bottom line: researching plants that are tolerant to your conditions will always help you get the best results with the least struggle . You know what they say about an ounce of preparation...

Best wishes for your garden! Cheers!