Our small Cambridge back yard is next to a black walnut tree and we want to expand our small single raised bed to several beds to develop a bit of a permaculture food mini-forest and increase our veggie production. We already have a significant container garden. Should the beds be fully lined with plastic (I’d rather not) to avoid Juglone contamination or will regular compost and mulch applications be enough to get the results I desire?
Thank you for contacting the Toronto Master Gardeners with regards to your question concerning growing vegetables near a walnut tree.
Black walnut trees exudes a chemical called juglone from its roots into the soil that is toxic to many other plants and kills them, thereby reducing competition for resources. Juglone is also leached into the soil from rainwater coming in contact with fallen walnut leaves, branches and decaying fruits. Plants located beneath the canopy of walnut trees are exposed to the highest concentration of juglone, but the roots extend well beyond the dripline of the tree and may affect susceptible plants at a distance about equal to the height of the tree- Growing vegetables near black walnut trees
The harmful effects of juglone on surrounding plants can depend on the soil and its moisture conditions. Good drainage may decrease responses to juglone, thus when planting near a black walnut tree enhancing soil drainage would be beneficial. There are a number of things that one can do to reduce the alleopathic effect of juglone. Regularly clean up all of the fallen leaves and fruit from the black walnut tree, keeping debris away from desired landscape plants. Maintaining high organic matter levels in the soil encourages healthy soil microbial populations that can metabolize toxins. Alleopathic Effects and Tolerant Plants
When establishing a vegetable garden near a walnut tree the key is to plant vegetables that are tolerant of juglone, these include lima and snap beans, beets, corn, onions, garlic, leeks, parsnip, carrots, cauliflower, soybeans, parsley, Jerusalem artichoke, melons and squash. Tomatoes, potatoes, peppers, eggplant, asparagus, cabbage, peas, rhubarb,petunias and other members of the nightshade family seem to be the most susceptible to juglone. The other key to success is to plant these tolerant vgetables in raised beds with barriers of wood, stone or concrete in the bottom which limits the growth of walnut roots into the bed. This article gives a number of alternate strategies to raised beds such as lasagna gardening and straw bale gardening