Q. I planted 35 tomato plants in my garden, and they are all turning yellow and wilting - they appear to be dying. It does not seem to be a typical tomato problem such as early blight. I’ve been growing tomatoes for years, and have never seen anything like this. What could be wrong with them?
A. Further conversation with the writer revealed that there is a large black walnut tree (Juglans nigra) growing in his yard. These trees secrete a substance called juglone that inhibits the growth of other plants near them. This phenomenon is known as allelopathy. It reduces competition for water and nutrients, and ensures their long-term survival. Other species of trees, including tree-of-heaven (Ailanthus altissima), sugar maple (Acer saccharum), hackberry (Celtis spp.), and American elm (Ulmus americana) also produce allelochemicals, but their effects pale in comparison to black walnut. Black walnut trees are so infamous for killing or damaging plants grown near them that pathologists have coined the term “walnut wilt” to describe the problem.
Butternut trees (Juglans cinerea), English walnuts (Juglans regia), hickories (Carya spp.), and pecans (Carya spp.) also produce juglone, but not as much as black walnut trees. Juglone is found in leaves, stems, nut hulls, inner bark and roots. It can affect plants growing under the tree or within the sphere of its root system. Remember that a tree’s feeder roots - those fine, hair-like roots responsible for absorbing water and nutrients - can extend out two or three times the circumference of the spread of a tree’s branches (drip line). Although plants growing immediately under the tree are most at risk, sensitive plants growing in the area of its extended root system can also be damaged or killed.
Susceptible plants are exposed to juglone through root contact, the decay of fallen leaves and hulls, or even rain leaching juglone from the leaves and branches to the ground below. Although the writer has grown tomatoes in his vegetable garden for years with no problem, the tree has grown over time, and the spread of its root system has increased as well. While tomatoes are probably the best-known victims of walnut wilt, juglone is injurious to asparagus, azaleas, blueberries, mountain laurel, potentilla, rhododendron, and many other plants when they are grown within the root zone of these trees.
There are steps you can take to reduce the allelopathic effect of juglone. Clean up fallen leaves, nuts, and other debris from black walnut trees regularly to minimize their accumulation in the soil. Incorporate compost, rotted manure, composted grass clippings, and/or cover crops to maintain a high level of organic matter in the soil. This encourages a healthy population of soil microbes that can help break juglone down and minimize damage to sensitive plants. Avoid using compost that contains black walnut debris unless it has been broken down thoroughly. If you compost actively - that is, you turn the pile frequently and keep it moist - the compost can be used in about eight months. If you use the lazy method of composting - that is, pile the materials up and let them sit - you should wait at least a year before using it. You can test the safety of the finished compost by starting tomato seedlings in it. If they do not die, the mulch is safe to use.
The writer is more interested in growing tomatoes than keeping the black walnut and is considering having it removed. It is a large tree, and its roots cover so much of his yard that he does not have another place where he can grow tomatoes. That is an option, but it will take at least two years - possibly longer - for all of the roots to die, even if the stump is ground out. If you choose this route, be aware that black walnuts are valuable trees. Large, straight-growing specimens can be worth a lot of money. Investigate options for selling the tree, rather than paying someone to remove it.
If you are less determined to grow a sensitive crop like tomatoes, be aware that there are plants that grow well near black walnuts. Kentucky bluegrass and tall fescue seem to thrive in association with black walnut and butternut trees, so planting the area in lawn grass is an option. There are also quite a few other plants that will grow successfully near these trees. They include the ones listed below.
This list was adapted from walnut toxicity publications by Michigan State University Extension, Virginia Cooperative Extension and Penn State University.
Source: Purdue University Cooperative Extension Service
Vegetables: asparagus, cabbage, eggplant, pepper, potato, rhubarb, tomato.
Fruits: apple, blackberry, blueberry, pear.
Landscape plants: black alder; azalea; basswood; white birch; ornamental cherries; red chokeberry; crabapple; hackberry; Amur honeysuckle; hydrangea; Japanese larch; lespedeza; lilac; saucer magnolia; silver maple; mountain laurel; pear; loblolly pine; mugo pine; red pine; scotch pine; white pine; potentilla; privet; rhododendron; Norway spruce; viburnum (few); yew.
Flowers and herbaceous plants: autumn crocus (Colchichum); blue wild indigo (Baptisia); chrysanthemum (some); columbine; hydrangea; lily; narcissus (some); peony (some); petunia; tobacco.
Field crops: alfalfa; crimson clover; tobacco.
Vegetables: lima bean; snap bean; beet; carrot; corn; melon; onion; parsnip; squash.
Fruits: black raspberry, cherry.
Landscape plants: arborvitae; autumn olive; red cedar; catalpa; clematis; crabapple; daphne; elm; euonymous; forsythia; hawthorn; hemlock; hickory; honeysuckle; junipers; black locust; Japanese maple; maple (most); oak; pachysandra; pawpaw; persimmon; redbud; rose of sharon; wild rose; sycamore; viburnum (most); Virginia creeper.
Flowers and herbaceous plants: astilbe; bee balm; begonia; bellflower; bergamot; bloodroot; Kentucky bluegrass; Spanish bluebell; Virginia bluebell; bugleweed; chrysanthemum (some); coral bells; cranesbill; crocus; Shasta daisy; daylily; Dutchman’s breeches; ferns; wild ginger; glory-of-the-snow; grape-hyacinth; grasses (most); orange hawkweed; herb Robert; hollyhock; hosta (many); hyacinth; Siberian iris; Jack-in-thepulpit; Jacob’s ladder; Jerusalem artichoke; lamb’s-ear; leopard’s-bane; lungwort; mayapple; merrybells; morning glory; narcissus (some); pansy; peony (some); phlox; poison ivy; pot marigold; polyanthus primrose; snowdrop; Solomon’s-seal; spiderwort; spring beauty; Siberian squill; stonecrop; sundrop; sweet Cicely; sweet woodruff; trillium; tulip; violet; Virginia waterleaf; winter aconite; zinnia.