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 an allelochemical called juglone that inhibits the growth of other plants near them.

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. Butternut trees (Juglans cinerea), English walnuts (Juglans regia), hickories and pecans (Carya spp.) also produce juglone, but not as much as black walnut trees.

Juglone 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 also can 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 also injures 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 -- 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.

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:

Trees and shrubs

Black, red, sugar and Japanese maples (Acer spp.)
Serviceberry (Amelanchier spp.)
River, cherry birch (Betula spp.)
Eastern redbud (Cercis canadensis)
Fringetree (Chionanthus spp.)
Hawthorn (Crataegus spp.)
Daphne
Forsythia
Thornless honeylocust (Gleditsia triacanthos var. inermis)
Carolina silverbell (Halesia tetraptera)
Witchhazel (Hamamelis spp.)
Rose of Sharon (Hibiscus syriacus)
Hydrangea
American holly (Ilex opaca)
Eastern red cedar (Juniperus virginiana)
Sweetgum (Liquidambar styraciflua)
Black gum (Nyssa sylvatica)
Virginia creeper (Parthenocissus quinquefolia)
Mock-orange (Philadelphus coronarius)
Oak (Quercus spp.)
Black raspberry (Rubus occidentalis)
Elderberry (Sambucus spp.)
Lilac (Syringa spp.)
Arborvitae (Thuja spp.)
Canadian hemlock (Tsuga canadensis)
Elm (Ulmus spp.)
some viburnums

Perennials

Bugleweed (Ajuga reptans)
Hollyhock (Alcea rosea)
Jack-in-the-pulpit (Arisaema triphyllum)
European ginger (Asarum europaeum)
False spirea (Astilbe spp.)
Bleeding heart (Dicentra spp.)
Leopardsbane (Doronicum orientale)
Crested wood fern (Dryopteris cristata)
Trout lily (Erythronium americanum)
Sweet woodruff (Galium odoratum)
Perennial geranium (Geranium spp.)
Lenten rose/Christmas rose (Helleborus spp.)
Daylily (wild orange type) (Hemerocallis fulva)
Hosta
Siberian iris (Iris siberica)
Lily-turf (Liriope spp.)
Lobelia
Bee balm (Monarda spp.)
Sensitive fern (Onoclea sensibilis)
Cinnamon fern (Osmunda cinnamomea)
Garden phlox (Phlox paniculata)
Great Solomon's seal (Polygonatum commutatum)
Christmas fern (Polystichum acrostichoides)
Primrose (Primula spp.)
Lungwort (Pulmonaria spp.)
Bloodroot (Sanguinaria canadensis)
Live-forever (Hylotelephium (Sedum) spectabile)
Lamb's ear (Stachys byzantina)
Meadow rue (Thalictrum spp.)
Spiderwort (Tradescantia virginiana)
Trillium
Canada violet (Viola canadensis)

Bulbs

Glory of the snow (Chionodoxa lucilae)
Spring-blooming crocus
Spanish bluebells (Hyacinthoides hispanica)
Winter aconite (Eranthis hyemalis)
Snowdrop (Galanthus spp.)
Hyacinth (Hyacinthus orientalis)
Grape hyacinth (Muscari botryoides)
Daffodils (Narcissus)
Siberian squill (Scilla siberica)
Tulip (Tulipa spp.)

Annuals

Wax begonia (Begonia semperflorens)
Pot marigold (Calendula officinalis)
Morning glory (Ipomea purpurea)
Pansy (Viola x wittrockiana)

This list has been adapted from walnut toxicity publications by Michigan State University Extension, Virginia Cooperative Extension and Penn State University.