A walk through the woods can be one of the most peaceful and calming experiences – a place where you can find quiet for reflection and marvel at the beauty of nature. Little do most people know that some plants, especially one specific tree, wage chemical warfare against other plants to keep away potential neighbors that would compete for nutrients and sunlight.

The tree most skilled at chemical warfare is the black walnut. The black walnut tree (Juglans nigra) is a useful, yet often misunderstood tree. Prized for its excellent wood qualities for lumber and furniture, the nuts it produces are either loved or reviled by those who try them.

The flavor of black walnuts is hard to describe. I would say that they have an almost astringent flavor, mainly due to the high level of tannins in them. They aren’t my favorite, but I don’t mind them either. I’ve learned to accept them, unlike during my childhood when you knew which church lady’s cake to avoid at the potluck because you knew that she put black walnuts in everything she baked.

You could tell when someone was picking or cracking black walnuts, thanks to the tannin stains on their hands that just wouldn’t wash off. Black walnut trees have the interesting ability to excrete a chemical called juglone, which makes it nearly impossible for a number of plants to grow anywhere in its root zone.

Juglone works by damaging the tiny root hairs on roots that are responsible for taking up a great majority of the water and nutrients the plants use. Research shows that it also interferes with the interaction of the roots with mycorrhizal fungi that aid the plant in taking up nutrients.

This process is not just specific to black walnuts. There are several other plants that do this. The phenomenon, called allelopathy, occurs when an organism excretes something that inhibits the growth of other things around it. You could equate it to the Penicillium fungus excreting a chemical that kills bacteria around it. We harness that chemical to use as penicillin.

Some plants are especially sensitive to the chemical. Many vegetable plants, especially tomatoes, are sensitive. Some plants, mainly those that would grow wild in the woods, are not susceptible. Many grasses also have a hard time growing beneath black walnut trees (tall fescue and Kentucky bluegrass being the exception, except during periods of drought).

Plants that are also sensitive are: asparagus, cabbage, eggplant, pepper, potato, rhubarb, tomato, apple, blackberry, blueberry, pear, 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, autumn crocus (Colchichum); blue wild indigo (Baptisia); chrysanthemum (some); columbine; hydrangea; lily; narcissus (some); peony (some); petunia; tobacco, alfalfa; crimson clover; and tobacco.

All parts of the tree produce the juglone chemical, so the effects could spread beyond the perimeter of the tree from fallen leaves and branches. I would also suggest that you make sure any fresh woodchip mulch that you use (specifically that from local tree cutters) is free of black walnut. The juglone may break down after composting the wood chips for six months to a year, but I would still be cautious about its use. The wood will release the chemical, killing susceptible plants for a few years in the area where it is applied. Studies suggest that juglone will break down during the composting process, but I would check to make sure by starting a few tomato seeds on the batch of compost to see what happens. The following are plants that will tolerate growing near a Black Walnut: lima bean; snap bean; beet; carrot; corn;melon; onion; parsnip; squash, black raspberry, cherry, 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; 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; and zinnia.

For more information on gardening, you can reach me at s.dejaeghere@me.com