What Won’t Grow Under that Walnut Tree

​Those of you who live with black walnut trees know that the delicious nutmeat is a hard-won prize. You may not realize that the tree gives off a toxin, juglone, which prevents members of the nightshade family from growing near it. The toxin is given off by the roots, leaves and stems of the tree. Although most of us would rather not have deadly nightshade around, this plant family has many desirable members. Two branches of the clan we frequently want to cultivate are the solanums and nicotianas.

The solanum branch of the family includes potatoes, tomatoes, eggplant and sweet and hot peppers. It is hard to imagine the summer garden without some of these vegetables. The green parts of potato and tomato plants contain toxic alkaloids as do their poisonous cousins belladonna, henbane and mandrake. Surprisingly, one of our most popular flowers, the petunia, is also a solanum. Bunnies eat them by the flat, so I’m certain they are nontoxic.

As might be deduced from the name, the nicotiana group does include tobacco. These are ornamental tobacco cousins. The taller ones (3-5 feet) have sweet, night-scented white, yellow or pink flowers which are attractive to some moths. These annuals are quite striking in the back of the border and easy to grow from seed. They stand up fairly well without staking. The dwarf hybrids are interesting front-row flowers of pink, white, yellow and green. Frequently sold in mixed colors at garden centers, single-color series have become more available.

The black walnut root system may extend well beyond the drip line of the tree. As a rule of thumb, place sensitive plants at a distance twice the length of the branches from the trunk. Another guideline is to plant at a distance from the trunk greater than the height of the tree. Mature walnut trees are typically 30 to 40 feet tall. If the tree falls or is cut down, there may be residual toxin present in the soil for a year. Tomatoes are especially sensitive to it, so plant them farthest from the tree.

What will grow around the walnut tree? Fortunately three of the veggie clans are immune to juglone. Members of the carrot family, bean clan, and melon branch of the squash family can be grown near walnut trees. Melons and cucumbers are very closely related so both should do well in the root zone. The bean clan is one of the largest plant families. It encompasses not only peas, beans and clover but also lupine, indigo and mimosas!

The carrot cousins are defined by their flat flowers, which are composed of many tiny florets. This is a great family to include in a crop rotation scheme. One can plant carrots in the spring, adding cilantro, parsley and dill as the weather warms. Complete the gardening year with parsnips that last into winter.

Connoisseurs of homegrown salsa must beware of black walnut trees, but backyard farmers have ample alternatives to tomatoes and peppers.

Master gardener Ann Bartlett never lets lack of familiarity with a plant stop her from trying it in the ornamental beds around her home, although she takes care to plant it out of range of any other plant she suspects it harbors secret plans to assassinate.

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