Walnuts: a mixed blessing

Flower & Garden Magazine, August-Sept, 1996 by Marty Ross

Gardeners Who Live Under The graceful canopy of mature walnut trees usually find it a mixed blessing. Walnuts are handsome trees, but their roots release a chemical, juglone, that is toxic to many plants growing in their extensive root zone. Azaleas, rhododendrons and mountain laurels are among the plants most sensitive to juglone, but vegetable gardeners don't slip free: tomatoes and other crops also succumb to the poison.

The traditional advice for gardeners who have walnut trees on or near their property has been to choose understory plants that can tolerate juglone. Recent research suggests, however, that even tomatoes can be grown near walnuts if the soil is generously amended with organic material.

"Research seems to indicate that the more you can improve the soil, the less juglone toxicity is going to appear," says Jerry Van Sambeek, a plant physiologist with the U.S.D.A. Forest Service. Van Sambeek's research should help change gardeners' attitudes about walnuts.

Compost improves soil aeration and decreases the density of the soil. Bacteria, which break down the juglone, thrive in porous soils, Van Sambeek says.

"We're finding the soil can make a big difference," he says. "When we see toxicity, we're looking at soil that tends to be poorly aerated and holds too much moisture, and there tend not to be microbes in it."

Black walnuts (Juglans nigra) are large, elegant trees, native in this country from Massachusetts to Florida and west to Minnesota and Texas. The largest specimens may reach 150 feet, but walnuts are valuable for their dense, dark wood, and these old giants are rare in the landscape. In home gardens, walnuts grow to be 50 to 75 feet tall. English and Carpathian walnuts (Juglans regia), extensively grown for their nuts, are more common on the West Coast. Butternuts, hickories and pecans -- all members of the walnut family, or Juglandaceae -- also produce juglone. Because they produce less of the substance than walnuts, however, they are less toxic to plants growing in their root zone.

Walnut trees are late to leaf out in spring and among the first deciduous trees to lose their foliage in the fall. Their compound leaves are thin and aromatic and provide light shade. One source says the dried leaves can be mixed with straw for a flearepellant animal bedding. The nuts ripen in late fall and drop from the trees around the time of the first frost. Walnuts in their husks may be the size of small oranges. They gather a good bit of momentum as they fall, so when the nuts are ripe, don't park under a walnut tree.

Juglone is produced primarily by walnut roots. The amount produced varies by species, and some individual trees make more juglone than others. It is not soluble in water, so it remains behind after watering or rain. Its effect is intensified during a drought. Juglone also is present in walnut leaves and in the hulls. Rake leaves away from plants under walnut trees and toss them on the compost heap. An active, or hot, compost pile will break down the juglone in a couple of months. In cooler piles, allow walnut debris to decompose for a year. If you remove a walnut tree, the juglone should disappear from the soil within a year.

Before you blame walnut trees for killing your plants, look at the conditions around your garden. Light, moisture and exposure influence any plant's growth and performance. Squirrels are interested in walnuts, too, and mischievous squirrels have a bad habit of digging up young plants as they search for and plant nuts.

If you don't have walnut trees in your yard but lose plants mysteriously, look around the neighborhood. A walnut in a neighbor's yard may be close enough to cause problems. Walnuts are deeprooted trees, but their roots also spread widely, up to one-and-a-half times as far as the tree is tall. A 50-foot tree may affect plants in several gardens around it.

Some plants actually thrive under walnuts. Kentucky blue-grass always looks lush under the high canopy of walnut trees. Dandelions and chickweed, unfortunately, don't even notice when walnuts are nearby. Oaks, red cedars, maples and most native hardwood trees can live side-by-side with walnuts. Daffodils don't react to the presence of juglone, and beets, snan beans, lima beans, onions and forsythia have all shown normal or improved growth under or near walnut trees, according to research published in the Northern Nut Growers Association bulletin.

Lots of gardeners like walnut trees. If you're considering planting some, interplant them with other species, Van Sambeek suggests. Turn plenty of organic material into the soil when you plant, and add more every year. Losing the occasional tomato plant isn't so bad, but no one likes watching choice rhododendrons die.

RELATED ARTICLE: Juglone-Tolerant Plants

Certain plants are better able to resist the toxic chemical juglone produced by walnut trees. Horticultural educator Dave Robson, with the University of Illinois Cooperative . Extension Servicehas comDfled the followina list of plants that are not affected.