Roanoke Times
                 Copyright (c) 1995, Landmark Communications, Inc.

DATE: THURSDAY, March 28, 1991                   TAG: 9103270136
SECTION: CURRENT                    PAGE: NRV-8   EDITION: NEW RIVER VALLEY 
SOURCE: Lelia Mayton
DATELINE:                                 LENGTH: Medium

HOW DO WALNUT TREES AFFECT PLANTS?

We often get questions like these at the Extension office. If you have black walnut trees, you may want to clip this article to file for reference.

The toxic effect black walnut trees have on other plants is caused by a substance called juglone, which is produced in the roots. The roots of a susceptible plant must come in direct contact with the roots of the tree for juglone to take effect. Therefore, only susceptible plants growing close to the walnut and within the spread of its root system are harmed.

Actually, black walnut toxicity causes damage to relatively few plants grown around the home. Your vegetable garden may be put near walnut trees and most crops will thrive except peas, potatoes and tomatoes. However, trees use a large amount of soil moisture so you will need to consider this when locating your garden.

Grass grows very well near black walnuts except during periods of drought. In fact, when moisture is adequate, the grass may be better under walnut trees than other parts of the lawn. We are not sure of the reason for this, but it may be related to the basic (or higher) pH level in soil under walnut trees.

Most spring bulbs do well under black walnut. Some annuals to try include begonia, calendula and pansy. Tolerant perennials include hosta, sedum, violet and chrysanthemum. Wildflowers for the site include cranesbill, bee balm, bloodroot and trillium.

Avoid planting azalea, rhododendron, mountain laurel and blueberry near black walnut. These plants are quite susceptible to juglone. Most other shrubs seem to be tolerant of this material.

Blackberry, apple, pine and sassafras may be seriously damaged by black walnut. Herbaceous plants susceptible to injury are creeping buttercup, forget-me-not, lily-of-the-valley, peony and thyme.

Now is the time to apply miscible oil sprays to kill overwintering mites, aphids and scale. Use the oil on deciduous plants and hardy evergreens but not on needle-leaved species. Spray miscible oils when temperatures are above 40 degrees but not within 24 hours of a freeze. Because the oil kills insects by suffocation, avoid spraying on windy days to ensure that all surfaces of the plant are covered.