Black Walnut Tree Roots Really Produce Poison

Q. I have heard that foliage from black walnut and cedar trees is not good for plants. What can I grow under them?

A. The problem with the black walnut tree is not in its leaves but rather in what comes out of the roots. Juglone, a toxic compound exuded by the roots, has quite negative effects on an enormous number of plant species that might be growing in its vicinity, not the least of which are tomatoes and rhododendrons.

When the roots of such plants come in contact with the black walnut's roots, damage or death is sure to result. Finding out which plants will tolerate such an area probably will be a trial-and-error process.

Cedar foliage is not toxic to plants. What makes growing plants difficult to impossible in the vicinity of large western red cedars is their extreme competition for water and light. The soil underneath cedar trees can be very dry even after a rainy winter; however, if cedar trees are thinned and pruned up to allow more light to reach the ground, and if extra watering is carried out, most any plant should be able to grow in their vicinity.

Q. I've heard a lot about lime, but I don't know if I need to add it. What will I achieve?

A. There are essentially two kinds of lime, agricultural lime, or calcium carbonate, and dolomite lime. The former contains calcium, the latter is made up of both calcium and magnesium. Plants need both calcium and magnesium for good growth, but the only way to know for sure if either is in short supply is to have a soil test done by a reputable soil testing laboratory. Adding either one to the soil also reduces its acidity, but again, the only way to know if your soil is too acid for what you are trying to grow is to have a pH test run. This you can do yourself with a pH testing kit available at most nurseries and garden centers.

The only two areas of the garden where either of these materials might need to be added regularly is the vegetable garden, and perhaps lawns. Vegetables do best in slightly acid soils with pH's in the range of about 6.0 to 6.8. Lawn grasses for western Washington will perform quite well even in moderately acid soils down to 5.5. A soil test should be able to tell if your lawn area needs lime. A magnesium deficiency in either area should signal the need to add dolomite lime. Unless your yard is a peat bog, there is rarely any need to use lime for tree, shrub, and perennial growth.

If you'd like to have a soil test done by a university testing lab, send a self-addressed, stamped (52 cents postage), long envelope to WSU Extension, 612 Smith Tower, Seattle, 98104 and ask for the soil testing materials. A set of instructions on how to take a sample, the fees involved for the various tests available, plus a bag for you to send your soil to the lab will be sent to you immediately. If lime is needed, the test results will tell you, and exactly what kind and how much to apply. Lime is best applied in fall and winter.

Gardening runs Friday in the Scene section and Sunday in Home/Real Estate of The Seattle Times. It is prepared by George Pinyuh and Holly Kennell, Washington State University/King County Cooperative Extension agents, Mary Robson, Master Gardener program assistant, and volunteer Master Gardeners. Send questions to: Gardening, The Seattle Times, PO Box 70, Seattle, WA 98111. Questions of general interest will be answered as space allows.