Q: We have a very old, very large walnut tree in our yard. It is more than 80 feet tall and the drip line is at least a 20 foot radius from the trunk. On odd years, it produces so many walnuts that we fill multiple garbage cans. On even years, we hardly fill even half of one can. We have been told that the walnut tree impedes growth of other plants well out past its drip line. Is this true• We’ve been told that it emits some self-made chemical/hormone either through the root mass or from the leaves. Our garden is probably 50 to 60 feet east of the tree and we were told the tree could be negatively influencing our tomato plants. Your thoughts• I’d like to plant some low bush-type plants on the southern side of the tree (your column on blueberry bushes sort of inspired my thoughts). Is there any truth to what we’ve been told about the walnut tree impeding the growth of other plants around it• If there is, how do we overcome this?

A: The roots of black walnut trees produce a substance called juglone. Its purpose is to inhibit the growth of other plants and reduce competition. Unfortunately, it works really well.

According to the folks at Ohio State University, the juglone-laced area on a mature walnut tree is a radius of about 50 to 60 feet from the trunk — a huge area that unfortunately may include your vegetable garden.

Some plants are very prone to the negative affects of juglone and should be avoided all together. These include: apples, potato, azalea, rhododendron, and, unfortunately, tomatoes. And the full list of affected plants is much longer. Juglone is also produced in the leaves and fruits of walnut trees but in lesser amounts. Promptly collecting fallen fruits and leaves does help keep juglone out of surrounding areas but there is nothing you can do about the juglone produced by the roots.

If possible, you may want to consider relocating your veggie garden. Another option is to have the tree removed, though this will eliminate an excellent food source for squirrels and other wildlife as well as create a big change to your landscape (not to mention the cost). If the tree is removed, it reportedly takes two to three years for the juglone to fully break down.

As far as what to plant on the southern side of the tree, there are a few plants that are known to be tolerant of juglone. Though there are others, here is a brief list of some of my favorites:

Trees and shrubs: Eastern redbud, fringe tree, ninebark, Viburnums, Rose of Sharon, Euonymus, arborvitae Perennials: Astilbe, sweet woodruff, coral bells, bee balm, phlox, Siberian iris, most hostas, lamb’s ear Annuals: calendula, begonias, zinnias Veggies: squash, beans, carrots, melons

I would recommend you take a soil test in your vegetable garden and any other potential planting sites to ensure a proper pH and nutrient content. Many times you’ll find that the soil under a walnut tree is quite alkaline, which adversely affects plant growth. You can purchase soil test kits through the Penn State Cooperative Extension office. Details: 412-473-2540.

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