Q: I lost a lot of plants I planted toward the back of my yard the last two years. A friend told me it's probably because of the walnut trees that are growing nearby in the next lot. He said there's some kind of poison in walnuts. Could that be doing it?
A: Basically, yes. It's not exactly a poison, but trees in the walnut family (hickories, too) produce a plant chemical called juglone that interferes with the growth of some plants.
The chemical is in the roots, leaves and bark, and its effect can extend out as far as the tree's canopy (the area covered by its branches). That's because tree roots can reach out at least that far.
The closer you plant a sensitive plant to a walnut's trunk, the more likely trouble will start. Yellowing and wilting are the main symptoms, followed by plant death in the most sensitive plants.
According to Penn State's Department of Horticulture, black walnuts and butternuts are the two most common walnut-family trees that stunt growth in landscape settings. Hickories, English walnut and Persian walnuts also produce juglone but in slightly less concentrations.
Actually, this plant-killing effect isn't peculiar just to walnuts and hickories. Many other species produce other plant chemicals that affect potential competitors. It's a way for plants to defend their territory, so to speak.
Other good chemically armed space-defenders include sugar maple, tree of heaven, hackberries, sycamore. Cottonwood, black cherry, red oak, black locust, sassafras and American elm, according to Penn State.
The best way to deal with walnuts in the vicinity is to plant species that are tolerant of juglone.
Shrub examples include smooth hydrangea, St. Johnswort, juniper, spicebush, elderberry, maple-leaf and Korean spice viburnums, forsythia and witch hazel.
Perennials resistant to juglone include yarrow, hosta, iris, Shasta daisy, aster, liriope, astilbe, beebalm, phlox, bleeding heart, Solomon's seal, barrenwort, sweet woodruff, sedum and coralbells.
A detailed list and more juglone details can be found here: http://consumerhorticulture.psu.edu/files/landscaping_walnut_trees.pdf.