Walnut tree causes harsh growing conditions.
A black walnut is one of the most picturesque and resilient of our native trees. But when you have one in your own yard, you soon learn that a walnut tree presents a peculiar challenge: Some plants just won't thrive in the vicinity of a walnut tree.
The problem is that the tree produces a substance called juglone, which is toxic to many other kinds of plants, big and small. All parts of the walnut tree contain juglone, but the root zone can be particularly toxic to susceptible plants.
Tomato plants are common victims of juglone. The tomato will wilt every day, seem to recover overnight for a while, but then dies, usually before the season's first tomato ripens. Some other plants behave similarly, although their death may not come as quickly.
My yard is backed by a ravine filled with walnut trees, providing a crash course in walnut toxicity. A new hydrangea, for example, wilted flat to the ground every day, despite moist soil. As soon as I dug it up and transplanted it far away from the walnuts, the hydrangea recovered completely.
Many gardeners have shared with me their own success stories of what will and what won't grow near black walnuts, and there are numerous lists online from universities, arboreta, and botanical gardens for both juglone-susceptible and juglone-tolerant plants. However, no two lists seem to be in complete agreement, so choosing plants to share space with a black walnut is still a little bit hit or miss.
I think the wild card is that soil type and soil moisture also play a part. Another problem: I can't find any information, pro or con, for some of the more unusual plants.
Still, there are some understory trees such as redbud, American hornbeam and hophornbeam that I can vouch for. My magnolia Butterflies is also thriving, brightening the spring with its yellow blossoms.
Jetbead, American bladdernut, wafer-ash, and black-haw viburnum -- all really big shrubs -- seem to be unfazed by sharing space with a black walnut. A number of smaller shrubs are doing fine, too, including red-stem dogwood, Japanese kerria and alpine currant.
The list of shade-loving perennials you can grow beneath the branches of a walnut tree is even longer. It includes, for example, lady's mantle, bloodroot, bleeding heart, Solomon's seal, trillium, cranesbill, hosta, goatsbeard, foamflower, astilbe and all kinds of ferns and sedges.
Canadian hemlock and arborvitae are the only evergreens I've found that can tolerate both juglone and some shade. Where there's more sunlight, junipers are a good bet. Other evergreens aren't as tolerant of sharing space with a black walnut.
* Write to Jan Riggenbach at 2319 S. 105th Ave., Omaha, NE 68124. Please enclose a self-addressed, stamped envelope. Find more online at www.midwestgardening.com.