Black walnut trees kill almost anything you plant under them. In 1 C.E., Pliny the Younger attributed this to their heavy shade. Now we know they secrete a poison, called juglone. In 1937 Botanist Hans Molisch, president of the University of Vienna, coined the term “allelopathy,” from Greek roots meaning “each other” and “sensitivity.”

Some people use the term only to refer to inhibition of one plant by another’s chemical residues. But a substance that inhibits growth at one concentration, commonly promotes it at a much smaller dose. Moreover concentrations in the wild are typically minute, so that the effect of one plant is less important than the interaction between the chemicals in a community of various plants. The same substance can also stimulate during one part of a plant’s growth cycle, and inhibit at another stage or season.

Of course one plant can simply outcompete with another for sun, water, minerals ... even air. To rule out competition, scientists conduct experiments like one that grew grass and apple seedlings in two well-separated trays. Water drained from the grass was introduced into the apple tray; the seedlings wilted.

Grasses inhibit many woody plants, notably dogwood and forsythia. In turn, Sycamore inhibits kentucky bluegrass, bermudagrass, and annual ryegrass. Other trees which inhibit various plants include horse chestnut, firs, pines ... and hackberry, a sturdy native tree I was considering — until now — to replace my dying maple.

New England asters inhibit sugar maples; many native asters are allelopathic. (Hmm, what have the ones all over my garden affected?) Allelopathy orchestrates the phenomenon of plant succession in abandoned land; major players include asters, sunflowers and goldenrod.

Roses, lilacs, viburnums, mockorange, and barberry show considerable allelopathic activity. Strong allelotoxins persist from most plants of the Brassica (cabbage) family — including that old-fashioned, sweet-smelling annual, stock. (Dame’s Rocket, too; maybe I don’t want so much of it after all.)

Some invasive plants create allelopathic toxins in the ecosystem they’ve been introduced to, but not in their native one!

Peach, apple and citrus trees leave allelochemicals in the soil that prevent replanting the same kind of tree. Citrus trees actually kill themselves off — as well as two plants I’ve struggled with: chrysanthemums and clover. (I tried three times with Trifolium purpurascens, a bronze-leaved clover that makes lots of four-lobed leaves.)

How many of the garden failures I grieve, or the plants too successful I have to discipline, are due to this whole new factor I need to learn about?

Text and photos © Copyright 2004 Catherine Holmes Clark

• To see dame's rocket, look at the photo with last week's column.
• The definitive book on this subject is Allelopathy, by Dr. Elroy. L. Rice. (Orlando, Harcourt Brace Jovanovich, 1984 [second edition]) It was Rice who began research in the U.S. on the subject, with his studies that established the allelopathic basis for plant succession.
• The apple tree / grass experiments are those of Pickering (1917, 1919) at the Woburn Experimental Fruit Farm at Ridgmont, (??XX) UK. Rice described them on p. 103 of Allelopathy (2nd ed.)
• Plant Against Plant, by Ellen M. Silva, Extension Technician, Department of Horticulture, Virginia State University. Originally published in The Virginia Gardener Newsletter , Volume 8, Number 4. Lists of allelopathic plants, the plants most sensitive to allelopathic toxins, and plants which resist them. "purslane, ragweed, and crabgrass, also exude allelopathic compounds"
• Black Walnut Toxicity to Plants, Humans and Horses, by Richard C. Funt and Jane Martin - Ohio State University Extension Fact Sheet HYG-1148-93 - lists plants that will not grow under or near the trees -- and plants that will!
• Trees and Turf: are they Compatible? Describes allelopathy in some detail, however with the definition limited to inhibition.
• Allelopathic research on chickpea seeds in Chhattisgarh (India) region ; An overview, by Pankaj Oudhia © 2001,2002,2003. Mentions homeopathic effect of low concentrations.
• Allelopathy: from concept to reality, by M. An, J. Pratley and T. Haig, Charles Sturt University, Wagga Wagga, NSW 2650
• International Allelopathy Society - I was particularly impressed by the research section of this site.