Competition in the plant world is fierce as the various species battle it out for sunlight, water, nutrients and space. Plant use a variety of ways to get the upper hand, some actually resorting to chemical warfare to crush the competition. This process is called allelopathy.
Besides the lovely way the word “allelopathy” rolls off the tongue – uh·leeﹶ·luh·pa·thee – it’s an interesting process.
Allelopathy is the chemical inhibition of one plant species by another. The “inhibitory” chemical is released into the environment where it affects the development and growth of neighboring plants. It does this by affecting respiration, cell division, photosynthesis and water and nutrient uptake.
Allelopathic effects include leaf wilting and yellowing, stunted growth, or death of part or all of a plant. Allelopathic chemicals can be present in any part of a plant including foliage, flowers, seeds, fruits, pollen, stems, bark and roots.
The most well-known example of allelopathy is the black walnut tree. Gardeners and landscapers often struggle to find plants that will survive under or near black walnuts. The trees produce a chemical called juglone that can damage plants growing near them. Juglone is released through roots, leaves that drop to the ground, husks of nuts, and even from rain drops that drip through the tree.
You can find extensive lists of plants both susceptible to and resistant to juglone on the internet. There has been very little scientific study into plant sensitivity to juglone so the lists are mostly based on observation and should be considered guidelines, not guarantees.
Many factors affect juglone sensitivity, including level of contact, health of the plant, soil environment, and the overall site conditions. In general, allelopathic effects can be somewhat reduced by cleaning up fallen leaves and fruit, and maintaining high organic matter in the soil so that healthy soil microbial populations metabolize the juglone.
Along with black walnut, there are other plants that produce juglone including butternut, English walnut, shagbark hickory, sugar maple, tree-of-heaven, hackberry, American sycamore, cottonwood, black cherry, red oak, black locust and American elm.
Juglone isn’t the only allelopathic chemical, just the most well-known. Many other plants produce chemicals that inhibit competitors. You may have noticed that grass or other plants do not grow well under your bird feeder after the birds have dropped sunflower seed hulls. The hulls are allelopathic. Some invasive plants, including garlic mustard, emit chemicals into the soil from their roots that makes the growing environment inhospitable to other plants that would be more beneficial to the ecosystem.
But allelopathy isn’t all bad. In agriculture, growing a selectively allelopathic plant as a companion plant to a valued unsusceptible crop can suppress certain weeds while not disturbing the growth of the main crop, a great alternative to using toxic herbicides for weed control. In the home landscape, a mulch of shredded black walnut leaves, hulls or bark chips could provide weed control in an area of non-susceptible plants.