Chemical warfare in plants has good points

What if we used plants’ natural ability to repel each other in service of certain goals in the farm field or landscape?

Experienced gardeners likely are aware that tomato plants won’t grow well near walnut trees. Maybe you learned the hard way that walnut hulls in the compost is the kiss of death for anything to which you might apply that compost.

Or perhaps you observe that trees growing in competition with grass are often at a disadvantage. There is more than just competition for sunlight, water and space going on! Certain plants are uniquely equipped to wage chemical warfare on certain other plants.

Timothy Chick, on the faculty of Paul Smith’s College in the Adirondacks, noted how this repellent behavior might someday replace herbicides in the management of utility corridors and roadsides. He observed that native prairie plants naturally repel invading trees by several means.

One is through chemicals given off in the soil. Studies have shown prairie species used to prevent undesirable tree growth on Hydro-Quebec utility rights-of-way, on a Texas pipeline easement and on roadsides in Iowa. This is promising for situations where the expense and risk of herbicide application on vast and often remote acreages could be avoided, keeping the roadsides weed-free and the lights on.

A $5 Word

“Allelopathy” is the name of this repellent action that some plants have on certain others, and “allelochemicals” are the agents that are given off to air, water or soil in defense of a plant’s own space. It’s a strategy to reduce competition, and the walnut/tomato interaction is — we now find — just the tip of the iceberg.The University of Minnesota Extension reveals an extensive list of what repels what in a publication available online at For example:Sumac, rhododendron and elderberry fend off Douglas-fir

Aster and goldenrod repel several tree species

Kentucky bluegrass is unwelcoming to forsythia

Who knew? When we see the living world as an interconnected web, this work begins to reveal how some of that works.

Grass, trees

Along with competition for water, light and space, allelopathy complicates the mixing of trees and turf. For many years now, best practices have included separating them:Group trees together in a shared, mulched bed. Trees and grass both grow and perform better when they are separated from each other. Meanwhile, trees growing in a group can offer a higher level of environmental services than the same number of trees growing separately. Spacing can vary with design intent.Use ground covers in place of turfgrass around trees. Depending on sun exposure, you might consider Aaronsbeard Saint Johnswort, barrenwort or pachysandra, all of which are used successfully in our area.

Future tool

Those working with allelopathic plant interactions imagine a future where:Informed plant selection may help to reduce chemical use in the landscape.Thoughtful plant selection on cover crops and pasture may help to reduce chemical use, advancing sustainable agriculture.

Plants that will suppress tree growth may reduce the cost of pruning or herbicide application in conflicts between trees and power lines.

Advanced investigation of plant interactions could expand our understanding of “companion planting” and its practical benefits.

There is ancient wisdom in the notion that some plants do better or worse in the presence of certain other plants. You may have learned this at your grandma’s knee. It is terrific that there is now active research into how this works, and how it might help us to manage crops, landscape, and rights-of-way more sustainably in the future.