Nature’s Own Herbicides

By Michael J. Caduto

Anyone who grew up in a large family knows how much time can be expended competing for food, space, and attention. Who is going to be served dinner first, get the best seat at the movies, or enjoy the last dollop of ice cream? Someone may get short shrift, and it’s no different in the natural world.

Plants also compete: they vie for air, water, space, sunshine, and nutrients. And while they can’t protest or push to be first in line, plants do deploy chemicals to obtain an advantage over other species. This process is called allelopathy, which comes from the Greek words pathos, “to suffer,” and allelon, “of each other.” This phenomenon was first recorded in 300 B.C., when it was observed that barley, chickpeas, and other cultivated plants interfered with the growth of other crops. Allelopathy is responsible for the killed grass beneath bird feeders where the hulls of sunflower seeds have fallen. And it is why gardeners plant wormwood away from the herbs whose growth it inhibits, such as sage, anise, fennel, and caraway.

Many native plants compete with other species using allelochemicals. Black walnut and butternut contain juglone, which is concentrated in the nut husks, roots, and buds. This compound inhibits the growth of many plants, including apples, blueberries, and azaleas, and especially those in the nightshade family, such as peppers, tomatoes, and potatoes. Since a black walnut’s root system can reach well beyond the edge of the crown’s expanse, the ground is often bare far from the trunk. Hickories, pecan, and English walnut also produce juglone, but in smaller concentrations.

Allelochemicals act in myriad ways. Compounds that inhibit plant growth and seed germination are emitted from roots and can leach out of leaves, twigs, flowers, and seed casings. Some plants exude chemicals that suppress the activity of chlorophyll in the leaves of other species, diminishing their ability to use sunlight to create carbohydrates for growth. Other allelochemicals impede the ability of roots to absorb water and nutrients. The leaves of some species give off toxic gases.

Hay-scented fern is an indigenous plant that many foresters and ecologists long suspected of being allelopathic, because it forms broad swaths through which tree seedlings struggle to grow, especially those of black cherry and sugar maple. Studies conducted by the University of Vermont’s Proctor Maple Research Center, however, show that this fern is simply a strong competitor. Its dense crowns block sunlight, and the thick mats of roots out-compete tree seedlings for water and nutrients. White pine and black birch seedlings fare better in hay-scented fern beds because their spring leaves open before the fronds of fern unfurl.

Plants have probably used allelopathy to compete with each other since the early stages of evolution. Over time,  the dynamic between indigenous species reaches a sort of balance, with any particular outcome depending on a variety of factors, including the type of soil, the amount of rainfall, and the pressure of browsing animals. Among other native plants that are suspected of using some form of allelopathy are sugar maple, American elm, red oak, sunflower, cottonwood, American sycamore, black cherry, sassafras, and tobacco. In fact, extracts from the wood of red maple and eastern red-cedar inhibit seed germination at least as strongly as do extracts from black walnut.

Some plants that have been introduced to New England from other environments—so-called “alien species”—are also allelopathic. Garlic mustard, for example, creates a compound that harms the intricate relationship between the roots of certain native trees and the mycorrhizal fungi with which they associate in the soil. In this symbiotic relationship, found among birches, beeches, maples, some conifers, and possibly other trees, the plant roots that are penetrated by fungal threads pass water and carbohydrates to the fungi. In turn, the fungal threads expand the root’s ability to absorb nutrients from the soil. Garlic mustard interrupts this mutually beneficial connection.

Whether a plant is a native species or introduced from afar, allelopathic competition plays a part in its ecological role. From forests and wetlands to gardens and fields, allelochemicals contribute to the dynamic of plant interactions. Nature’s deceptively serene appearance belies the struggles being waged unseen.

What You Can Do

Instead of using pesticides that are harmful to people, soil and garden pollinators, you can employ the natural defenses of chemicals contained in plants to your own benefit by interplanting select varieties of vegetables, herbs, and flowers in your garden to help keep pests away. Some recommended books are: Great Garden Companions: A Companion-Planting System for a Beautiful, Chemical-Free Vegetable Garden by Sally Jean Cunningham (Rodale Press), and Plant Partners: Science-Based Companion Planting Strategies for the Vegetable Garden by Jessica Walliser (Storey Publishing). (Please support your local bookstore.)

Invasive garlic mustard (Alliaria petiolata) is allelopathic. It releases chemicals that impede the grow of other plants and harm the symbiotic mycorrhizal soil fungi that assist tree roots in absorbing water and nutrients. Photo: Michael J. Caduto.

Article adapted with permission from Through a Naturalist’s Eyes: Exploring the Nature of New England by Michael J. Caduto (Chicago University Press/University Press of New England).


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