Allelopathy in the Home Garden

You may have heard the gardener’s mantra of “right plant, right place.” This helpful phrase means that if you choose a location in your garden that already has the proper conditions for the ideal growth of a particular plant, the plant is likely to flourish. The opposite is true if you force it to grow in less-than-optimal conditions. Usually, this approach works. However, sometimes, no matter how much care has been given to a plant, it just refuses to thrive for no obvious reason. At this point, a gardener will review an endless list of possible causes that may include things like variations in microclimate, overlooked required growing conditions, an unknown exposure to an overspray of herbicide, or invasion by an invertebrate yet to be found. If this scenario has happened to you and the end of the list has been reached without finding a cause, consider that it is the plants growing next to the suffering one that could be the culprits.  

Some plants have the ability to affect their immediate neighbors by producing and releasing toxic chemical compounds into the soil or air. When these chemicals are absorbed by a neighboring plant, they can inhibit growth or even kill the plant. This action allows the toxin-producing plant to reduce competition for available resources and dominate the utilization of these resources, which were intended for your plants. The toxin-producing plants are said to be allelopathic, which is the ability of a plant to release toxins that suppress the growth of plants in its vicinity.

One of the most commonly known allelopathic species is the black walnut (Juglans nigra). This stately Pennsylvania native shade tree provides hardwood for making furniture and produces walnuts for human and wildlife consumption. These trees produce a highly allelopathic compound called juglone in their leaves, stems, and fruits, with the highest concentration in the roots. If a plant that is sensitive to juglone comes into contact with juglone, the plant may show signs of yellowing leaves, wilted foliage, stunted growth, and, ultimately, may die. There are varying degrees of sensitivity to juglone. Vegetables in the solanaceous group, including tomatoes, peppers, and potatoes, are some of the least tolerant of juglone. Ornamentals, such as rhododendrons and azaleas, also are sensitive. Fortunately, there is a long list of plants that are less sensitive or not affected by juglone, including the squash family, beans, onions, and ornamentals like Forsythia, mock orange (Philadelphus spp.), and trees such as oaks (Quercus spp.), hawthorn (Crataegus spp.), and Eastern hemlock (Tsuga canadensis).

Black walnut trees are not the only plants capable of allelopathy. Many common garden plants that you may not suspect as the culprits in the demise of their neighbors are able to produce allelopathic compounds. These include sunflowers (Helianthus spp.) and goldenrod (Solidago spp.). Some of the most insidious invasive species may have allelopathic properties allowing them to out-compete the native plants. These include garlic mustard (Alliaria petiolata), tree of Heaven (Ailanthus altissima), and purple loosestrife (Lythrum salicaria).

Gardeners can manage allelopathy within their gardens by learning which plants may be allelopathic and which plants are susceptible to plant toxins. With this knowledge, you can design your garden accordingly.  Site gardens a good distance away from the allopathic tree and plant species. Do not plant sensitive plants directly under allelopathic trees or shrubs; move plants past the root zone (50 to 60 feet away for a mature black walnut). Raised beds lined with weed barrier fabric and filled with fresh topsoil may be used as an alternative. Where allelopathic species such as black walnut are established, removing the tree(s) is an option. Another tree or shrub can be planted in its place after several years to ensure that any remaining roots have decomposed. Juglone is also in the leaves and nut husks that drop to the ground. Rake leaves, hulls, and stems away from planting areas to avoid juglone exposure. Do not plan on using black walnut fruit and leaves as mulch.  Black walnut leaves can be composted. To be safe, compost black walnut leaves for at least six months or use on non-edible plants.

To learn more about using black walnut trees in the landscape, visit Penn State Extension: Working with the Black Walnut and Landscaping and Gardening Around Walnuts and Other Juglone Producing Plants.