What Is Allelopathy?

Plants That Kill Other Plants

Black Walnut (Juglans nigra) leafy twig with fruits

David Hosking/Getty Images

Allelopathy comes from the Greek words allelo (one another or mutual) and pathy (suffering). It refers to a plant releasing chemicals that have some type of effect on another plant. These chemicals can be given off by different parts of the plant or can be released through natural decomposition.

Allelopathy is a survival mechanism, that allows certain plants to compete with and often destroy nearby plants, by inhibiting seed sprouting, root development or nutrient uptake. Other organisms, such as bacteria, viruses, and fungi, can also be allelopathic.

The term allelopathy is usually used when the effect is harmful, but it can apply to beneficial effects too. And even when the effect is harmful to plants, it can be a benefit otherwise. Think of how corn gluten meal is used as a natural herbicide, to prevent weed seeds from sprouting. Many turfgrasses and cover crops have allelopathic properties that improve their weed suppression. Or how about the way the fungus penicillin can kill bacteria. These are all seen as beneficial to humans.

You've probably heard of the problems experienced growing plants near black walnut trees. All parts of the walnut tree produce hydrojuglone, which is converted to an allelotoxin when it is exposed to oxygen. The roots, decomposing leaves, and twigs of walnut trees all release juglone into the surrounding soil, which inhibits the growth of many other plants, especially those in the Solanaceae family, like tomatoes, peppers, potatoes, and eggplants. Even trees and shrubs, like azaleas, pine trees, and apple trees, are susceptible to juglone. On the other hand, many plants are tolerant of juglone and show no ill effects at all.

How Do You Know If Allelopathy Is the Problem With Your Plant?

Unfortunately, there are no telltale symptoms of allelopathy, but you can often deduce it. For example, when your azalea dies, even though you think it has the ideal growing conditions, and you replace it with a new, seemingly healthy azalea that begins declining soon after planting, take a look at what is growing nearby. There may be no black walnut in sight, but there are other culprits. Different plants are affected by the allelotoxins of only certain plants. Kentucky bluegrass is allelopathic to azaleas, for example.

Another example many of us have experienced is how nothing seems to grow under the bird feeder that had sunflower seeds in it. All parts of sunflowers contain allelopathic toxins that inhibit seed germination and seedling growth. The impact is so apparent that they are being studied for their use in weed control.

Invasive Allelopaths

A disturbing side note to be aware of is the way invasive weeds can use allelopathy to choke out the competition. In many areas, garlic mustard's (Alliaria petiolata) rapid spread seems to point to an allelopathic ability. Other non-natives, such as purple loosestrife (Lythrum salicaria) and knapweed (Centaurea maculosa) also appear to gain an edge with allelopathic toxins.

What to Do About Allelopathic Plants

First, you need to be aware of what nearby plants could be allelopathic. There is a list below of some common landscape plants to watch for.

However, don't panic because you could possibly have warring plants in your yard. They can peacefully co-exist if they are kept at a distance. And the quality of your soil can be a factor in how long the toxins are held. The heavier the soil, the longer the toxins are trapped. Well-draining soil will move the toxins below the root zone of nearby plants.

Having healthy soil with plenty of beneficial organisms also seems to help. It's amazing how many amazing things fungi and bacteria can do for your soil. They can break down, dissipate or convert the toxins into something more benign. On the other hand, there are micro-organisms that aid in the allelopathic process. That's nature.

Bottom Line

Allelopathy is nothing new. The term is attributed to an Austrian professor, Hans Molisch, who coined it in his 1937 book "The Effect of Plants on Each Other". However, humans have been aware of it much longer. Records from the ancient Greeks and Romans talk about plants being toxic to one another. Pliny the Elder is often cited for noticing the detrimental effect of black walnuts and calling them toxic.

And allelopathy does not necessarily imply that something is out of whack. A certain amount of allelopathy goes on in natural systems. Allelopathy is thought to have a hand in how forests restore themselves. Although plants sometimes simply compete for the limited available resources of water, sunlight, and nutrients, without resorting to chemical warfare, ongoing research is studying whether the two conditions may have more to do with each other than formerly thought. Research also seems to show that the more a plant is stressed, either beforehand or because of the allelotoxin, the greater its reaction to the allelotoxins.

So it's a fascinating but confusing issue. There does not appear to be an exhaustive list of allelopathic plants, probably because there is still a lot of research to do. However, here are a few commonly noted allelopathic plants and their victims.

  • Asters and Goldenrod: Tulip poplar, red pine, sugar maple
  • Broccoli: Other cole crops
  • Forsythia: Black Cherry, Goldenrod, Kentucky bluegrass, Sugar Maple and Tulip Poplar
  • Junipers: Grasses
  • Perennial Rye: Apple trees, flowering dogwood, and forsythia
  • Sugar Maple: White Spruce and Yellow Birch