Plant Allelopathy: An Alternative to Toxic Chemical Herbicides

Plant Allelopathy: An Alternative to Toxic Chemical Herbicides
External Guest Writer
April 27, 2021

Plant allelopathy is everywhere, yet very few people are aware of this fascinating phenomenon. On the one hand, allelopathy can lead to slower seed germination and plant growth - both undesirable factors in a gardener's or farmer's book. 

Alternatively, allelopathic plants are very effective weed suppressors, if used in the right way, even having been dubbed "Mother Nature’s own weed killer.”

In this article, we’ll discuss everything you need to know about allelopathy — from its benefits, dangers and all the way to its great potential.

What is Allelopathy?

Allelopathy is the 'chemical warfare' between two plants, with the goal being that one plant will suppress the other and thereby gain an advantage. In other words, it is the biological phenomenon of one plant inhibiting the growth of another plant.

In the phenomenon of plant allelopathy, allelopathic plants create adverse conditions to other neighboring plants by reducing their seed germination and seedling growth. The allelopathic plants are very effective in weed killing and are sometimes known as "nature’s weed killers".

Why Is It Important?

Plant allelopathy is incredibly important because it is effectively a natural alternative to harmful chemical herbicides and other environmentally damaging chemical compounds that are widespread in conventional agriculture.

Herbicides and other synthetic compounds can be toxic to the soil, they can encourage resistance to build up in the target plant which in turn will require even stronger and more toxic chemical compounds to kill, and can force the propagation of unknown genetic mutations in these target plants, as well.

Eco-friendly alternatives to harmful herbicides are vital to mitigate the possibly disastrous consequences of continuing to use such substances, and plant allelopathy is a natural solution to weed management that doesn't interfere with the natural order of things.

How Does Allelopathy Work?

Competition for resources is a very common phenomenon across all ecosystems. Different species as well different members of the same species will frequently compete for any number of resources including any combination of sunlight, nutrients, water, space, or prey. This competition is the basis of plant allelopathy. Allelopathic plants use chemical tools to win the competition for resources in a few different ways:

  1. Release allelopathic compounds from their roots into soil - Allelopathic plants release chemical compounds - known as allelochemicals - from their roots into the soil, and these compounds suppress or even kill neighboring plants when they are absorbed. These allelochemicals alter chlorophyll production in neighboring plants (an essential part of the plant cell where photosynthesis takes place), thereby slowing or stopping their ability to produce energy from sunlight, eventually leading to their death. 
  2. Release allelopathic gases via pores in leaves - Alternatively, allelochemicals can be released in gaseous forms from the small pores of allelopathic plants' leaves. Neighboring plants then absorb these gases unwittingly and as a consequence, are suppressed or killed.
  3. Allelopathic compounds decompose into soil via dropping leaves - When leaves drop from the allelopathic plants to the ground, they are subjected to decomposition; when the leaves decompose they release their noxious chemicals as a way to inhibit the growth of other neighboring plants.
a tractor operating on a field

What Are the Advantages of Allelopathic Plants?

Allelopathic plants can be strategically introduced into agroecosystems where they can serve a range of beneficial purposes:

  • As a natural herbicide - Allelochemicals can be extracted from plants and used to create natural herbicides or pesticides. Various allelochemicals classes including alkaloids, flavonoids, cyanogenic compounds, cinnamic acid derivatives, benzoxazines, and ethylene and some other seed germination stimulants can be used, and are readily phytotoxic to many unnecessary plants.
  • In companion cropping as a weed suppressant - An allelopathic plant that only suppresses certain other plants, but not all, can be used as a companion plant with another crop plant. The selectively allelopathic plant will suppress certain weeds, while leaving growth of the main crop undisturbed. 
  • To trigger "suicidal germination" - Some allelochemicals stimulate growth of certain weeds by mimicking the weeds' actual stimulant compound. Farmers can use allelopathic plants to trigger the germination of weed seeds dormant in the soil, so that they can grow out and be removed before planting of the main crop.
  • To control weeds with residues - Selectively toxic plant residues can be managed in a proper manner to control weeds efficiently. Using allelopathic crops in crop rotation, cover cropping with smother crops, using phytotoxic mulches etc. can be the examples of some good allelopathic residue management practices.

Harnessing the Allelopathy Potential

Rounding off on a word of caution: allelopathic plants can sometimes create persistent problems in the soil if not managed properly. For example, the residues of allelochemicals may exist in the soil for a long time after the plant is removed, resulting in soil sickness and reduced suitability of the soil for general plant growing. 

Allelopathic plants must be used carefully, but present great potential for natural weed suppression.

Read More: The Ecological Sustainability of Slash and Burn Agriculture