HORTICULTURAL TERMS
by Cheryl Carmichael, Master Gardener

Allelopathy: noun: a-lee-low-pathy. the ability of one plant to produce and release chemicals that inhibit the growth of other plants either by inhibiting germination or growth; a form of chemical competition.
Allelopathic: a plant that produces toxic substances to restrict the growth of other plants.

Competition for space, water, nutrients and sunlight is the basis for allelopathy. Some plants use their chemical tools to win the competition to gain an advantage for available resources. Some work through their root system releasing chemicals that suppress or kill plants, often through interfering with another plants' chlorophyll production, causing withering and death. Some allelopathic chemicals are released as a gas from their leaves. The gas is absorbed by the leaves of other plants causing suppression of plants' vigor or death. When leaves fall from allelopathic plants they release their noxious chemicals as a way to inhibit seed germination or plant health. While most allelochemicals are stored in the leaves, allelopathic properties can be found in roots, bark, flowers, fruits, seeds, pollen and foliage.

Allelopathic plant pairings can operate as herbicides and pesticides in agriculture. There are numerous crop pairings that can provide weed and bug suppression in order to reduce herbicide and fertilizer use. For those of you who plant vegetable and fruit crops, a new, well-regarded book is available outlining pairings that can be useful to you, with good explanations for choices.
Walliser, Jessica: Plant Partners: Science Based Companion Planting for the Vegetable Garden.

Sources
Reza, Shamim (2016). “Plant Allelopathy”. Permaculture Research Institute.
Walliser, Jessica (2020). Plant Partners: Science Based Companion Planting for the Vegetable Garden.