Allelopathy is a form of chemical competition among plant species. In other words, some plants inhibit the growth of another plant by secreting chemicals from leaves, stems, flowers, roots, or fruits into the soil. These chemicals either kill off existing plants by inhibiting root or shoot growth or cause them to be unable to germinate and become established. To make matters slightly confusing, not all species of plants are inhibited, some are actually tolerant of the chemicals but it is species-dependent.
So, if you’ve ever tried to grow groundcovers or other plants under or close to a certain tree or shrub and wondered why nothing would ever grow in that spot, it could be due to allelopathy. On the other hand, it could be due to one of several other factors say, shallow root systems or not enough light or poor nutrients. Although allelopathy is sometimes hard to prove through scientific measures, that doesn’t mean it doesn’t exist.
Allelopathic plants are found everywhere in the world, and in fact, are often used in crop agriculture for weed control. Plants native to the US like the black walnut (pictured above), box elder, sugar maple, black locust, or wax myrtle are allelopathic as are many exotic-- even noxious--weeds like garlic mustard, the smooth brome that’s invaded the mid-west, or lantana, a vine that’s often found growing in commercial citrus tree orchards in Florida. Even broccoli can be allelopathic, prohibiting growth of other cruciferous vegetables planted after it is harvested.
Photo source: www.donnan.com