Allelopathy refers to a plant releasing a chemical that inhibits the growth and development of a nearby plant. These chemicals are referred to as allelochemicals.
The most common example of an allelpathy plant is the black walnut. Dozens of plants are sensitive to the allelochemical juglone, which is produced by all parts of the black walnut tree.
Garlic mustard, an invasive weed, is also exhibiting strong allelopathic tendencies.
The most common example of allelopathy is the black walnut tree and its negative effect on tomatoes growing in its vicinity.