Allelopathy is the secretion by plants of a toxic substance into the soil in order to prevent other plants from growing near them. This trait is most commonly exhibited in desert plants, where water is scarce and competition from other plants for it is a matter of survival.
Establishing allelopathy in plants
When a plant suspected of being allelopathic is tested, one method is to take portions of the plant as an extract or slurry and to use various concentrations of the material to see if, when added to soil, it will inhibit the growth of plants or seeds of other plant species started in that soil. Since organic debris such as stems and leaves are known to leach nitrogen out of soil, and decreased nitrogen itself will reduce plant growth, nitrogen fertilizers are often added to the soil in order to counteract that general effect.
Phenolic content of plants
The phrase "one man's weed is another man's wild flower" is an accurate summation of the subjective nature of desireabilty of plants. However, when agricultural crops are grown and yields are reduced by overgrowth of "volunteer" plants, or non-native plants crowd out endangered species in their last remaining native habitat, the undesirable nature of the offending plant is not simply subjective. Since gross changes in vegetation in natural habitats can also affect animals and soil, invasiveness in plants is a concern in ecology. Allelopathy is one attribute that adds to a plant species ability to be invasive, allowing it to establish colonies in areas of land that are already inhabited by other growing plants.
Examples of allelopathy in invasive plants
Verbesina encelioides (A. Gray, Asteraceae) is a perennial weed common in uncultivated fields in semi-arid India that often invades crops in cultivated lands.
Centaurea maculosa Lam. (European spotted knapweed, Asteraceae) is an invasive plant in North America.
Allelopathy in ornamental horticulture and garden design
Ridenour, W.M., and Callaway, R.M. 2001. The relative importance of allelopathy in interference: the effects of an invasive weed on a native bunchgrass. Oecologia, 126: 444-450. doi: 10. 1007/s004420000533.
Allelopathic potential of Verbesina encelioides root leachate in soil Inderjit, Chikako Asakawa, K M M Dakshini. Canadian Journal of Botany. Ottawa: Oct 1999. Vol. 77, Iss. 10; p. 1419 (6 pages)