Allelopathy

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.

Invasive plants

The phrase "one man's weed is another man's wild flower" is an accurate summation of the subjective nature of desireable 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 undesireable nature of the offending plant is attributed to its invasiveness. One nmodality that weeds, non-native plants and other plants knwn to be invasive is allelopathy.

Centaurea maculosa Lam. (European spotted knapweed, Asteraceae) is an invasive plant in North America.

References

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.