Some weeds have special dispersal mechanisms, which contributes to their success. Weeds and crops share many characteristics, which makes it likely that they would be found near each other. Weeds and crops both rely on human intervention for establishment and weeds thrive in disturbed habitats. Practices that tend to favor crops also tend to favor weeds, since they may be similarly adapted to the same habitat. Weeds impact the lives of humans by affecting the health, crops, and domesticated animals of the land.
Weeds reduce crop yield by competing for resources and reducing food, feed, and fiber quality.
The second mechanism has been termed allelopathy. Allelopathy is now defined as any negative plant response, mediated through chemicals produced by another plant. This broad definition includes chemicals produced by algae, fungi, actinomycetes, and other microbes that may also exist together in the rhizoplane. Allelopathy is separated from other forms of negative plant interference in that the detrimental effect is exerted through release of a chemical by the donor plant. There are numerous ways by which allelochemicals are introduced into the environment.