Crop or desired plant species co-occur with undesired species and thereby the co-occurring species become weeds. This human-imposed quality is based on the perception that there is an interaction that results in some negative effect of the weed on the crop or desired species. At odds with this perception is an evolutionary perspective that would expect co-occurring species to select traits to minimize the interaction in a shared and limited resource pool as well as selection for traits to outcompete co-occurring plants. So which traits are more likely to be selected remains a question: Those that minimize interaction or those that intensify the interaction through competition (Grime 2006). It is quite possible that some co-occurring species may not be interacting in competition for resources, or competition may be minimal or indirect so that control of the undesired species is not required. Clearly, it becomes important to gain an understanding of the potential interaction among desired and undesired species and use that knowledge to instruct a management decision. All too often the decision to manage is reduced to selecting the best tool (usually herbicide) to get rid of the weed driven by the precautionary principle. The precautionary principle applied here is that when in doubt about the interaction between the weed and the crop one should control the weed. Unfortunately, controlling weeds can be expensive and can have environmental side effects. So there are trade-offs in the decision to control weeds and thus knowledge about the interaction between desired and undesired species should be gained to make the best decision about weed management (Swanton et al. 2015; Maxwell and Luschei 2004).
Maxwell, Bruce. "Weed-plant interactions." Integrated Weed Managment (Book) (December 2017): 1-15. DOI: 10.19103/AS.2017.0025.02.