Weed biology is a cross-disciplinary science
Weeds are plants defined by their occupation of habitats that have been modified or created by humans for human purposes. They are by definition plants that cross boundaries created by us. Some are truly unwanted plants, others may have more-or-less useful roles in agricultural ecology, depending on context... and some may be useful as medicinal plants, fodder for stock, or as minor food plants.
The study of weeds is inherently a cross-disciplinary science that involves anthropology, ethnobotany, crop history, plant domestication, ecology and agriculture as well as all the practical research areas involved in managing weeds - from the engineering of harvesting machines to the design of herbicides.