[[1]] are the common name for undesired plants that might naturally develop in a field. 

In a field, many other undesirable plants may grow naturally along with the crop. Weeds are the common name for these unwelcome plants.

They are harmful to the crops as they compete for nutrients with them.

Weeding is usually done to keep unwanted plants from competing for resources like sunlight, water, and nutrients with desirable plants. Weeding can be done manually with hoes and cultivators or chemically with weedicides.