Unwelcome growth is a term used to describe the rapid and uncontrolled spread of something that is not wanted. In this case, it can refer to WEEDS, which are plants that grow in an area where they are not wanted. The crossword solutions for the term 'unwelcome growth' are WEEDS (5 letters) and WEED (4 letters). Both words refer to the same type of plant, but the former is plural while the latter is singular.