Weed spectra vary widely by crop and growing region. While some weeds flourish over wide geographic areas, local environmental conditions and production practices exert big impacts on weed populations, weed species, and degrees of difficulty of control. For example, a survey of corn from the Southern Weed Science Society’s 12 states included 47 weeds. The list was narrowed to 10. Listed below are some of the most troublesome weeds most commonly cited by university weed-control specialists. They are listed in no particular order.