Notes: How do you know if you have a herbicide resistance problem? Herbicide resistance should only be suspected when 1) other causes of herbicide failure have been ruled out; 2) the same herbicide or herbicides with the same mode of action have been used year after year; 3) one weed that is normally controlled is not controlled while other weeds are; 4) healthy weeds are mixed with controlled weeds (same species); and 5) a patch of uncontrolled weeds is spreading. Herbicide resistance should be confirmed by collecting seed from a suspect plant and subjecting the plants grown from this seed to various rates of the suspect herbicide(s) by your local extension or research specialist.