Residual herbicide solution remaining in agricultural spray tanks after application can, if not thoroughly cleaned out, cause injury to the next field sprayed. This is particularly true for the auxinic herbicides dicamba and 2,4‐D, for which rigorous tank‐cleaning procedures are mandated, including a triple rinse of the tank. A faster and more reliable procedure for deactivation of pesticides in the spray tank has been demonstrated at the scale of commercial agricultural sprayers. The process utilizes a well‐known waste treatment technology known as Fenton chemistry. Cleaning takes about 20 min—much faster than a triple rinse, although triple rinsing remains the approved process for dicamba. For dicamba and other spray mixtures, the deactivated spray solution can be sprayed directly onto sensitive species such as soybeans without injuring the plant. Co‐pack kits of hydrogen peroxide and iron sulfate solution sized for commercial sprayers in safe packaging have been developed that enable reliable deactivation of spray tank herbicide residues under real‐world working conditions for applicators.