The effects of aqueous root extracts from five species of the family Papaveraceae on the growth of cyanobacteria, algae, and other non-target aquatic organisms were investigated to evaluate their potential use as algicides or cyanocides in the aquatic environment. Dicranostigma lactucoides and Sanguinaria canadensis featured the highest toxicity while Macleaya microcarpa was found to be the least toxic to all aquatic organisms tested. The Chelidonium majus extract had the best properties as a potential algicide or cyanocide because of its significant toxicity to phytoplankton and lower toxicity to non-target aquatic organisms as compared with the other Papaveraceae family members.