The term POISON PLANT can be summarized by two crossword solutions: SUMAC (5 letters) and IVY (3 letters). SUMAC is a genus of flowering plants in the family Anacardiaceae, which includes poison IVY, poison oak, and poison SUMAC. IVY is a type of climbing or ground-creeping evergreen woody plant that is found in many parts of the world. Both of these plants are considered to be poisonous, as they contain an oil called urushiol which can cause skin irritation and rashes.