Answer: I understand your concern but the only way you will release toxic fumes is if you attempt to burn the poison sumac. Cutting into it will make the sap flow but as long as you don't breathe it in or get it on your skin, it should not cause a problem. Roundup? Poison Ivy Plus Tough Brush Killer will work against poison sumac, roots and all. Be sure to read and follow label directions. Best wishes with your project!
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