|Triclopyr, in formulations called Garlon 3A, and Garlon 4, is a selective herbicide for broadleaf and woody plants and will not kill grasses contacted by the spray. It is absorbed by plants and is carried to the root systems. These herbicides have low soil activity, so do not pose a threat to groundwater if applied as directed. Triclopyr, the active ingredient in Garlon, degrades rapidly in soil. It poses a low toxic threat to mammals, birds, fish, and invertebrates. Itís also relatively safe for humans. Even so, you should be careful. Always wear gloves, and glasses or goggles and follow the directions on the label to the letter. The label says: "Danger! 3A can cause irreversible eye damage"
You can look online for the biological effects if Triclopyr. Garlon 3A is used by national parks, forestry departments, and golf courses all over the U.S. to control invasive species of plants, and is made by Dow AgroSciences.
This info was easy enough to find online to kill your sumac. However, finding a gallon of Garlon 3A or 4 is tough. It isn't sold at Home Depot or Lowes. You might find it at a farming supply store but I did find it online. There is a lot of good info about how to kill trees with or without chemicals on:
If you do just have sprouts, I would hand pull or dig them up. Reason? It is a lot cheaper. A 2.5 gallon jug of Garlon 3A is $221.00 plus shipping at:
--Garlon 4 costs even more at $282.00 per 2.5 gallon jug.
It is up to you if you feel you want to spend the money, from what I have read it is an excellent herbicide for very stubborn plants and trees. It ought to be for that price. Good luck.