Hi Donna:
Garlon 4 and its generic equivalents will work on sumac (Rhus spp.). However, it is important to note that volatilization of ester (i.e., oil-based) herbicides is very common during warm weather.
Therefore, I would encourage you to wait until late fall or winter before applying this ester formulation herbicide to your sumac plants.
Tricloypr ester can be purchased at local farm stores and elevators.