Bottles are cheap.
Harper’s mix is 50/40/10 water/garlon3/imazapyr and works very well
Be careful using the imazapyr if you have oaks in the area you are trying to save. I use 60% garlon and 40 %water if oaks are in the area that I don’t intend on killing. Depending on tree size you can also try basil bark treatment with diesel and garlon.
As long as you don't have any major runoff it's not likely you'll have any issues with non-target trees. Root grafting between species is very rare.
Only way I'd let a sweetgum live is it was saw log size and I knew a timber harvest was upcoming in the next year or so. They produce too many seed to leave untreated.
