What kind of over the counter herbicide should I used for hack and squirt? Target species is sweet gums and oaks.

Glyphosate will work fine if you mix it strong enough. Other herbicides may work faster. I killed a big sweetgum beside my house using 50/50 gly and water. It took it about 2 years to fully die but I was trying to kill the gum without harming other trees close by.

Pathfinder 2 and Tordon RTU are a few others that'll do

Garlon 4 if you can get it.

Thanks guys. How long does it typically take before the leaves start dropping? Am I looking at a year or a few months?

It’ll take a couple months for the leaves to turn. It’ll take 1-2 years to die and fall. I use pure glyphosate concentrate. Don’t skimp on the number of hacks. Don’t do it during drought.

CNC is pretty much spot on from my experience. Next year it will put leaves on but it will have some dead branches and will look really stressed. In year two it will be really struggling if not fully dead. Mileage may vary depending upon the size of the trees being killed. Mine was a mature sweetgum over 30 feet tall and it took over 2 years to fully die.

It's more expensive but Triclopyr (Garlon 3a) will do a better job on hardwoods than Glyphosate from my experience. I used a 50% solution on some sweetgums last summer and it took forever for them to start browning up and they still have some green leaves trying to come back this year.