Good afternoon, all. I have a meadow-type hillside in my front yard that is densely planted. In that area a number of saplings have been growing over the years, mostly sweetgum and willow oaks. I have been cutting them down, but they regenerate very quickly. It's not an easy area to access without trampling plants in the summer, so I've cut them down spring and fall. Spring I tried cutting them down and spraying the little stumps (0.25 cm - 2 cm diameter) with a bit of RoundUp. When they re-grew and leafed this spring I tried RoundUp on the foliage, but sparingly because it's hard not to spray all the neighboring plants.

None of this has worked, and the saplings are growing fantastically. What's the best herbicide application method that you would recommend, and what timing in the year, to kill these saplings but minimize collateral damage? Thanks!