It will work much better in late summer early fall, as the herbicide will be pulled down into the roots of the tree, vs. the spring when most of the herbicide will be taken up into the limbs and leaves. Roundup(concentrated) will work, but there are better herbicides out there for trees. Also, it doesn't matter how far you drill into the tree, the only part of it that is living, is the bark.(inner and outer layers) Once you get past that it's just dead wood. Taking an axe or hatchet and making a gash in the bark(with a downward swing, so as to form a small shallow pocket so to speak) will let more area exposed that will take up the herbicide, as opposed to drilling holes. It's also a lot easier and quicker to do it with the axe, than it is to drill holes in all of the trees