For woody plants the thing that works best is making downward chops or cuts through the bark with a hatchet and pouring in some full strength Round-Up or Brush Killer. This puts the killer in the Cambium Layer which is just under the bark and is where nutrition, etc moves down to the roots to kill the plant. The interior wood doesn't transfer it. There are pesky trees here such as Quaking Aspen and Poplar that sucker badly if you don't kill them this way before removing. If you just cut them down the roots put up countless new plants.
