Depends a lot on what type of tree/bushes you have. After being mowed off, especially if they've been mowed off for several years, they can be very difficult to kill because of the advanced root system.
If you let them grow a year or two and then apply Remedy mixed with diesel fuel from the ground up to 18 or so inches, completely covering the stem, that will usually smoke the tree.
I use a 4 gallon backpack sprayer and mix 1 quart of Remedy to 3 quarts of diesel. I use just enough pressure to kinda dribble the mix along the stem as opposed to high pressure where over spray affects the surrounding area to much.
Surmount is another lethal herbicide, but is very expensive.