Unless i have drilled into the base of the tree and used full strength gly, I have had no luck.
I have used diesel & Garlon4 at 3:1 diesel heavy on buckthorn with great success. I put it if a cleaned out dish soap bottle and run a ring about 4-6" above the root collar of the tree.
be careful, the garlon4 will get into the soil and can attack other freindly trees & shrubs.