As posted, Glypho or round up works very good. I use a 3 % solution (1¼ gals of glypo to 40 gal water) in a 3pt sprayer on my compact tractor. I soak the brush and it defoliates it and kills it. Some buckthone is 12ft tall and it seems to go down easily. I killed small brush nucent trees also. I use this solution to go down fence row where the buckthone, boxelder and prickley ash is trying to invade my side of the fence. It's cheap and doesn't kill larger established trees. But, it kills all grass also. I do have grass that moves in during the season - so be carefull the next season to spray only the brush and not the grass. It takes a couple yrs, but the grass will take over and grow in.