When you clear land of buckthorn, a very dense wood, you are inevitably left with deep strong roots. After cutting off the tree, I drill a half inch hole a few inches down into the trunk. I then pour a little straight Roundup into the hole several times over a weeks time. The root drinks the roundup. When the root is dead, I pound in a metal rod for wind protection, then mound charcoal around it, douse with charcoal starter and light. Then I slip over a tube of furnace sheet metal with a couple holes in the bottom for draft. The root burns to below ground level and nature does the rest.
Buckthorn is so invasive and hard to get rid off, and it grows so fast. It pops up every where in the yard. Some of them along the fence, next to a tree that I can’t dig up the roots easily. These I have to cut the new growth twice a year to keep it under control. If I missed one, next time I see it, it will have good size branches. A lot of work in the spring time.
I’ve killed thousands (and thousands) of buckthorn seedlings/bushes/small trees. The only way that I could have effectively done so was to spray the bottom 2’ of trunk or so with a mix of 11 oz. Crossbow in 2 gallons of diesel. I apply the mix with a garden sprayer. I usually get 2-3 years out of a sprayer before the diesel destroys the seals.
I have spent well in excess of 100 hours actively spraying. It is a fight that I will ultimately lose because nobody else in the area attempts to control the stuff.
Drilling a hole and filling it with salt works impressively well and no repeat treatment necessary, but if you want stump removal too then putting a metal rod, chimney and diesel down the salt hole and burning it sounds awesome. The salt hole fills with water generally.
