I have watched a few videos from the U of M. I know you guys like to cut down buckthorn and then treat with Triclopyr. I have a very large oak tree that has five 3"-4" buckthorns close to it's base. The buckthorns are approx 2'-5' from the base of the oak tree. Can I cut the buckthorn and treat with Triclopyr or Glyphosphate that close to the base of the oak tree?
My recommendation would be to use a high concentration of glyphosate (Roundup) - at least 25% - and paint it on freshly cut stems. It should be somewhat safer than triclopyr. It kills anything green or freshly cut, so it still must be used with caution. If you want an extra level of safety, wrap the trunk of the oak with plastic while you are painting the buckthorn stems. Read here:
A mature oak should not be harmed as long as the glyphosate does not contact any fresh wounds. Still, to be safe, be careful when painting the buckthorn cuts.
Read here about chemical control of buckthorn: