Dan Saufferer lives in Minnesota and is helping to eliminate Buckthorn:
http://www.swnewsmedia.com/ede...
It seems that transporting for the purpose of eliminating plants that you are destroying is not the same as transporting a potted Buckthorn plant home from the store; how does the law read? Is it only live plant material that cannot be transported?
I did some quick research:
There is an effort to eradicate Buckthorn at Birch Island Woods. Maybe you could contact the folks there to get advice. They are near enough to your location that you might consider joining one of the volunteer days to help cut and pull the free material for your projects.
This newsletter (from 2007) contains a link for more information:
http://eminnetonka.com/documen...
www.eminnetonka.com
A more current newsletter here:
http://eminnetonka.com/documen...
One article mentions using the cut Buckthorn - volunteers are permitted to take home the cut wood:
"...one advantage for those who’d like to craft something out of wood. Many re-sprouted stems have grown very straight..."
Hope this helps.