Sound like Dewberry. Grub it out and be prepared for a longterm battle. If you use such things an herbicide might help. Good luck!
I have a field full of hardhack, dewberries, blackberries, alders and various young trees where eventually I'd like a garden waiting for me in central Maine. I've been fairly good about getting it mowed once or twice yearly - but that's it so far. Makes me tired just to think of it.