He's going to fight a plant. All right, that's not that weird; Thor had studied the flora colossi when he'd learned more of their language than the Allspeak could translate out of sheer curiosity. Granted, the Groots tend to be a bit more... mobile. Have legs, he means.

This plant... it's in a pot.

But it seems like it can hop all right, so sure. Why not.

Thor approaches, unarmed, looking bemusedly down at the strange creature. "Then let us begin. Take your best shot," he challenges.