Why's that? If you kill them before they explode they don't come back, and most 13th level characters should be able to finish this off within 10 rounds (never known a fight to even go that long).That's at least CR 16.
Cuz it's infinite. Also, until they find a fire-based creature they only reproduce 1 of themselves. So basically all they are doing is moving along as the same plant.Edit: Actually, why haven't these over-run the plane of fire yet?
Say nothing of it! I am the Tribble We Do Not Speak Of!also, your avatar, is that from 'the village'?