Wall of Thorns

When the druid casts this spell, a tangled wall of briar vines erupts from the ground, bristling with sharp, deadly thorns. Running into or attempting to force a way through the thorns will cause 8 hp damage, plus 1 hp for every point of armour class. A creature may be caught within the wall of thorns when the spell is cast, and if this is the case it will take damage immediately and must fight through (taking damage a second time) to gain enough freedom of motion to chop at the thorns. It takes four turns to cut through a 40 ft thickness of a wall of thorns.

Normal fire has no effect upon a wall of thorns, but magical fire can ignite the entire growth, creating a Wall of Fire (as per the spell) that burns away to nothing in two turns.