Like so?
Thorny Defense (Ex): When a Tiny or larger creature moves within 5 feet of a firethorn, the plant strikes out with all its thorns. Treat this as a 5-foot-radius burst. Creatures within the area take 1d12 points of piercing damage. A successful DC X Reflex save halves the damage. The save DC is Constitution-based.
Twice per round?
Besides flavor text, what remains?