Teach it Ancient Power and level it up to level 33.
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It can evolve as early as Level 33 in Pokemon Diamond, Pearl, Platinum, HeartGold and SoulSilver but it needs to know the move AncientPower.
Yes; Tangela evolves into Tangrowth by leveling up Tangela when it knows Ancient Power.
Tangela does not evolve in Fire Red.
Tangela > level up while knowing Ancientpower > Tangrowth
tangela evolves when it learns the move ancient power then you get tangrowth... :D