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Tangrowth can learn Leaf Storm through Shiftry as well. Shiftry learns it at level 49 and can then breed it onto Tangela.
A way to get Tangrowth at a low level
It's possible to get a Tangrowth at level 2, right? All you need to do is hatch a Tangela, teach it Ancient Power, and feed it a Rare Candy or level it up and you have a level 2 Tangrowth. Should this be listed in the Trivia section or something?