Frenzy Plant inflicts damage and then forces the user to recharge during the next turn. Unless this attack misses, the user will always have to recharge.
An elderly man, much like the woman on Cape Brink, appears on Route 228 in the Battle Zone, and will teach it to the three fully evolved Template:Type2 starters of the previous generations, as well as Torterra. He will also require that the Pokémon that is to learn the move has a high happiness rating.
Mew, though repeatedly said on the Pokédex to be able to learn any move, is not able to learn Frenzy Plant because it can only be taught to Grass-type starter Pokémon.