In Diamond, Pearl and Platinum, an elderly man, much like the woman of Cape Brink, appears on Route 228 at the Battle Zone, and will teach it to the three fully evolved Grass-type 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.
In Black and White the Move Tutor that can teach Frenzy Plant appears in a house on Route 13, and will teach it to the four fully evolved Grass-type starters of the previous generations, as well as Serperior. Like in previous generations, this move tutor will require that the Pokémon he teaches the move to has a high happiness rating.