UPNetwork -> Pokédex -> Generation 3 -> Abilities

Overgrow - Grass moves become stronger when it has low HP.

001 - Treecko - Grass

002 - Grovyle - Grass

003 - Sceptile - Grass

307 - Chikorita - Grass

308 - Bayleef - Grass

309 - Meganium - Grass