Grass moves are super-effective against Water, Ground, and Rock type Pokémon.
Grass moves are not very effective against Fire, Grass, Poison, Flying, Bug, Dragon, and Steel types.
Making them statistically weakest type, with the weakest combination being Grass/Psychic Pokémon.
Water, Electric, Grass, Ground-types are not very effective and have a disadvantage against Grass Pokémon.
Fire, Ice, Poison, Flying, and Bug types are super-effective and have a high advantage against Grass Pokémon.