There are a few poisons whose effects are increased because water causes the poison to spread.
@The_Doctor Think about it this way. You are poisoning the plant itself instead of the ground that it grows in. The plant can't really filter poisons if that happened.
Flying types are usually in the air, while Grass-types are usually in the ground.
About Rock's weakness to Grass: there was a reason for that, but I don't remember.