I guess I have to quote this from Madoka Magica..
Sayaka: You dont plan to use those grenades when Im fighting up front, do you? Im a melee fighter in case you didnt know. And I dont like the idea of a grenade exploding in my face.
Homura: Uh, okay. But that's the only weapon I have TTnTT.
Mami: Then why not steal a gun? You have that time magic of yours, it should be no problem for you to steal one.
Summary:
Grass would not like the possibility of being hit.