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Deals damage. If one of the user's allies chose to use Fire Pledge or Water Pledge this turn and has not moved yet, they take their turn immediately after the user and the user's move does nothing. Power triples if this move is used by an ally that way, and a sea of fire appears on the target's side if the first move was Fire Pledge which causes non-Fire-type opponents take 1/8 of their max HP as damage at the end of each turn, or a swamp appears on the opponent's side if the first move was Water Pledge which causes the Speed of all opponents to be reduced by 1/2. These special field effects last four turns. Does not consume Grass Gem when used.
Grass Pledge is fairly useless in singles play, as the Pokemon that have access to it have superior options such as Giga Drain or Energy Ball. However, in doubles, it can be used together with Fire Pledge to stack up residual damage on your opponent's side of the field, assisting in wearing down the opponent. When used together with Water Pledge, Grass Pledge can also be used to slow down the opponent; this is helpful to assist slower Pokemon on teams that would rather not run Trick Room.
Giga Drain is a more powerful alternative that also heals the user.
Energy Ball is a more powerful alternative that also has a chance to lower the opponents Special Defense by one stage.
Leaf Storm is a much more powerful alternative that also cuts the user's Special Attack after use.