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If you are using the default battle system, make a new Terrain called Appearance Grass or somethin', then make that the Terrain for your grass tile. Under Terrain just boost the Enc Multiplier to 200 or something around there.
If you don't like that method, go to your map, right click the heirarchy, and select "Creat New Area." Box off your grassy regions and select which pokemon will appear there.