Your weed needs water to grow. However, while your weed will still grow when the game is minimized or off, none of the water will be used. So if you want to grow as much weed as you want without using much water, then your best bet is to plant it, then minimize or close the game window for a long enough time to let the weed grow. No matter how much or how little water you had put in, as long as at least a bottle was put into each pot, the weed will grow.