Water in the morning. On hot summer days, watering wilted plants and dry grass in the afternoon does nothing because the hot, dry sun quickly evaporates the water. The best time of day to water is early in the morning when the outside temperature is the coolest. This gives the plants a good supply of water to face the heat of the day, and there is less evaporation. If you can't water in the morning, wait till after sunset to turn the sprinklers on.