no hard set rule says you have to water in the am. But if you grow outside. It would be like watering your grass mid day. you stand a chance of the grass burning from the sun. If you water to late you stan a chance to get fungus, mold. Compare to grass if you water at night you could get mushrooms growing. At least where I'm at it can be a problem. Inside I water when ever in the veg room and in the am or when the lights come on in the flower room.