It's a myth that water droplets on plant leaves will focus the sun's rays and burn holes in the leaves. So you can water any time of day without harm to the plant from the sun. However, watering during the middle of the day is more wasteful than watering in the morning, because warm and windy conditions cause water to evaporate quickly. Water early enough in the day that the plant dries off before nightfall. If the foliage stays wet for a long time, diseases are more likely to develop.