This gardening myth stems from the notion that sunlight is "magnified” by the water drop on the leaf to cause a leaf burn. If this notion were true, all gardeners would cover all their plants prior to every rainstorm and farmers would not be able to prevent widespread "leaf burn" after rain clouds gave way to sunshine. The root of this notion may have come from the effects of applying poor-quality water high in dissolved salts. As water drops evaporated from leaves, the salts left behind cause a leaf burn.
The bigger question here should be when is the best time to water plants and lawns to be “water wise." The best time of day to water a lawn is when solar radiation, temperature, and wind are minimal and humidity is high. This reduces the amount of water lost to evaporation and maximizes the water that penetrates the soil. Since prolonged leaf wetness may promote some diseases, late evening should not be the normal watering time. To facilitate the removal of dew and get maximum water efficiency, the best time to water is early in the morning.
