"People think water droplets can magnify the sunshine and burn plants," says TOH landscape contractor Roger Cook. "It's impossible!" If you come home on a hot day and see drooping plants or a lawn turning blue-gray, then get out the hose. A quick watering right away will help, not hurt, the plants.
The reason you shouldn't water when the sun is strong is that the water evaporates too quickly. "It wastes water," says Roger. So follow any quick emergency watering in the daytime with a longer, deeper irrigation the next morning. Don't do it in the late evening. The water won't evaporate enough overnight, putting plants at risk of developing fungal disease.