Among the most persistent of the numerous gardening myths is the one that insists you should never water plants, and especially not lawns, in full sun, especially on a hot day. Otherwise, the drops of water landing on the leaves will act like a magnifying glass and burn the foliage.
But you simply can’t burn a leaf with a drop of water.
Remember your childhood experiments with a magnifying glass. To burn a hole in a piece of paper (or roast a helpless ant: kids can be so cruel!), you’ll recall that you had to hold the lens a certain distance from the surface to be heated so as to concentrate the sun’s rays. Well, a drop of water sitting directly on the leaf is simply too close to do the job.
At any rate, drops of water falling on leaves in full sun evaporate too quickly to cause any damage whatsoever to the plant.
A Myth… with a Beneficial Side Effect
So, watering in full sun won’t burn foliage, but it’s still not the best time to water, so maybe the myth is useful… indirectly. That’s because when you water in full sun, especially on a hot day, the moisture tends to evaporate before penetrating very far into the soil. So watering at noon on a sunny day will be less efficient than watering in the early morning, at the end of the day or on a cloudy day: less water gets to the plants, plus you waste more water.
The ideal time for watering is early in the morning. Why? Well, that’s usually the coolest time of day and the soil too will also be cooler. This minimizes evaporation and essentially all the water applied will enter the soil where it will be readily available to plant roots. Also, leaves moistened in the morning dry out quickly, well before nightfall, reducing the risk of disease. (The spores of most foliar diseases germinate best on foliage that remains moist for long periods.)
Evening is the second best time to water because the sun is no longer beating down, so you’ll be reducing evaporation. Still, there’ll be more evaporation than in the morning simply because the soil and foliage are still quite hot. If you water in the evening, try to do so without getting the leaves wet (easy enough to do if you use a soaker hose), because foliage moistened in the evening tends to stay damp all night, increasing the risk of disease.
On the other hand, there’s no need to be more Catholic than the Pope: if your plants are seriously short of water and the only time you can water is in the heat of the day, go for it. It’s better to save your plants even if your watering efforts are somewhat less effective. Just water a bit longer to allow the water to penetrate well into the soil and thus reach the roots of your thirsty plants.