As I was learning to garden, I repeatedly heard the “experts” telling us not to water in the middle of the day. The prevailing wisdom was that any water droplets on the foliate would act as little magnifying glasses, burning tender leaves. (Think of using a magnifying glass to start a campfire, and you get the idea.)
Then, we all learned that this was a gardening myth. Water droplets are too close to the plant tissue for sunlight to focus on the leaf and cause any damage.
And now I’m reading a study by Gabor Horvath at Eotvos University in Budapest, Hungary, and published in the journal New Phytologist, that contradicts the myth status. Apparently, he and his colleagues ran everything through a computer, then did some actual hands-on experiments. Their results show that, while smooth leaves indeed escape unharmed, fuzzy leaves are another matter. The hairs keep the droplets suspended above the leaves, allowing just enough of a gap that the sunlight actually burns them.
So, on one hand, smooth leaves won’t be burned by water droplets. On the other hand, hairy leaves can be burned. And on the third (?) hand, hairy leaves tend to shed water droplets, so that lessens the danger.
Watering min mid-day has other drawbacks, however. Much of the water evaporates before it can soak into the soil, wasting a limited resource. Also, on really hot days (like today), plants slow down their metabolism, reducing photosynthesis, so they can close their stomata and reduce water lost to evapotranspiration. They won’t be able to absorb as much water at midday as they can when temperatures are more moderate.
Of course, you can avoid all these issues by watering by soaker hose or using a drip system instead of an overhead sprinkler. There’s a reason why our municipal water utility prohibits the use of sprinklers between 10 a.m. and 6 p.m., but allows drip irrigation at any time.
At this time, the best advice is to water early in the day, before the sun gets hot. That will cut down on evaporation. (Watering in the evening can leave foliage damp overnight, encouraging fungal problems.) However, if a plant is obviously being stressed from lack of water, don’t wait until later. Water it right away!
One more note: some plants, particularly those with large, flat leaves (think squash) tend to wilt on hot days even if the soil around them is well-watered. They purposely let their leaves sag to reduce the surface area exposed to the drying sun, conserving water. As long as they perk up again by evening, they’re likely fine. If the wilting persists, however, check the soil under the mulch (you are using mulch, right?) to see if they need a drink.