Is watering plants under a hot sun bad? A couple of people have told me that it's a bad for plants to be watered in the middle of a sunny day, i.e. it's not just that you waste more water due to evaporation, but that there are actually bad effects on the plants itself. I've heard this claim specifically for watering grass. The explanation I heard is that it causes the plants to open up pores or otherwise behave as if it were raining, but the fact that the sun is actually shining leads to burning or other damage. But there are certainly other possible explanations. **Can it cause harm to plants to water them under a hot sun?**

The argument as I know it goes somewhere along the lines of "water drops act like lenses that focus sunbeams so that they burn the plant". Several experiments have failed to reproduce this, though (link in German, sorry) - a drop of water, even a perfect half-sphere simply doesn't act as a lens. However, there are circumstances where it might happen, and that appears to be if the structure of the leaf surface is not flat but irregular or hairy. In that case, water drops might "hover" above the actual leaf surface, and sunbeams might be focused enough to cause damage.

This is not to be expected with grass, though.