The question comes up often. I have asked that question several times of people in the horticultural field and have always received a reply that essentially said, "I don't know". In summer, I tend to water during the hottest part of the day, and I water straight from the hose. Only a very few woody plants are in shade, the rest are in full sun all summer. My opinion is, it's beneficial because it cools the soil and increases evaporative cooling.

Watering with ice cubes is a growing craze that seems to have a number of people speaking out against it; yet, all I've heard is that "it shocks the roots". While I wouldn't use ice cubes to water (for a number of reasons), I think I would want to actually understand what, if anything, a sudden flood of cold water actually does to plants physiologically, and how does one determine 'how cold is too cold' and does it vary by plant and/or the current root/soil temp at the point in time when the plant is watered.

If you find anything you think is reliable, please post so we can evaluate and potentially share with others when the question arises.

Al