I am not sure, but I see no one has answered so I will give you my feeling on the subject. This is just my opinion for what it's worth.

There was a feeling that by watering in the afternoon, in the hot sun, you can damage plants because the water droplets magnify the suns rays and can damage the plant leaves. I recently read that this MAY be a myth. Afterall, mother nature sometimes gives us morning showers immediately followed by sunshine and the plants seem to be able to handle it.

For efficiency sakes, however, I read that when you water in the afternoon it's less efficient because some of the mist of the water is evaporating before it hits the plant. I am not sure how much of an issue that is, however.

At least for home lawns, and most plants, they say to water well and perhaps less often, rather then providing frequent light waterings.

Interested to hear other people's opinions on the "when to water" theory.

Good luck,
Glen