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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?

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I've broadened the scope of the question to include all plants, not just grass. – Fabian♦ May 5 '11 at 8:23
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I've heard that too. But, I've read (and through my own gardening experience) that the alternatives are worse since watering after sundown or in early in the morning lets the water sit on the leaves and ground and promotes the growth of harmful molds & fungus. – Booji Boy May 5 '11 at 13:06
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I want to confirm or debunk the idea that I shouldn't water the grass in the middle of the day. Ideally there'd be a study answering it definitively, but also just debunking all the known theories would do. – Ganesh Sittampalam May 5 '11 at 14:36
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@Garnesh: The "don't water in the middle of the day" thing is often pushed to save water rather than to protect the plants. The logic goes that a larger fraction of the water is lost to evaporation if applied during the hot, sunny part of the day than if applied during the cool of the morning or night; and consequently less total water has to be applied to maintain the health of the plants. – dmckee May 6 '11 at 21:42
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@dmckee: I appreciate that's one argument against doing it, but I deliberately excluded that in my question as I don't personally care about it. – Ganesh Sittampalam May 6 '11 at 21:44
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up vote 13 down vote accepted

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.

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I don't think this is a fully complete answer. You covered the explanation for one of the possible mechanisms of how the effect works; but what I (and possibly the OP) are interested is whether there is an observable (and reported in a study) effect in the first place. E.g did someone actually run a review/experiment on 2 sets of plants, watered at different times, and observed the effects. – DVK May 5 '11 at 13:53
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@DVK: I think we need to call the mythbusters ;) – Trufa May 5 '11 at 15:06
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@DVK: I know, I was hoping for more answers here myself. – Tim Pietzcker May 5 '11 at 15:13
I agree that a more general answer would be nice, but it's been a few days now so I'll accept this one at least for now. – Ganesh Sittampalam May 10 '11 at 7:32
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Let me pose another process by which the anecdotal "scorching" might occur when watering during sunlight hours that sounds plausible:

Let's say the water has a significant amount of some compound (e.g. salt or some other chloride) dissolved in it and some of the water sits on the leaf under the blazing sun so evaporates in place on the leaf it leaving behind a residue of undissolved salt. Just like adding salt to other moist things it dehydrates and ultimately kills the cells, leaving a burn mark (and thus weaker plant) behind.

Horticulturist Linda Chalker-Scott takes on the "don't water in the day" myth in an article published on her WSU page and explains that the salt problem is not due to the water being on the leaf, but by it being absorbed normally (i.e. via roots) and by becoming too concentrated inside the plant because it has a deficiency that hampers its ability to deal with the salt.

Apart from this, the article is a good debunker for the whole idea that watering during the day causes leaf scorch.

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applying pure logic: would plants have ever survived climates in which rain and sunshine follow each other quickly if they were so fragile they'd burn/scorch if they were exposed to sunlight when wet? – jwenting Dec 14 '11 at 8:17
In this case, how did so many cultures develop this myth? – vsz Nov 27 '12 at 16:26
@jwenting and vsz I think the difference between natural forces and hand watering is the nature of the water. For example, when my rainwater tanks empty in November (as they just have now) I start watering with mains water (pumped 100km from a river after filtering, etc) which almost certainly contains more hard metals and other compounds not typically present in rain water. This could explain the high salt content when growing plants artificially (i.e. gardening) vs allowing nature to take its course. – Lisa Nov 27 '12 at 23:25

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