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I've always heard not to water your lawn during the day and that the fine water droplets will evaporate before it is absorbed by the lawn (either in the air or while sitting above ground). But how bad is it? Is there a direct correlation to the temperature outside?

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The drier and hotter it is, the faster water will evaporate. However, the water droplets on the grass will not soak into the ground anyhow. They will stay on the grass and evaporate (unless it starts raining). Heat just makes this happen faster. You need to make sure water soaks the ground properly when watering, no matter the temperature. So in other words: It won't make a difference. (+1 for asking "how much" instead of "should you water during the night" or similar nonsense questions. I don't know how much, so this is not an answer, but a comment). –  Lennart Regebro May 29 '11 at 9:11
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2 Answers

The answer to the question is do not water your lawn during the heat of midday. Watering at night will eventually lead to Fungus on your lawn. The best time to water your lawn is in the early morning between 4 and 8 am. It has time to soak in to the soil and do the lawn good. Watering in midday is wasting water. It evaporates quickly. Twice a week should be enough. Make sure that you can put a half inch of water on your lawn each time you water. Put out a rain meter to measure amount of water.

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I can answer the evaporation portion of the question. The most important factors to consider are:

1) the relative humidity of the air about your lawn

2) and the temperature of the water itself.

All the other factors will be constant for your lawn or are not correlated with day or night in general. (For instance, wind will hasten evaporation, but it can be windy in the day or at night in most locations.)

At night, air temperature is lower than during the day (usually!), which means the temperature of the water is likely to be lower, which means evaporation is slower. Additionally, lower air temperature means lower water vapor pressure in the air, which means higher relative humidity, which means slower evaporation. So all things considered, water on a lawn evaporates more slowly at night than during the day. (And that matches common sense. Whew!)

The gardening question is will the slower evaporation of watering during the night make watering at night better for your lawn relative to the amount of water used. I don't know for sure, but I believe that is correct with one exception. Too much water in the soil can cause plants to be unable to absorb nutrients and can cause root rot. If you flood your lawn at night, it might cause problems. But if you water at night, you should be able to use less water to get the same results than if you water during the day.

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I would add a 3rd important factor - the way you apply the water. A fine spray will have an enormously higher rate of evaporation on a hot, windy and dry day than say a leaker hose. If sprayed into the air, the smaller the water droplets are, the less the water temperature matters, and the more the air temperature matters (the rate the water warms [or cools] is related to the ratio of the surface area of the droplets to their mass, as well as the temperature difference between the water and the air) –  Highly Irregular Dec 2 '11 at 9:13
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