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I've always heard that watering plants if the temperature goes a few degrees below freezing will help prevent them from freezing, but I've never quite understood the physics behind it. Can you guys help me to understand this a bit better? Thanks!

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-1 Your questions seem to have very little to do with physics. "Are all physicists like Sheldon Cooper" "How does watering your plants ..." "How does a drip in a pipe ..." What's next? "What is the physics of furballs", "Why do gremlins turn mean when wet"? These last two are obviously facetious examples, but you get the drift. – user346 Jan 1 '11 at 20:28
This really belongs on popular natural science I think... shame the site doesn't exist yet, or I'd migrate it there. – Noldorin Jan 2 '11 at 20:34

2 Answers

up vote 3 down vote accepted

If the temperature is not much below freezing, the rate of heat transfer from your plants (and particularly from the earth around their roots) is low, if there is a lot of water present, the high heat of fusion means that it will take a long time to actually freeze much of it. So maybe the plant makes it through the night without too much damage.

Note that if it doesn't warm up enough the next day the second night will kill them because it starts close to freezing.

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While this may be true, a better option is actually to spray your plants with water. When water freezes, it releases heat (a little counterintuitive, I know, but that's why you have to put water in a cold place to freeze it - you have to take away heat). So, if your plants have a thin layer of water on them that freezes, it actually helps keep them warmer. Additionally, the layer of ice, being a good insulator, will then help keep the plant warmer through the cold spell.

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