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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asked Jan 1, 2011 at 20:06
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$\begingroup$ -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. $\endgroup$– user346Commented Jan 1, 2011 at 20:28
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$\begingroup$ This really belongs on popular natural science I think... shame the site doesn't exist yet, or I'd migrate it there. $\endgroup$– NoldorinCommented Jan 2, 2011 at 20:34
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3 Answers
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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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2$\begingroup$ As someone who often had to get up at three in the morning to spray water all over the apple trees as the temperature dropped, I can attest to the effectiveness of this technique. $\endgroup$ Commented Apr 7, 2015 at 17:40
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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.
answered Jan 1, 2011 at 20:44
dmckee --- ex-moderator kittendmckee --- ex-moderator kitten
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It's actually got a lot more to do with chemistry than physics: it takes energy to break chemical bonds, and energy is released when said bonds are formed. Essentially, the most important factor here is the intermolecular forces at work between water molecules: hydrogen bonding. The hydrogen bonding that causes water to solidify releases energy into its surroundings as they are formed. So, as water begins to freeze, hydrogen bonds are being formed which releases energy to the surrounding area and actually warms the environment. So, spraying your plants with water before a cold night will prevent frosting, and the subsequent death of said plants, because the water will heat up your plants as the temperature drops.
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$\begingroup$ Do you know whether/where this has actually been studied? Not whether it works, but precisely how it works, and to what extent the different possible mechanisms actually contribute to preventing the plants from freezing? I can imagine many reasons (more water quantity means more time to freeze; increased heat energy required to form bonds at the freezing point; preventing evaporation from the plant; sprayed water may be warmer than the air/plant; etc.) but quantifying them is another matter. $\endgroup$ Commented Dec 15, 2021 at 20:07