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!
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.
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.
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.