Mom always told me that spraying my plants with water before a hard freeze would save them - how would that work, exactly?

The idea is that the ice on the plants would insulate them from much lower temperatures. But I live in California, I have no idea if it works.

You can also simply cover them with a tarp

In Florida, when temperatures are expected to go below freezing, farmers will spray the strawberry fields, to coat them with ice. This keeps the temperature at a relatively safe 32 degrees Fahrenheit, instead of exposed to the much colder air.

I find it very unintuitive to think about this chemistry concept in terms of freezing, but far moreso to think about boiling.

As water heats to 100 degrees C, it boils. While it’s boiling, the temperature stays at 100 degrees C, because the energy is now being used to convert the water to steam rather than to increase its temperature. Once it’s fully converted to steam, it may continue heating up. Here’s a graph of water temperature over time that shows this – the temperature plateaus for a little while at 100 degrees C while it converts to steam: http://www.chemteam.info/Thermochem/Time-Temp-Graph-4.GIF

The same thing happens in reverse when water freezes, which means that the plants will stay at 0 degrees C for a while rather than rapidly falling to the temperature of the below-freezing air. Once frozen, the ice can drop below 0 degrees C, but the process of freezing delays this cooling.

This does work! Winemakers in cold places like Germany and northern France do this to protect the vines.

I would think you still need to protect the roots from cold weather. I would trust the tarp idea.

Do tarps work?