I've never done that after a frost but I do make everything is well watered if a frost has been forecast. Well hydrated plants can stand up to the stress a lot better.

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Rosie z7a

We're supposedly getting our first frost tonight already! I had to go cover everything! I also read that placing milk jugs of water by the plants will heat up small spaces. The jug will give off heat as it turns the water from liquid to frozen. So I put a few of those in, too.

I've never read that about watering early in the morning to prevent frost damage. But I did hear of doing it before dusk on the night just before the freeze. I'd be interested to know if it really works! Please do let us know the results if you try it.