Practice Other Protective Methods
If you’re not keen on chemically treating your pots, or if it’s just not practical, the only solution to prevent frost damage is to wrap them up.
Use protective plant covers. The traditional method was to encase them in straw secured with hessian. Today we use horticultural fleece or polythene bubblewrap to enclose both pot and plant. It’s worth noting that there are various grades of horticultural fleece; a standard garden grade should protect down to 26.6 degrees Fahrenheit (-3 degrees Centigrade), you can find heavier grades that will protect to 21 degrees Fahrenheit (-6 degrees Centigrade).
The ultimate in frost protection, though, is one of the new zip-up frost covers, which completely envelop both the plant and container, protecting the roots, foliage and the pot. Made of breathable fleece, it should protect to 17.6 degrees Fahrenheit (-8 degrees Centigrade). It can be fully zipped and has a drawstring toggle to keep pots snug and frost free.