Sissy, we are in California, between planting zone 9b and 10. We discussed cold with the guy we got our filter from. He lives in San Jose and runs a pond construction and maintenance company. He said it wouldn't be a factor with our weather. He maintains multiple 2 foot deep exposed fiberglass breeders without insulation or heat and he said his koi do great through the winter. They are closer to the bay than us but our weather stay extremely fair through winter with only a couple frost days and no snow ever, except once in 1996 for a few minutes.