My guess they are in gallon pots? I had mine out in 18 - 19 degrees but not the wind chill that you describe. I had them covered with frost cloth and moved up by the house and they were fine. But that only lasted for a few days. The hardy zone is 5-9 or 6-9 depending on the type. I'm sure that is in the ground so in a pot they would be more tender.
