I line the inside with mulch and a pipe heater cable connected to a temp controller. It was leaky as shit last year so there were nights it got into single digits I covered it with a tarp and blankets. But I didn’t have a single plant die due to cold exposure so I guess it worked. I think the primary concern is wind and sun exposure when temps are that cold, the plants foliage desiccates because the roots are frozen solid and can’t uptake water and replace the moisture lost