Well, I think how cold depends on the size of the container and the material of the container. They seem to be able to take temps down to about 24-25F overnight quite readily even in the smaller containers. My day temps tend to rise above freezing during the daylight hours and I probably wouldn't trust leaving a small container out if the duration of the freeze were longer than overnight. Larger pots and pots that are naturally thicker take more cold than a 6" plastic pot, e.g., but if it were going to be a deep freeze for several days I wouldn't chance it personally. Since I don't risk it, I'm not sure just how cold they might handle and still return. This winter has been pretty mild, so I have some stacked up against a south facing rock wall that I haven't brought inside at all. My weather here in Texas isn't going to be comparable to yours in Michigan, but sometimes my location is subject to a cold spell down into the teens and sometimes single digits and staying below freezing for several days. Most things get moved into the garage for those spells.
