For very low temperature monitoring, down to -40 C, I would consider the kuomostat wireless sensor tags. These are neither zwave nor zigbee, but seem to do really well as single value sensors across a wide range of temperatures. You do have to also buy their receiver unit. There’s no direct integration with SmartThings, but a number of community members have created indirect integrations. They do have an IFTTT channel, for example. And an open API. Just nice little devices. (Although I understand customer service is pretty much nonexistent.)
You can check the forums for either kumo or wireless tag and you should find some integration discussions.