Our weather is very similar. I've never covered collards and don't mulch them. They were fine at the 19f we had a short while back. They are really hardy. They are the best tasting and cook faster after a frost.Was 7b, but I think it's changed to 8a now. Dec 2022 we got down to 7°F one night, & the ones that were cut off at the soil survived & came back in the spring. But those temps aren't normal here & it didn't stay that cold very long. I use heavy leaf mulch & frost covers & we've gotten down to 19°F so far this growing season.