In that microclimate, not unusual. Kale and spinach are even hardier than mustard and I still have both in the garden. The kale is totally unprotected and doing very well. The spinach is under a screened box which we protected early in the winter with a quilt, but quit when the spinach came on so good in the greenhouse. It still looks fine too. I have had spinach survive 0 F in the open garden with a mulch of oak leaves. Picked a nice mess of it during March that year.

It is more or less normal. Some years mustard greens will make it through the entire winter, though you will see that as the number of daylight hours lengthen in late winter/early spring, they begin to bolt.

Normally, most varieties of mustard greens can tolerate temperatures down into the low- to mid-20s or even lower, as long as they have had time to become hardened off. So, a night-time low of 20 or 22 degrees might damage or kill them in November if it is the first time those particular plants have been exposed to a sub-freezing temperature, but it might not bother them nearly as much (or at all) if it happens in January or February after they've already experienced cold temps in the low-30s or mid- through upper-20s.

Cold-tolerance is different for each variety. Southern Curled and Red Giant usually can tolerate temps only down to the mid-20s, but Tatsoi can tolerate low temps down to roughly 10-15 degrees, and some of the varieties from Even Star Farm like Tender Tat, Even Star and New Star often tolerate cold temperatures down to roughly 6 to 12 degrees if they've been able to gradually harden-off to ever-increasingly cold temperatures over the course of the winter.

When the plants are on the south side of a structure, they sometimes can tolerate temps that go lower than they generally can withstand out in a more open area.

As cold-hardy as the mustard greens are, they generally are not the most cold-tolerant vegetable. The most cold-hardy vegetables include brussels sprouts, collard greens, spinach, parsley, kale and carrots (those that are already growing are cold hardy, but carrot seed itself doesn't germinate well in very cold soil).

Dawn