The chard, parsley, and celery I have NOT covered during cold weather looks the SAME as the covered ones

The covered (with plastic, loosely) and uncovered sections were right next to each other.  Everything looks fine, and, everything looks the same.  Some nights went down to -4, -5.  So, it would appear there's no need to cover chard, parsley, and celery at those temperatures, provided the plants are healthy to start with.