Late frost/ freezes are a common occurrence in non-water protected areas in the Eastern, Central, and Southern USA. They vary from year to year in severity and damage. Most native trees suffer very little or no noticeable damage until temps get below 28F degrees and even then it depends upon how far along they are in their growing cycle. Duration is also a key factor. I have seen it go below 28F several nights in a row with highs in the 40's and 50's and there was no discernible damage due to the freeze being only 4 or 5 hours long each night. The bad years are when it is below freezing by say 10 pm and remains there until 8 or 9 am the next morning. So 28 degrees for 2-4 hours is no big deal but 28 degrees for 8-10 hours+ can do real damage.
My Mother always said that the purpose of a late freeze is to give the trees - fruits, nuts, mast crop - a needed break from over-fruiting and this is a normal part of nature's cycle. That being said... every 50 years or so we get a devastating late freeze like April, 2007 that actually kills down to the wood. I hate those types of years.