Carrie no, 31 briefly overnight shouldn't be a concern - you could sheet but I don't think necessary.

My own had foliage buds opening about three weeks ago when we got down into rare-for-March mid 20's overnight, three nights in a row. I did cover with sheets, but two nights since then it's reached 29 and I've left them uncovered, they haven't skipped a beat.

I got three nikko's bare root and potted them up. One barely leafed out before dropping them. The two others were leafing out just fine so silly me sat them outside for more sun. I guess it got colder/windier than I expected - something like 35 F and those hydrangeas really didn't take it well. They suffered frost damage and those nice leaves wilted and fell over. (It got a tray of peas that were doing good, also). My question- will these sorry excuses for hydras sprout out new life? Each of them is simply one stem in a pot, and I am not seeing swelled buds or anything. Should I take the clippers and cut off a bit to encourage growth? Thanks.

tazebell, just let them be for now. Plants don't need you to cut off parts at this point to encourage growth - they have already gotten the message that they need to do so thanks to Mother Nature's nip. Cutting now risks the chance that you cut too much. Let them sprout new buds and then you can prune off any dead looking tips afterwards.