Ed, Possibly both. Here in Western Mass overnight temps have rested in the 50's too, but 4AM temps have been low thirties. You don't have to have frost to croak your plants just sufficient chill and some desiccating breeze. My plants have sulked from the low morning temps, but have survived. Best until transplant time for me has been partial shade in the daytime, indoors overnight. And, of course, a weak feeding of frost defeating fish emulsion. Gareth