Whyizzit, after I've let down my guard on a new plant I'm babying through winter, a whimpy spring storm kills it?!
Many plants are most vulnerable to cold in late winter and early spring. What doesn't phase it during winter's coldest days can kill it in March.
Plants become gradually hardened off during fall and early winter. By midwinter they are unperturbed by cold, even sub-zero temperatures up to their species' hardiness limit. Once a plant begins to soften its tissues as days lengthen, however, it won't receive any cues or time to harden off if a late winter Arctic blast comes through.