The replies so far have all been focused on temperature. That is certainly an important consideration. However - I think soil type / drainage is at least equally important. My New Jersey garden has a heavy clay-ish soil. No idea why Fritillaria imperialis is happy and multiplying by offsets but let's just ignore it for the nonce. My Connecticut garden of happy memory had that mythical high organic, moist but well drained gardener's holy grail and everything grew wonderfully well. My opinion - cold wet soil will kill geophytes that would have survived equally cold but better drained soil. Judy in New Jersey where today is - again - overcast with rain, rain, rain (at least it doesn't need shoveling) and the first galanthus and eranthis are in bloom