I agree that chances are it is not nematodes in this case, Elaine, but (unfortunately!) there are plant-parasitic nematodes that can survive freezing and live through the winter even here in Canada. They are able to kind of "freeze dry" themselves to survive the cold. It depends on the species whether they can do that, though. One example that does survive and cause plant damage in Canada is the root lesion nematode {Pratylenchus penetrans).