All north american CP species have some type of dormancy requirement.
Some plants die back to a resting bud, while others just stop putting
up much new growth. Many are difficult to winter, among them the
filiform sundews, D. rotundifolia, and D. intermedia. I lose some
each winter, but the survivors do well. The problem is rot. If
the plants got cold enough I probably wouldn't have rot problems,
but where I'm at we have cool temps and high humidity, so the plants
get mold sometimes. This is only a problem on the bud-forming
drosera. Flytraps stop growing and will often die back until you
think they are dead. I don't have the temps available, but any
US plant which forms a bud should be able to survive freezing temps,
particularly if mulched so that the plant itself doesn't freeze.
It's ok if the roots on bud-forming plants freeze as new roots
should grow in the spring. Flytraps and sarrs die back, but do
not form tight winter buds, so if their roots freeze the plant
has a greater chance of dieing.
Several people on this list have reported US species surviving
hard freezes and many inches of snow, providing the plants have
12" or so of dry mulch on them to keep them from taking the
worst of the freeze. You typically lose a few plants this way,
so the key is to enough extra plants that the evolutionary process
kills off the weak ones and the strong ones live to reproduce
in the spring :-).
Seedlings are a different story, and probably need to be protected
until they are reasonably mature. Mature plants tend to survive
temp extremes better, so if you make a mistake you don't automatically
lose your plants.
My US species all live outdoors in the winter in large potted bog
gardens. During the winder the pots fill up and cover everything
with about 1/4" of water. Despite this all of the large plants
and most of the small sundews come back each spring. The only
exception is D. filiformis types, which I do not do well with
at all. They seem VERY susceptible to having grey mold appear
on the resting buds.
Robert