Hmmm...how cold are we talking about here? S. purpurea grows in some
locations in Iowa, where it can get up to 105 degrees in the summer, and
they deal with it. But S. minor and flava are not likely to take the
kind of winters that one finds in Canada and N. Minnesota - down to -30
or -40F. In Seattle we get cold snaps down to 6 or 7F every so often,
and Sarracenias are killed off if they get frozen. Certain species do
seem less hardy, especially S. minor and S. alata. S. leucophylla
surprisingly enough is hardier than some. But I don't believe they are
"equally cold hardy."
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We should keep in mind these plants tend to grow with lots of
water which helps keep sudden temperature changes to a minimun.
When grown in pots they are much more at Jack Frost's mercy. I
am not sure about the absolute low a particular species can take
but all Sarracenia can (should?) be frozen. Perhaps some
species have forms which are less hardy. I don't think cold
kills in as much as not protecting the rhizomes from exposure.
Purpurea makes a protective rossette around the stem as the
phyllodia of oreophila does, same for the floppy winter leaves
on flava which lay down and some cover the rhizomes. This
doesn't quite hold true for leucophylla, whose stem is not
covered by a morphological quirk, but as you say minor and alata
(my addition is psittacina) seem less hardy well they also have
no such natural protection. These plants are also found farther
south than the winter adapted species. Could be the S. alata
and S. minor are more evolved or evolved in warmer climate than
S. leucophylla, hence their lack of phyllodia. I know I didn't
even consider S. rubra but I'm not that familar with it and this
is just coming off the top of my head anyway.
Dave Evans