Debbie's information about Alaska is interesting.
One other variable is timing. Usually our temperatures gradually get
colder and plants fortify themselves for the winter. Occasionally we
will get an arctic blast while everything is still in full growth. I
can remember a fall when the trees hadn't turned colour yet - their
leaves were still green, and got frozen onto the branches so we had
brown-leaved trees all winter.
In England, and perhaps other European countries too, gardeners worry
about spring frosts when plants have begun growing again.
I agree with those who said we should note the plants that grow well
in our normal conditions.
It is reassuring to have a reliable source of information about
hardiness. The usual U.S. zone information is not very helpful to me
here on the Pacific coast.
One set of books that I have found consistently reliable is by Roger
Phillips and Martyn Rix, published in the U.K. Their numbers may not
be at all helpful to those of you with a continental climate, but
perhaps they might. You may find it helpful to look at their
recommendations for plants you are very familiar with, to see if you
can trust them with advice about plants that are new to you.
For my conditions, if they say a plant is OK at -15 C (5 F), the
plant will be completely hardy, at -10 C (14 F), it will live most
years, but get killed in a bad winter which happens perhaps every ten
years or so.
I see I should not have been growing confusa outside. I didn't check
first.
Here are a few samples of iris from their book Perennials Volume 1:
confusa: -5 C 23 F
clarkei: -15 C 5 F
delavayi: -15 C
wilsonii: -15 C
lazica: -15 C, leaves killed at -10 C
unguicularis: -15 C
cristata: -20 C -4 F
setosa: -25 C -13 F
versicolor: -25 C
Diane Whitehead
Victoria, British Columbia, Canada
maritime zone 8, cool Mediterranean climate
mild rainy winters, mild dry summers
