Hardiness really is a very complicated concept, more complicated than just
minimum temperature. I think we all have had experience with plants that
are supposedly not hardy for our area surviving, and the opposite with
supposedly hardy plants for our area dying after their first winter. What
is more confusing is the fact that 5 out of 10 of these plants live and the
others die. Open winters with no snow cover, previous year overcropping of
fruit, unhealthy or stressed plants, etc. are just a few other factors off
the top of my head that can have an effect on a plants ability to withstand
the winter. It is why I never say "This plant is hardy to -40F", instead I
say "This plant has survived -40F"
A process called supercooling prevents the formation of ice particles
forming within the plant tissues itself and each plant has this ability to a
greater or lesser extent. Supercooling does not work if the temperature
goes below -40F however, which is why in the northern Canadian prairies you
can actually see a division of where certain plants flourish and are
non-existant based on only the winter temperature lows.
The difference between -10F and -30F is important. Tests at the U of
Minnesota in a freezing chamber with grapes have shown that wild grape
(vitis riparia) and some other riparia hybrids have this supercooling
ability to around -40F, but at lower temps, ice formulation still occurs and
the tissue dies. Other labrusca and vinifera grape hybrids burst at lower
temps.
This is why I believe a few years of testing for hardiness (at least in the
north) should be done at different sites before any breeder releases a plant
to consumers.
Sorry for the rant....
Dean Kreutzer
Regina, Saskatchewan Canada
USDA zone 3
[NAFEX] Blackberry Hardiness Question,
Dean Kreutzer, 10/19/2004