hardiness, cold temperatures

Kenneth Hixson khixson@nu-world.com
Mon, 27 Dec 2004 12:11:18 PST
Dear members:
         To quote John Bryan,
>It is an interesting point you mention regarding plants being dry or wet
>to survive the colder temperatures.
         I think the situation is a little more involved than just being wet or
dry--first, is the damage being caused by actual freezing--ie, are the cells
of the plant being ruptured by the expansion of the water in the cells of the
plant?  Plants that have been dry for some time usually survive this problem
better than plants that are well hydrated, because plants which have been
dry have their cell sap more concentrated, which acts as "antifreeze".  Also,
there is less water available to rupture cell walls.
         If the damage is caused by a cold wind blowing over the leaves of the
plants while the ground/stems of the plants are frozen and unable to
transport moisture up to the leaves, a dry plant suffers more damage than
one which is well hydrated.  This is typical of shrubs which are "burned"
by a freeze, and while it can kill, often it just looks unsightly and the plant
will recover.  It is usually recommended that plants be well watered
prior to freezing weather.  I might also mention that there have been people
who manually removed all the leaves on a borderline plant, and the plant
survived the winter when other plants of the same thing nearby which
were not defoliated, didn't.
         Sometimes plants-usually shrubs again, are damaged by "barksplit",
where the bark of a plant will literally split and the cambium layer will
separate from the rest of the stem, and the plant-usually a shrub or tree,
will look fine but will later die when the leaves transpire the moisture in the
plant above the "barksplit".  In my area, Rhododendron davidsonianum is
notorious for suffering barksplit, often in April, long after the coldest 
of winter.  In this case, planting on the north or east may make a 
Planting facing east is the worst possible location, because the morning
sun shining on the frost damage will finish splitting what might otherwise
have slowly recovered.  It is sometimes possible to tape barksplit--simply
use tape to tightly bind the cambium layer back to the stem, and the plant
may survive.  I have plants here which have suffered barksplit, and it is still
visible, but the plant survives.  If the plant is valuable enough, a graft 
below the freeze damage to above the  freeze damage may save the life
of the plant, if not the appearance.
         Other plants-in my area, Camellias and Daphne as examples, will
be top-hardy, but the roots of plants grown in containers will freeze and the
plant again will look fine but will later die, usually in spring just after
expanding  the spring flush of growth (leaves).  This is the bane of
nurserymen, who sell what looks like a healthy plant, which dies soon after
being sold and planted by a homeowner.  You can imagine what the
homeowner thinks of that nurseryman.
         I thought it was 1988 here in Oregon, but perhaps it was 1990, that
we had a long, mild fall, with virtually no freezing weather, then in early
November it abruptly dropped to  +18F, and hundreds of rhododendrons died.
These same plants which were killed at +18F, had only the year before survived
+5F in December, after a more normal fall pattern.  Obviously, it isn't just
the absolute temperature, but the condition of the plants themselves, which
determines the temperatures they can survive.
         I knew one rhododendron grower who used to claim his plants
were hardier than plants from other nurseries, because he used a
Nearing frame (an unheated coldframe) to propagate cuttings.  It is
true that rooted cuttings from a coldframe/Nearing frame are better
adapted to the outdoor temperatures than plants from a greenhouse,
but climate adaptation is not the same as being hardier.  At the end
of the growing season, his plants were only as hardy as the plant itself,
regardless of how it had been propagated.
         To tie this to bulbs, I have found that lilies in containers will 
and die--not in December, when it is coldest, but in April, just after the
current seasons' stem has started growing.  A lily which may have survived
+5F in December will be killed at temperatures in the upper twenties in
April.  People from areas where they have snow-cover often suggest that
keeping the container covered and thus dry, will prevent loss, but my
experience is that isn't the whole answer.  Keeping them dry may help,
but perhaps because it also keeps the plants dormant later in spring.
Covering them may actually make the situation worse if the cover is
"tight" enough that it is warmer than the surroundings.
         Daffodils being grown in containers will be killed by temperatures
much higher than they would survive in the ground, simply because the
roots are killed.  I'm not sure of the exact cause, just that it happens.
Jane undoubtedy has more experience than I, but bulbs in containers
should be in "plunge" material, not just in a covered cold frame or bulb
frame, to protect the roots.  The "plunge" material is more important
than the coldframe.
         Hardiness is a complex subject, and it is easy to misunderstand
what is actually happening.  Sometimes more than one thing is happening,
and sorting out what effect did which damage is difficult.


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