[nafex] what determines cold hardiness?

redherring at tnaccess.com redherring at tnaccess.com
Fri Mar 29 15:18:58 EST 2002

    Lemons and limes won't go dormant till night time temps drop into the
40's for a sustained time.  Since Florida seldom has such cool nights for
any length of time,  wintry blasts tend to really damage them.  Oranges,
tangerines, and grapefruit develop dormancy at higher temps, and thus tend
to weather freezes better.  Early citrus that has finished it's crop in Oct
or Nov will be less active than Mursott trees, which produce of gourmet
quality fruit in Feburary.  Persimmons seem to be limited more in their
northern range by whether the fruit can ripen than by whether they can take
the winters.  This is one of those examples of, well, yes I can grow it
here, but will it crop?  Hardy citrus can SURVIVE considerably further north
than it can crop.  At the far end of the citrus scale, trifoliate orange
goes dormant when nights are in the low 60s.  Winter can hardly catch it off
guard.  This creates some weird effects, like someone trying to grow
trifoliate in NM semidesert where nights are 60 nearly all summer long and
the trees barely grew.
    While peaches are hardy in TN, they tick off the "chilling hours" when
it's between 34 and 45 degrees, and we usually have plenty of those hours to
trigger blooming before all the frosts are done.  I have a volunteer peach
evidently from further south with maybe a 600-700 chilling hour requirement,
whereas my local seedlings probably require 900 or so hours.  So the tree
will grow, but seldom fruit.  Camellias seem to do the same kind of thing
here, and forget Magnolia stellata.
    I find the parallels between hardy citrus and roses to be quite amazing.
Once blooming roses seem to sit quietly while our fake springs alternate
with wintry returns.  Hybrid teas (with subtropical type genes) come out,
freeze, and come out again,.... enough freezes and they don't do so good.
Up in Canada, they are talking about Z2 roses, and saying the important
thing is to select for plants that respond NOT to temperature, but only to
day length.  That makes me wonder about whether persimmons trees are doing
that.  They seem so dang smart, not to bloom till about June here!
    There was some discussion a couple years ago about kaki persimmons
grafted onto native rootstock in the deep south.  They would all come out
fast and then freeze and turn black and die to the graft line.  I was
reading about this effect on new growth on camphor trees, and really
wondered if the kaki freeze damage was merely due to unhardened wood.
    I have come across comments that plants that can stand heat and drouth
are often more frost hardy than their fellows.
    Bernie's laments on hardiness in the far north remind me of some
properties of water.  I suppose you have all noticed that frost patterns on
windows look rather like the leaf patterns of evergreens. Turning that
thought on it's head, isn't it interesting that hemlock and spruce and
pines, etc, all look sort of like the patterns that frozen water makes?  If
they adapted the shape of their leaves to the shape of freezing water, then
ice crystals may form and dissapate harmlessly without disrupting the leaf
structure.  Not that I think we can do anything to teach this to other
plants, but maybe there are engineering applications that humans could use.

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