Re: Deep freeze

Paul,

The roots of most if not all plants are more sensitive to cold damage
than the tops. In a windy cold like we are having right now, the wind
will conduct the heat away from the pot rapidly, and the smaller the
pot, the faster the soil temperature will approach the air temperature,
whereas there is always residual heat moving (albeit slowly perhaps) up
from deeper layers to warm the soil itself. Plants in containers are a
lot more vulnerable than plants in the ground.  

In 1990, we had some manzanita (A. patula) from cuttings taken at about
3000 ft. that were about a year old in gallon pots. I was greatly
relieve that the tops had survived the 16F temps (it is routinely that
cold at 3000ft around here, anyway). In late January, as the leaves
began to need water, we found that the roots had been killed by the
frost, as with hundreds of other plants that are hardy when planted in
the ground. There are lots of plants which can take freezing
temperatures above ground, but will die if their roots actually freeze. 

I presume the roots just do not have a mechanism, like the tops often
do, of removing water from their cells to concentrate the solutes inside
the cell and prevent crystal formation. (Have you noticed that most
frost-hardy vegetables, for example, wilt deeply as they freeze? The
water in the cells has been released into the cell walls and will be
reabsorbed later.)

Also, plants which are marginally hardy are much less so while actively
growing. I have lost citrus trees to a mere 27F night while they were in
a flush of growth, yet the same top/rootstock combination survived the
1990 blast without even a dead branch (lost all leaves, though) when it
was thoroughly hardened.  Recent divisions and newly propagated plants
are almost always a lot more sensitive since they are busy growing to
establish themselves.

Nevada City is going to get pretty cold tonight, I guess. Hope you don't
lose too much. I've got sprinklers going in several places, and two inch
thick ice already. All the faucets (30+) are dripping, and the babies
are tucked away in the greenhouse with a small propane stove. Maybe it
won't get as cold as they say.

Gary Matson, Far northern Sacramento Valley. 

"paul@nevco.k12.ca.us" wrote:
> 
> Do plants in small pots — like those newly home from the nursery, or
> recently potted plant divisions — have more problems surviving a freeze
> such as this? Does the size of a pot matter in damage/survival?
> 
> Also, given that soil temperatures throughout most of Northern
> California will not remain frozen for long, how much does the freezing
> and thawing of soil damage plants?
> 
> — Paul Harrar, Nevada City, CA (Zone 7)
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