Freezing bulbs: Duration vs. low temperature

In my solar heated greenhouse, pots near the glazing typically freeze several times a winter (they've done it once already this year at an outside temperature of 0oF). They have always thawed by early the next day, with no harm done to the roots, including bulbs, though on more tender species top growth may look bedraggled. Recently, I have strayed a bit from my pure ecological principles by installing a propagating pad in that chilly southwest corner, controlled by a thermostat that kicks in at about 34oF, which corresponds to 32o or so in that corner. I grow a fair range of geophytes.
Jim Jones

-----Original Message-----
From: Lee Poulsen <wpoulsen@pacbell.net>
To: Pacific Bulb Society <pbs@lists.ibiblio.org>
Sent: Tue, Jan 15, 2013 3:53 pm
Subject: Re: [pbs] Freezing bulbs: Duration vs. low temperature

I've noticed the same thing about the difference in native hemisphere. But I
attribute this to the mediating influence of what could be (and sometimes is)
called the Southern Ocean. Any frigid cold air masses moving north from the
south polar regions has to cross a great deal of liquid (and presumably
above-freezing) water before it can reach land areas (South Island, New Zealand;
Tasmania and the southern regions of Australia; South Africa; and southern Chile
and Argentina). So this will tend to warm up any frigid air mass much more by
the time it reaches land again than the parallel case of frigid north polar air
masses moving south towards all the populated northern hemisphere land areas
(USA, Europe, China, etc.) The north polar frigid air masses mostly ever travel
over very cold and dry land on their way south, so they remain far colder far
longer than their equivalents in the southern hemisphere. So I think that at the
same latitude north vs. south, on the same side of the continent, you're
probably going to see more examples of colder extreme low temperatures in the
northern latitude than at the equivalent southern latitude.

[Furthermore, due to the same physical cause--i.e., the huge mass of water
pole-ward of southern latitudes vs. the mostly land mass pole-ward of northern
latitude regions--you find that the humid sides of the continents (i.e., the
eastern sides) have much warmer nighttime temperatures during the summer in the
northern hemisphere than do the equivalent latitudes on the east coasts of the
southern hemispheres, even right along the ocean coasts. Compare Nanjing or
Shanghai, China vs. Sydney Australia. Or Charleston, South Carolina, USA vs
Buenos Aires, Argentina. And yet, southern hemisphere cities can grow much more
tender plants than the equivalent latitude northern hemisphere cities. Because
they never see low temperatures as cold.]

As for the clear plastic vs. nonwoven (or even woven), *opaque* fabric, I think
the explanation is due to the blackbody radiation effect from physics. This is
typically a problem during cold clear dark windless nights. (If it's really
windy, you either don't quite reach temperatures as cold as you do under similar
conditions with no winds. Also, when the air is frigid but it's very windy,
coverings of any kind aren't as effective.) We call it "radiation frost", but
it's due to the principle that the ground or the surface of uncovered plants
"sees" the dark black sky of space (which has a temperature of around
-270°C/-454°F), so since the surface is much warmer than space, it will
spontaneously start radiating that heat energy as infrared radiation towards the
sky. This loss of energy causes its temperature to drop, even below the air
temperature around it, which is why you can sometimes get a killing frost even
when the air temperature is above freezing. (This effect can gradually cool the
air temperature near the ground as well.) The fabric (or cloth or overhead tree
leaves or eaves of a house) block this visibility, so there isn't this energy
radiation effect occurring. Also, I believe all of these "opaque" objects are
opaque to infrared, so the infrared radiation can't travel through the cloth or
other object. On the other hand, clear plastic allows more infrared to radiate
than cloth does, and the cold black sky is visible to the ground and plant
leaves through the clear plastic so the blackbody radiation effect is more
effective than through cloth. I forgetfully saw this principle at work earlier
this winter in my vegetable garden when I used some new plastic-tunnel-with-hoops
thing I saw and purchased earlier last year on several pepper plants one day
when they were predicting very cold (but not freezing) low temperatures. There
was a still surviving tomato plant next to them that was far too big for the
plastic hoops, but I threw some Reemay over it just because it was handy. I
think the low temperature was about 36°F/2°C at the garden that night. There was
a lot of radiation frost that killed all the exposed tomato and pepper plants in
the other garden plots of the community gardens. My covered pepper plants turned
to mush as well but the one covered tomato plant was still green and alive the
next day. (It's dead now; yesterday morning's low temperature was just too cold.
Even downtown Los Angeles dropped to 34°F/1°C.) BTW, this is why you will
sometimes see a species get killed growing in the ground out in the open when
the same species growing under a nearby tree does not after a cold clear
windless night.

--Lee Poulsen
Pasadena, California, USA - USDA Zone 10a
Latitude 34°N, Altitude 1150 ft/350 m

On Jan 15, 2013, at 11:23 AM, Jane McGary wrote:

I've noticed a pattern over the years: "marginal" bulbous plants that
survive cold snaps are most likely to come from the Northern
Hemisphere, especially the Mediterranean, west and central Asia, and
the west coast of North America. Plants from South America and
southern Africa are less likely to recover

I agree that plastic film alone should not be used, and
especially clear plastic.

The nonwoven fabric

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