FROSTS
Kelly Irvin (Tue, 13 Nov 2007 10:38:02 PST)

Frost is the ice crystals that form on plants and other objects and may
not necessarily occur right at 32°F, but never above that temperature.
It can happen lower than, and it's all related to dew point, some sort
of relationship between outside temperature and humidity in the air.

The ice crystals themselves (the ones we see as frost) do not
necessarily cause the damage to the plant, not even in relationship to
being quickly warmed by the sun, but they do say for sure that the plant
surface temperature has reached at least as low as 32°F. Now, what comes
into play is the frozen cell itself.

Whether the plant is damaged at this point is entirely related to the
plants adaptability to the nasty ice crystals which are actually forming
within the cell itself. Plants highly adaptable to freezing move the H2O
in varying degrees between the cells, leaving the interior of the cell
with a higher concentration of all other components, which means less
crystallization within the cell, meaning no swords puncturing the cell
membrane. I would imagine the sun rapidly warming the surface of the
plant could mean that the cell wall becomes pliable enough for the still
crystallized ice to puncture it, when, otherwise, a slower warming might
remove the crystallization of the water more evenly with the softening
of the carbon-based walls.

I have a lawn that has bermudagrass mixed in with various fescues and
ryes. When we get our first frost, whether the sun hits it or not, the
bermudagrass leaves die and the fescues and ryes look fine. The
bermudagrass does not transport water out of the cells quickly enough
(or not at all), so the leaf dies because of massive internal puncture
wounds. This will also happen to the bermudagrass at 32°F, even if no
frost forms, because the cells are not adapted to any self-protection
mechanism for freezing temperatures.

I have a tendency to believe the actual reason the frosted plants
exposed to morning sun seem more tender is likely a reflection of the
temperature changes they experienced the evening and night before. They
were also likely exposed to a quicker transition of temperature in the
first place. A plant in full sun, also got evening sun, and the
temperature drop would have been more drastic. A plant on a west, east
or south wall experiences residual heat from the wall and may not react
as well as a plant that has not received direct light. A more sudden
transition from warm to cold does not allow as much reaction time for
water transport between the cells. I guess I am suggesting the damage
has already been done even before the sunlight shines on the frosted
surface.

Mr. Kelly M. Irvin
10850 Hodge Ln
Gravette, AR 72736
USA
479-787-9958
USDA Cold Hardiness Zone 6a/b

mailto:kelly@irvincentral.com
http://www.irvincentral.com/