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 

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

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