Subject: Re: Old Wives, water droplets was FROST

Tony Avent
Wed, 14 Nov 2007 08:26:57 PST

I don't have an old wife, but will tackle these tales as it relates to 
agaves.  We have two water issues that cause severe foliar damage.  If 
we have several days of rain in the fall or early winter as the growth 
is slowing, followed by bright sun, we can get terrible leaf scorch due 
to Oedema.  .…. 

The other water related damage occurs after either an ice or snow storm 
that leaves frozen water on the leaf surface.  When the sun reappears, 
the tissue beneath the ice is scorched due to the magnification of the 
suns rays through the water.  No damage occurs where the ice or standing 
water is in the shade.  Over the years we have seen such damage on other 
plants, but none as severe as we see with agaves.

Tony Avent
Plant Delights Nursery @
Juniper Level Botanic Garden
9241 Sauls Road
Raleigh, North Carolina  27603  USA
Minimum Winter Temps 0-5 F
Maximum Summer Temps 95-105F
USDA Hardiness Zone 7b
phone 919 772-4794
fax  919 772-4752
"I consider every plant hardy until I have killed it least three times" - Avent

Robt R Pries wrote:
> Jonathan wrote;
> " It doesn't really make sense, as I think about it,
> and I suspect you'll
>  agree, that the sun's heat would lead to increased
> chilling of the
>  leaf under where the waterdrop was located--it
> wouldn't evaporate any more
>  rapidly than the sun's heat caused it to vaporize."
> I am a bit on shaky ground since I am not a physical
> chemist, but as I understand the phenomenon. Sunlight
> can increase evaporation and although there is a
> direct relationship between light and evaporation,
> calories may be drawn from the surrounding
> micro-environment and not just the sunlight as the
> water changes state. Since there is a proportionally
> huge consumption of calories required some of these
> may be drawn from the leaf. The sunlight may not be
> the only source of heat for the evaporation and may
> facillitate the process by affecting surface tension
> and other factors. Thus the immediate area of the leaf
> may be chilled because of its donation of heat to the
> process. Perhaps we are both saying the same thing.
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