Darryl wrote:

I have two questions about zone hardiness:

1) why does a plant even care how cold it is once the temperature drops
to, say 10 F? Why would a plant be damaged more by -40F than -20F? Is
it the depth of the frost, or something else?

2) In the colder zones, does the snow really protect the plant from
cold, or just from the wind? I would think that if the air temperature
is 5F all day, wouldn't the ground even under the snow also be either
5F or colder? I don't understand how "insulation" can help when
there's no heat source: if you wrap an ice cube in a mitten in a
snowstorm, the ice cube will still be cold.

Some plants have more dissolved sugars and nutrients in their sap
than others. The greater the concentration of dissolved matter,
the lower the freezing point.

Some plants have thicker bark over their sapwood. The more
insulation, the less damage from freezing.

etc, etc

A layer of snow keeps colder winds off the plants. That's why some
gardeners build cages with tree branches around smaller shrubs, to
trap the snow and create an insulating blanket. Under that snow
blanket, the temperatures might reach 20°F (the temperature when
the snow fell) while the air above has dropped to 0°F. Also, cold
air is more damaging when it is very dry. The snow helps maintain
some moisture in the air under it; even if it's not melting, the
snow can give up moisture through sublimation (direct
transformation from solid to gas).

--

David E. Ross
URL:http://www.rossde.com/

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