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?

Have you ever been in a room and thought it was too cold, while someone else
in the room thought it was warm enough? Or maybe vice versa? Ever see
someone walking down the street in shorts and a t-shirt while you've got the
heater on in your car? Does your dog look for a jacket before going outside
in the winter?

Why is it hard to understand that different plants have different
tollerances?


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.

Under the snow there is a heat source. It's the Earth. The temperature at
the core is believed to be somewhere around 3000-5000 C. It certainly isn't
that warm near the surface, but it still is a heat source. There is also bio
activity in the soil that's producing heat as well. There may also be some
heat produced by underground utilities. A sanitary sewer, for example, can
be pretty darn warm thanks to all the bio activity! There are plenty of heat
sources under that snow cover. Enough to melt the snow? In some places, yes.
In some places, no.

--
Warren H.

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