"Darryl" wrote in message
oups.com...
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.

Thanks.


Snow protects in a number of ways. It keeps the plant thinking it's cold.
That's important for plants which form next season's buds before the snow.
If the snow melts and the plants start growing too early, buds can be
damaged. Sometimes the plant dies, or maybe its flowers are ruined. Snow
also helps keep the ground from heaving (shifting or lifting) due to
repeated thaws and freezes. That can cause mechanical damage to roots, or
just expose them to cold, or air, which can dehydrate them. Depends on the
plant.

As far as why some plants seem bulletproof while others are fuzzy, someone
else will have to explain that. I imagine it has something to do with
antifreeze. :-) I mean, how the hell does a tiny crocus put up with as much
obscene weather as it does?