Climate Zones.
Rodger Whitlock (Thu, 27 Oct 2005 22:05:36 PDT)

On 27 Oct 05 at 10:41, John Bryan wrote:

The USDA zones were devised to assess the winter hardiness of
woody material and are based solely on average winter low
temperatures.

They are not, iow, *climate* zones. Climate involves a great
deal more than just the winter low temperatures.

For example, consider rainfall. Not only is the annual rainfall
important, but also its distribution over the course of the
year. We get abt 20" of rain a year here, but it mostly falls
in the Dec-Feb period and our summers, though cool, are usually
bone dry.

The same amount of water spread out over the year would give
entirely different growing conditions. Concentrate it in the
summer, not the winter, and we might even be able to grow those
touchy east Asian plants that, if they don't desiccate into
nothingness in August, rot away in February. (Tricyrtis in
Victoria? A complete failure in my former swamp.)

Other factors: wind; exposure to sudden sharp outbusts of
severe weather (as in Portland, Oregon when vileness swoops
down the Columbia River gorge or here, where we get "arctic
outflows" of seriously cold weather). Also summer heat units
(degree-days) and length of growing season (frost-free period).

And others.

Even your exposure affects climate: the north side of a hill
often has a very different climate from the south side.

--
Rodger Whitlock
Victoria, British Columbia, Canada
Maritime Zone 8, a cool Mediterranean climate

on beautiful Vancouver Island