Howdi,
I just realised that, like the NewCrops mob out of Queensland
Uni, hardiness zones might also be not as well known as I thought
they were. I'd just kind of assumed [again] that this was part
of that nebulous construct ... Common Knowledge.
So for the listeners at home that don't already know and use
this zoning method -- read on.
The USDA (Dep of Ag) came up with these things years ago, and
their big claim to fame is that, unlike most standards that come
out of the USA, you can actually use them out of the USA.
Two temperature thresholds are defined -- a Heat Zone and a Cold Zone,
with the former being the number of days with temps > 30C, and the
latter indicating the minimum (averaged) temperature.
Wikipedia [1] has some good oil on this stuff, so head there for a
listing of the zones. My lowest winter average temperature is
around -4C, and I tend to get about 90 days a year with temps > 30C,
so that puts me in Cold Zone 9b, and Heat Zone 7. Hmm, looking
at that site, there's no reference to Heat Zones -- which is something
that Diggers[2] use in their seed catalogues. The other seed mob I
use - Eden Seeds - doesn't use anything, so you're left to scan the
descriptions for the words 'frost' and 'hardy' and hope for the best.
Of course this has all kinds of limitations, but as long as you think
about what this information actually tells you, and take it as a
very rough guideline about the climate in that particular place,
then it has some use. Certainly the most useful thing for me is the
CZ rating - anything above a '10' means it's not frost hardy, so I
can pretty much cross it off my list.
Some of the obvious limitations include ignoring the following:
o microclimate / local geography
o aspect
o altitude
o latitude / hours of daylight
o (yearly distribution of) rainfall
o prevailing / strength of winds
o frost-free months (or just lengths of seasons)
o average hours of cloud cover
o low temp duration -- all night long, or just briefly at 4am
o influence of nearby mountain ranges, oceans
o number of *really* hot days : low 30's != high 40's
o inverted requirements -- some plants, for example, not only
are hardy to very cold or very hot temperatures, but actually
*need* them in order to set or ripen fruit -- think stone/pome
as varieties that must have chill to be productive, and think
grapefruit as a tree that really needs desert-hot temperatures.
The climate zone system typically targets plant survival, rather
than plant productivity (the history of these zones was to satisfy
the average ornamental lover, not your home produce gardener).
The Sunset mob [3] attempted to make a better zoning system, and
predictably fell prey to the World = USA fallacy. It does try to
factor in things like growing season, rainfall, humidity and so on,
but the very act of assigning zones to entire states demonstrates just
how applicable a zoning system it is in reality. Having an arbitrary
number of zones (33 in the Sunset schema) also reduces the efficacy
of any such scheme. It makes sense to have a string that defines the
key characteristics of your climate, including all the important
influencing factors, but the fact that such a system isn't in use
gives a hint as to how hard it is to design one.
In .au you can point people at the weather records for your
region, which assumes that you're conveniently close to the nearest
BOM station. This is good for indicative purposes only -- for example
I am somewhere between Mudgee, Jerrys Plains and Scone - but not
really near any of those places in a geographic, altitude, or
even climatic sense. Mudgee is about 300 metres higher than I am,
and a hundred kilometres further inland, and consequently its climate
is much harsher than mine. The other two - Jerry's [4] and Scone [5]
are also about 100 km from me, and are worth looking at concurrently
to get an idea of just how much the recorded climate of two proximal
regions can differ. Open them both up, same window size, overlaid,
and then alt-tab between the two .. and you'll see what I mean.
In this particular case it might be heavily influenced by the quantity
of data recorded at the two sites -- Scone (higher elevation) has much
milder winter temps, but has been recording data for only 5 decades,
compared to Jerry's 1884-onwards dataset. Global warming theorists
can have a ball with this stuff.
Anyway, the BOM climate averages for your part of the world makes
fascinating reading, and they're a good starting point, as well as
being a source of comparative data for other growers.
Jedd.
[1] http://en.wikipedia.org/wiki/Hardiness_zone
[2] http://www.diggers.com.au/GrowingGuides.htm
[3] http://www.sunset.com/sunset/garden/article/1,20633,845218,00.html
[4] http://www.bom.gov.au/climate/averages/tables/cw_061086.shtml
[5] http://www.bom.gov.au/climate/averages/tables/cw_061089.shtml