In article IlX4a.169127$iG3.19825@sccrnsc02,
Anne Middleton/Harold Walker wrote:

"Nick Maclaren" wrote in .
And the USDA zones are a damn-fool idea anyway, as they measure an
extreme value rather than a central one, and that is NOT a
reliable//////////////Bottom line.......I find them very useful when
combining with other factors........far from being a "damned fool
idea"........I would not consider growing a plant listed as zone 8 min. when
I am in zone 6 (regardless of other factors)........I also admit the a plant
listed for zone 7 might not be in my grow list for other climatic reasons.

The reason that they are a damn-fool idea is that the average minimum
winter temperature is directly relevant ONLY to plants that rely on
their above-ground parts overwintering, and is marginal even for them.
Using the average minimum daily average would make it relevant (a) to
more of them and (b) to plants that overwinter underground.

In the USA (an almost wholly continental climate), the soil never
freezes in zone 8, let alone zone 9. In the UK (an almost wholly
maritime climate and MUCH further north), it does, readily. It has
been dropping to -4 to -5 Celcius here (borderline USDA zone 8/9)
and the ground has frozen hard, probably a couple of inches down.

In the contiguous USA, there is a STRONG correlation between the average
minimum winter temperature and the average minimum daily average, but
it does NOT extend outside that restricted ranage of climate types.
In the UK, there is a very poor correlation, and there is almost none
with the USA.


Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:
Tel.: +44 1223 334761 Fax: +44 1223 334679