In article ,
Roger Van Loon wrote:
I'm sure there are very many serious plant growers in the UK. And I'm
equally sure that, when they do look up a plant, and the info says
"Z8" or "Z9" or "Z10", that this is, for them, very useful info
(combined with other factors). It surely must be one of the factors
that let them decide wether or not they can try it outside in their
own specific location.
Surely you wouldn't try out Z10 plants in your backyard?

We do. Next question?

As a good rule, in the relevant ranges of USDA zones 7 to 9, there is
a VERY POOR association between the USDA zone categorisation and the
hardiness of the plants on the UK. In fact, a good proportion of
USDA zone 6 plants are too delicate for the UK :-(

If the average minimum temperatures (which means: amount of frost) do
not play any role whatsoever where you are, OK , then I'll shut up.

No, actually, it DOESN'T mean "amount of frost" in any reasonable
sense, and that is part of the point. It refers to the extreme low
in an average winter, which is NOT the same as what most gardeners
mean by amount of frost, especially not in the UK.

[ In this context, may I draw to your attention the connotations of
the terms "a short, sharp snap" and "a hard frost"? The former is
often colder than the latter, but the latter implies that more things
actually freeze. ]

Of the major factors affecting the hardiness of plants, it is one
that is relevant to only a small proportion of plants, whereas there
are others that affect far more. And a good proportion of those
other factors are both measurable and applicable across a far wider
range of climate types. Some of the most important to the UK are not
easily measurable, unfortunately.

As I have posted before, with evidence, and been flamed for so doing,
the traditional indicator is the latitude at which the species is
found growing naturally. This is a bad indicator, but is certainly
no worse than the USDA zones.

And, again, as I posted, if the measurement were the average minimum
daily average rather than the average extreme minimum, it would be
VASTLY more useful and VASTLY more widely applicable - as well as
being equally easily measurable. It is perfectly reasonable to say
that ignoring this fact is a damn-fool idea. It was and is.

The fact that the information can be used, with unreliability and by
an expert, does not make it a good thing for UK gardening purposes.


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