Subject: Hardiness and cultural needs of ornamentals

From: [email-masked] (Stavros Macrakis)
Date: 03 Oct 1995 14:00:02 GMT

Plant hardiness is (in the US) usually expressed in terms of USDA
Zones (based on earlier Arnold Arboretum Zones), which correspond to
average annual minimum temperatures.  Clearly there are other relevant
macroclimatic factors, such as continental vs. oceanic climate
(cf. RHS Dict'y), summer stress (temperatures, humidity), etc. as well
as microclimatic factors (wind exposure, ...).

For that matter, I would have thought that the relevant issue is not
the _average_ minimum temperature, but some percentile minimum
temperature.  For long-lived trees, you would want to know a
low-percentile temperature (a 1st percentile minimum would give you an
average hardiness expectancy of 100 years); while for perennials, you
might want a much higher percentile (a 25th percentile minimum would
give you an average hardiness expectancy of 4 years, which might well
be acceptable).  And then, unseasonal frosts are apparently far worse
than minima in mid-winter.

The Sunset publishing companies defines a system of 20+ zones for
California alone based on several factors.  Has it been done for other
areas of the country?  How thoroughly have plants been classified as
to hardiness in these various zones?  Is it in fact useful?

Then of course there are cultural requirements such as drainage,
shade, soil chemistry, and so on....

Most plant references (RHS Dict'y, RHS/AHS Encyclopaedia, Hortus,
Dirr, ...)  and commercial catalogues give indications on plants'
preferences but these references often seem to disagree on winter
hardiness, on shade tolerance, on required soil acidity, etc.  For
that matter, they usually don't mention whether they're referring to
bud hardiness, wood hardiness, or root hardiness, and it is rare to
see all three given, although they are relevant to plants which can be
grown either as woody plants or as herbaceous perennials.

So, in a nutshell, what is the state of the art in understanding
ornamental plants' climatic and cultural requirements?  What are
considered definitive sources?

Thanks,

	-s

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From: [email-masked] (Katherine Sedwick)
Date: 4 Oct 1995 21:44:42 GMT

The Sunset zone system covers ~40% of the USA, the Western 
portion.  For anyone living in that region, the _Sunset
Western Garden Book_ is THE gardening book to buy.  Not
perfect, but invaluable.  Mine is falling apart from use.

Kate Sedwick