Nick Maclaren wrote:

I tend to use the Sunset book myself, when referring to USA conditions,
but even that needs a fair amount of interpretation.

I reckon the Sunset Western Climate Zones deserve a favourable mention in
the FAQ.
They are more nearly applicable here than USDA zone (minimum winter
temperature only).

And also perhaps include a list of other regions with broadly comparable
approximately temperate maritime climates (if a bit warmer). eg New
Zealand, parts of Pacific NW, Channel Islands

I see nothing wrong with specifying minimum tolerable winter temperature
for a plant in degrees Celsius.
(that clarifies what is being stated) USDA Zones sound plausible but are
misleading in the UK.

It usually isn't the fairly modest UK minimum temperatures that kills
things so much as the long drawn out sunless grey cold misty days that let
moulds and fungi attack weak immature and unripe wood on tender plants.

Regards,
Martin Brown