Re: Hardiness and heat zones

**Larry Maupin l*@sbcglobal.net**

on 6/26/03 9:49 PM, John MacGregor at jonivy@earthlink.net wrote:
> There are a great many more factors influencing the hardiness of a plant
> apart from the lowest minimum temperatures it will endure (which is all that
> the USDA map takes into consideration).  For one example, the amount of sun
> a plant receives to harden it off before it is subjected to freezing is of
> extreme importance, which is why many plants that are fine in parts of
> northern California where temperatures are actually colder than they are in
> England, will not make it even in Kent unless they are grown against a
> south-facing wall and unless the summer and fall have been particularly
> sunny.  Other limiting factors are the amount of moisture the plant receives
> before, during, and after a freeze, the duration of the cold temperature,
> the amount of snow cover, etc., etc., etc.  The USDA map addresses none of
> these.
> Like the Sunset zone system, the USDA system has undergone refinement, but
> it is limited by its own definition of its mission and by the need to
> squeeze  so much information into a single small-scale map.

I agree totally, John. As I mentioned at the end of my post, "don't ask
about microclimates and maintenance variables." All regions, all cities, all
greenhouses and all residential landscapes have microclimates which must be
factored in by any competent landscape designer. The zone maps, no matter
which one you use or prefer, is just a starting place. Real world situations
must be factored in. For example, I live in Dallas, Texas, which is listed
as USDA zone 8a (probably should be 7b, but that's another story). I've
successfully overwintered zone 9 plants in containers on the south side of
my house. And northern gardeners can mulch and cover zone 5 roses to
overwinter them in zone 3.

Maintenance is a huge factor also. You mentioned sun exposure, which relates
to its drying effects on foliage and soil. Many people around here quit
watering everything in winter and wonder why their plants are dead in
spring. A desiccated plant will die before cold winter temperatures reach
its hardiness zone temperature.

As you said, the USDA system has its limitations, but it is widely quoted
and is generally workable if everyone will read what it really means and use
it as a starting point in their landscapes.

As an experienced California gardener, what is your opinion of the Sunset
zone system? Should we all be adding the Sunset zones to our stories?

Regarding refinement of the USDA map, I heard last summer that the list was
being revised significantly -- has anyone heard of a new USDA map? It was to
include more tropical zones and other changes.
-- 
Larry Maupin
Maupin Photography
Freelance Garden Writer/Photographer
Member, Garden Writers Assoc.
Dallas, TX   214/341-3933

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