New hardiness maps are out

Jane McGary
Mon, 02 Jun 2003 09:18:54 PDT
Tony Avent and John Bryan have both made good observations on the 
unwieldiness, to put it mildly, of North American "hardiness zone" maps so 
I will not launch my usual tirade on this subject at this time. Using 
postal codes (American, "Zip codes") to map hardiness is a good thought, 
but here in the mountainous West, there can be climatic variation even 
within a Zip code where one code covers a large rural area. For instance, 
I'm a zone colder than the small town where my post office is situated, 
because I'm 1000 feet/330 meters higher up.

However, I'd just like to emphasize that with bulbs, you have to look at 
when they make their grown (below as well as above ground) before any other 
consideration, and also at their summer heat requirement, in the case of 

The best gardening advice I ever got was "Try everything," and that's the 
best advice I can give.

It's impossible to guess precisely even from natural habitat conditions 
what a geophytic plant will tolerate. For example, visitors are surprised 
to see Fritillaria pluriflora growing, flowering, and setting seed in my 
bulb frame, since they've been told it is highly habitat-specific and 
requires stiff clay soil--yet I grow it in my regular gritty mix. Doug 
Westfall apparently has the same thing going on with his Calochortus 
luteus, which in the wild grows in very dense rocky clay, yet flourishes 
for him in sand+pine needles. The answer to these paradoxes is that we are 
managing the water so that we replace the moisture-sealing quality of the 
clay with some other factor that keeps the bulbs from desiccating in 
summer. As for Doug's advice to keep the plant from freezing, I would add 
that in the wild, C. luteus routinely experiences freezing conditions (down 
to the low 20s F) without snow cover.

Those who would like to ignore zone maps should enjoy the book '"Rock 
Garden Design and Construction," funded by the NARGS and due to appear in 
October 2003. It includes many chapters on techniques for extending the 
range of plants in the garden, including a section on regional climates and 
how to deal with them. I don't know if Timber Press has inserted a zone map 
into it -- they did into "Bulbs of North America," at the behest of NARGS 
officers and over my protests -- but plenty of the contributors make snide 
remarks about "zones."

Jane McGary
Northwestern Oregon, USA

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