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
summer-growers.

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