1936 usda zones, US

penstemon penstemon@Q.com
Mon, 08 May 2017 08:19:33 PDT
>One thing about USDA hardiness zones, be they from 1936 or 2017: they 
>refer to the  hardiness of woody plants. Nothing else. They have proven 
>very useful in the cultivation of other plants, but do not take into 
>account other important factors, notably the annual pattern of 
>precipitation, which is just as important as temperature to the 
>successful culture of bulbs in the open garden.
True. In Santa Fe, NM, a couple of weeks ago, I noticed large flowering plants of Cystisus scoparius everywhere. In Denver there are none, and yet winter lows are roughly the same. The only explanation I can think of is that Santa Fe receives at least twice the precipitation in autumn, which helps with the manufacture of cryoprotective sugars. 

>I always wonder why dwellers in the southwestern deserts of the US 
>(notably Phoenix!) don't use oncocyclus irises as bedding plants.

Different rainfall patterns; it rains in Phoenix in the summer. Not a lot, but more than none. Rain combined with heat might be fatal to the roots. The oncos grow their leaves in winter and receive twice as much rainfall in the wild, in winter, as Phoenix receives all year. 
The sand in which desert species like Iris mariae grow is wet to a considerable depth in April. 


Bob Nold
Denver, Colorado, USA
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