OK, the toughest family of clumping bamboos that I have in my collection are the Fargesias. Fargesias come from the the alpine conifer forests of west and southwest China. These are beautiful in delicate sense (in appearance only) and some can be found in mountain elevations as far up as 8,000 feet. On average, these bamboo can take -20 f per the respected American Bamboo Societies source list. The toughest of them all is Fargesia dracocepela (also known as Dragon's Head Bamboo) that lists a minimum temperature at -23 f. Pictured above is the closely related White Dragon Bamboo (Fargesia dracocephela "White Dragon") growing in my garden.
For running varieties, here are some tough cold tolerant varieties:
Phylostachys bissetti at - 15f (very wind tolerant as well)
Phylostachys aureosulcata "Spectabalis" at -10f (yelow culms with green stripes and red at nodes - I have this in my collection, very pretty.
There are a few more that are at -5f to -10f cold tolerance.
So, check out the American Bamboo Societies source list online and you'll be amazed. Photos of these bamboos can be found at http://bambooweb.info/.
Many thanks to Kate for bring this question up, hope its useful or at least interesting.
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