Perhaps you haven't been out here, perhaps you don't know much about the Sierra Nevada. But we happen to have within our range the highest peak in the contiguous 48, Mt. Whitney. We also have a rather respectable number of high peaks relative to range in the country as well.
Can you explain how 28F in the Rockies is truly different than 28F in the Sierra, because the last time I checked temperatures are temperatures are temperatures. How would my snow be all that different from your snow? In other words, why would my plants withstand, while sitting in 1gal pots, weeks of freezing to below freezing temperatures, down into the teens if it were simply cold temperatures that kill them?
:)
Here's a photo from April 2010 of a group of clones that I didn't want to feed or tend anymore, so I stuck them outside. Did that the previous December. And there they are in April, having lived through what are the worst winter months for us in the Sierra, January through March. We got temps down to 15F or so several times that winter. These girls didn't get fed (it shows), watered (it shows) or tended in any way (it shows). They're not dead, though.
I called them whoors for their persistence. I took the photo because I couldn't believe they were still alive.