I've seen them in Santa Fe, NM which is the same zone as you. They are commonly used here in abq(zone 7). Although it's likely wetter in the winter where you are, which is a factor. Other tree yucca's that can take zone 6 better are:

yucca elata
yucca rostrata

Thanks for the info.
I know of Elata but I heard the Rostrata isn't quite as hardy. The problem is when I go to different web sites for info on hardiness I always get different answers.

I realize this is an old thread, but I was just perusing a few from a while back and this one was interesting...

Yes, there are indeed plenty of Y.faxoniana in Santa Fe, and well north, in fact I've seen them thriving in Denver. They probably are less hardy than Y.elata, which I've seen naturalized in Denver, but as I understand it, once they've attained some size, they don't seem to mind the cold.

Anyway, by now you've probably moved on in your gardening plans (it has been 8 months!), but it would be interesting to know if anybody is growing this species in Ontario.

I know that yuccas(need really fast draining soil) as well as any plants that are marginal benifit from extra protection and a hardining off period starting generally after first frost which means holding back water and fertilization at least a month befor first frost, they say alot of western plants wont grow where rainfall exceeds 20"but there are many success stories of large trunking yuccas agaves etc with protection I've even heard of people in the great lakes region growing agaves with just a tarp over them, whats next?the moon...a hot plate a large glass bubble an oxygen tank and a 250,000mile extenion cord should do it lol,anyway the first parts true.

Faxonia is equally as hardy as the Yucca elata or rostrata if grown in a desert situation. These are all true desert grassland to quite desert conditions plants that need to
be grown in fast draining soil. The large yuccas here in the Boise Valley are never watered nor fertilized, and thrive in our volcanic soil. They do not like the caliche or hard pan [clay] soils. Their roots tend to stand in the hard pan and rot. The more sand and volcanic soil the better. I would suspect that the faxonia can stand temperatures down to -25F or so like the rostrata, thompsonia and elata.

I live in zone 5b/6a in Springfield, IL. My Yucca Faxoniana rotted after this winter and it was protected from moisture. I protected it by putting a garbage can over it. It's possible that moisture built up in the garbage can and that's what caused it to rot. It had excellent drainage as well.

I've tried yucca rostrata as well. It also rotted. I'm trying one more yucca rostrata that I got from www.coldhardycactus.com, we'll see how that goes. From what I've heard, Yucca elata is the best (hardiest) trunking yucca for cold/wet climates. I have a small yucca elata. I am going to bring it in this year and then leave it out permanently next year.