I'm don't know the answers to your specific answers, but I do know that Washingtonia robusta & filifera will have severe leaf burn with a single night below 25F. I live in Fresno (170 miles south of Sacramento) in Sunset zone 9, which is the thermal citrus growing belt at the base of the Sierra. We get lows like those about once every 8 to 10 years. If those kind of low temperatures were routine, I think either of those two would have trouble being a long term plant.

The interesting thing about the southwest is the fact that there is a lot of solar radiation which does increase the cold tolerance of palms like washingtonias in those areas. I've heard of washingtonias survive near 0F temperatures in the southwest even though they would usually be dead at 10F-20F anywhere else. I think by you, washingtonias will both need enough protection to keep them at 20F and up during the coldest nights of the winter. Livistona chinensis has about the same cold tolerance as washingtonia robustas and my livistonia gets damaged in the low 20s but has always come back with no problems since it has never seen below 20F (except for one year where it did see a night of 18F when the lights failed).

Trachys would look their best with protection by you. My unprotected trachy survives my winters (coldest it has seen is 5F), but looks awful. My other trachy that gets protected so it stays above 15F looks much better and grows much faster. It's worth protecting them when young. I will probably stop protecting mine after this winter unless temperatures are forecasted to go below 10F.

Good luck!
-Alex

I am in approximately the same winter temp zone, Carson City...but much wetter. As you may know, the East is currently in the grip of an arctic outbreak. Unprotected are my Needle Palm and Sabal minor (one may be S. Louisiana). I did succumb last night to protecting my Trachys. Smaller, juvenile ones get a plastic garbage can over them. The larger ones get X-Mass lights and a few layers of a light insulating blanket. Then too, there is all the snow! That too is insulating. Was dreading to protect them but it only took about 15 minutes...better safe than sorry. Have not had to do this in a few years. Those little lights generate more heat than you might imagine. One year, I used bubble wrap but that was a mistake...worse than unprotected. Just not sure how any of these cold tolerant palms survive in a much more arid climate. I know Sabals love water, Trachys have good wet and cold tolerances, and Needles too like it moist. You might also have to shade then more from the hottest sun, compared to those grown in the East. Maybe try European fan palm. I know they are rated to 8 (maybe 7b), but you drier climate will help with that one. Good luck.

Thank you everybody for the feedback, it is much appreciated (:

@tropicalzone7
What materials do you use to protect your trachys? and I know you said to keep the washy's above 20F, but what materials would I use to go about doing that?

I protect everything with christmas lights, a light frost cloth, and a plastic garbage can and it's been pretty successful. My larger palms have 6 foot tall wooden stakes holding up the garbage bin and frost cloths covering everything under the garbage bin so it stays as protected as possible. As of this winter I am using C-7 and C-9 christmas lights and it has done a great job keeping everything very nice and comfortable. We went down to 12F last night but my Livistonia didn't go below 49F inside it's protection which is only the christmas lights and a plastic garbage bin (no frost cloth in that one).
My oleanders are my biggest plants that get protection. For those I use christmas lights, a frost cloth, and a plastic garbage bag (not can) when it's snowing or raining to keep the moisture off them. I take the plastic off as soon as the precipitation stops since plastic and plants don't go well this time of the year.

Good luck!
-Alex