I hear that they are only cold hardy to the 20s, but yet they grow naturally in an area that does get colder than 20F. I think they have the potential to be very cold tolerant palms (or at least zone 8b palms), but they seem to be tender when young. I wouldn't try it in the ground until its a decent size and I would then protect it below 25F (but not before letting it outside through some nights in the upper 20s to see its response).
Good luck!
-Alex

I know that T. wagnerianus is hardy in 7 and T. martianus--no way. I have all of these but only have had the fortunei in the ground. Guess I was thinking of pushing those limits again. But based on your input will keep the latisectus containerized--at least for the time being. Thanks.

I think latisectus is equivalent to martianus in hardiness... mine got burned equally during our freak freeze nearly 4 years ago (25F for over 7 hours). Both lived but lost most leaves..

Both my Latisectus and Martianus were unharmed from 2 nights or 23f.

guess I got some wimpy palms

Well,they were on the porch and had seen gradually colder nights before those temps hit.

I would say zone 7 is pushing it a bit too far for these. -5c (23f) is pretty much the limit, if exposed to radiational frost with these temps the leaves will fry. Mine saw lower temps (6c) but they were given protection. In short zone 7 with no protection....not a chance.

I also think hardiness improves with age... the T martianus at Huntington were fried that year, too, but not nearly as badly, and they were about 20' of trunk at the time... mine was a 2' tall seedling fully exposed in the middle of the lawn.