" There is also a very large, and interesting plantation of them in Maui, not the southeast, but still a really warm humid climate" Nope! Wrong again! The redwoods there are mostly planted above 5000', a climate where dewpoints are always << 60F, thus nights are going to be in the 50s at worse, and even 30s and 40s in winter. Highs probably never hit 80f, even during the worst heatwaves. Not at all a "really warm humid climate". A few are a bit lower, down to 3000', but their growth seems to become abnormal. Even there, summer are as cool or cooler than Boston's!

Here is one I didn't know about. Looks like they LOVE the high summer rainfall, and deep sandy soils of southeastern Virginia! The best Cryptomeria I've ever seen on the east coast (probably, anywhere! I've never been to JP or CN) was somewhere in Norfolk but I doubt I could ever find it again. http://bigtree.cnre.vt.edu/detail.cfm?AutofieldforPrimaryKey=660 Time for those folks to install a lightning arrester for that tree! Or make sure the church spire is well grounded so that it diverts the charge LOL.

" I wonder how one would do in pure muck soil where taxodium grow, if inland enough where saltwater flooding isn't a problem." Not well! It would last about a week in summer before the root rots would take it out! In the rest of the southeast where they do grow, they definitely only grow on well drained soils. I have had one die from root rot. In spite of being hexaploid, they are not invincible. FWIW, there's a 'trick' about the PNW - coastal CA climate. I saw rhododendrons almost in the bottom of swales at the old Greer Gardens in Eugene, OR.* You would NEVER see them growing somewhere like that in PA, NJ, or MD. But...the time plants like rhodies and redwood are most susceptible to root rot organisms is in the summer. That's when most of those disease organisms grow best...at temps >= 15C. I see redwoods in the native environs growing in soil that appears year round damp, but guess what? It's only going to be inundated with water, during California's wet winters! Likewise with the rhodies that would experience wet (but not underwater, mind you) soil conditions at Greer...only in winter! There's just NO WAY for Eugene to have 3" of hot wet rain in late July, on a single day, as we can have! (or, now that I think about it, at Sonoma Horticultural Nursery, which is called something else these days, the same is true. Although Polo did put some rhodies on the lowest plains of his property in impromptu little raised beds. That might be necessary because he was so mild in winter, root rot organisms could still grow well during the wet season.) Now that a nursery has supposedly grafted Sequoia onto Metasequoia, maybe people in the southeast can plant them in poorly drained soils with abandon. We'll see. * - although, TBH, it wasn't much to see and not even as nice as the peak of NJ Rarefind around the time Hank died in 2009, I'm glad I got to see it in 2011 and meet the famous Mr. Greer. Greer might have had rarer, more tender cultivars in zone 8a, but Rarefind had just more plants, and better displayed, period, in their 6b garden.

Simple answer: sempervirens can be grown in the southeast and can take the heat. giganteum does not like high humidity heat, and grows much slower in east coast summers than it does out west with dry summers. Can be grown, but will not thrive and the amount of maintenance required usually makes it more of a specimen tree for arboretums and botanic gardens than anything else. Basically, you want to grow a tree in the redwood family requiring less maintenance and much more ease, and you live on the east coast, stick with cunninghamia, cryptomeria, taxodium or metasequoia (or go for the gold with glyptostrobus if you live in zones 8 and above). not sure about athrotaxis or taiwania on the east coast, any experience?