Trachycarpus is a genus of roughly 8 species of fan palms from south-east Asia. It includes the Pacific Northwest's best known palm, T. fortunei, commonly known as "windmill palm". It is generally cold-hardy in all but the very coldest places west of the Cascades, tolerating temperatures down to around -17°C/0°F when mature (though with some damage). However, it is not widely known that T. fortunei is surpassed in beauty, elegance and potential ornamental use by some of the other species of Trachycarpus. These other species remain rare because some were recently discovered, or because seed has not been made available.
T. takil comes from the western Himalayas, and is possibly the best all-around species, being larger and more vigorous than the Chinese T. fortunei, and probably just a couple degrees more cold-hardy. It has stiffer leaves and is probably more suitable for exposed situations. T. wagnerianus, which has been confused with T. takil in California, also has very stiff leaves and is especially attractive when small. It is also a couple degrees hardier than T. fortunei and very useful in smaller spaces.
T. latisectus and T. martianus are two extremely attractive and tropical looking, large leafed, smooth trunked species from the lower Himalayas, but they are somewhat less hardy than T. fortunei and only really suitable for sheltered areas. Two forms of T. martianus exist: one from the Khasia Hills is probably not hardy at all north of Southwest Oregon; one from Nepal is more likely to be hardy but is not quite as robust. These species have not performed all that well in the Southeast USA, and the verdict is out on their ultimate hardiness.
T. nanus and T. princeps are two very rare species from small areas of south-west China. T. princeps was discovered only a few years ago and has leaves with stunning silvery-white undersides. T. nanus, an attractive dwarf species, has been known about for years, but only been made available recently. There is also T. oreophilus, native to the top of the highest mountain in northern Thailand, which was also discovered very recently, and T. 'wilailak' which looks pretty much like T. oreophilus but has a yellow stripe on the petiole. All of these palms may be a bit more difficult to accomodate in cultivation, as they seem somewhat tricky to grow for various reasons. There are also references to T. caespitosus, a mythical species that forms suckers, but as of yet no one has really been able to prove such a palm actually exists, though it is not out of the question.
Trachycarpus generally like part shade, and require lots of summer water. Too often T. fortunei is seen planted in full sun with less than the ideal amount of water, and it looks unattractive and often yellowish. Larger species such as T. takil are probably more suitable for planting in the sun in climates with dry summers, as is T. wagnerianus. And if they are given copious amount of summer water, T. fortunei can even look pretty good in full sun, sometimes. Trachycarpus (except T. wagnerianus) also does not perform well or look good in wind-exposed, coastal situations. Of course, Trachycarpus might also be tried east of the Cascades in very mild places, but they should not be expected to last through prolonged episodes of subzero temperatures.
My general estimate of Trachycarpus
T. takil: -20°C/-5°F
T. wagnerianus: -19°C/-3°F
T. fortunei: -20°C to -16°C/-5°F to 3°F depending on the seed origin.
T. nanus and T. princeps: not known, but probably similar to T. fortunei
T. oreophilus: not known, but probably just a bit less hardy than T. fortunei
T. latisectus: perhaps -11°C/12°F, though young plants have shown damage at much higher temperatures
T. martianus 'nepal form': probably around -11°C/12°F, but not really known
T. martianus 'khasia hills form': about -6°C/-21°F.