Arenga is a genus of attractive feather palms from southeast Asia. There are a number of species in the genus, some of which are grown in tropical and subtropical regions. A. engleri, a relatively small species, has been tried periodically by some hardy palm enthusiasts. It is hardy to about -5°C/23°F, or maybe just slightly lower, but is probably not hardy enough to be a viable long-term proposition in the Pacific Northwest. It is an attractive little palm however, coming from Taiwan, and does not seem to suffer from the lack of heat in our climate. Arenga tremula is also said to be somewhat hardy.
Recently a species has been discovered in Bhutan and southeast Tibet called A. micrantha. It is a clustering species, forming multiple trunks and growing to a modest height, but nevertheless impressive. Its hardiness is not yet known in cultivation, but it ought to be somewhat hardy since it experiences frosts down to -6°C/21°F in its native habitat. Because of its clustering habit, it is possible that this palm could recover from the base if the entire top froze, which would be advantageous in places that experience many consecutive mild winters between hard freezes. It should be tried in sheltered microclimates in the Pacific Northwest.
A general comment on Arengas can be made that the species which have been tested in cultivation are somewhat hardier and more cool-tolerant than might be expected based upon where they are native. Considering A. micrantha, this could mean that A. micrantha also shares this characteristic and is going to be quite hardy, or it could mean that A. micrantha is the one species that grows at the climatic limit for this genus and is not much hardier than A. engleri. Only time will tell. The recent discovery of A. micrantha also suggests that there may be more Arenga species that have not been discovered yet in Asia - who knows, there may be one that is even more cold hardy?
Arenga seed requires heat above room temperature to germinate well. Seedlings are extremely succeptible to spider mites, and should not be kept in a dry environment for long periods without very careful attention.
I would rank Arenga hardiness
roughly as follows:
A. micrantha: could be anywhere from -6°C/21°F to somewhat lower, perhaps -9°C/16°F
A. engleri: -5°C/23°F
A. tremula: -2°C/28°F