Dypsis is a large genus of single-trunked and clustering feather palms from Madagascar. It includes D. lutescens, a well-known clustering species often used as a patio plant in milder Mediterranean climates, and D. decaryi, the triangle palm. Both of these can only tolerate a few degrees of frost and do not stand a chance outdoors in the Pacific Northwest.
D. decipiens and D. onilahensis grow at high elevations in Madagascar and are quite a bit more cold hardy. D. decipiens, a magnificent, single-trunked species, especially has a reputation for being very cold hardy in California, supposedly tolerating temperatures down to -10°C/14°F without damage, but it does not always turn out to be quite that hardy and these reports are to be questioned. D. onilahensis, a clustering species, is less well known, but not likely to be any hardier than D. decipiens. Both of these species are very xeric and should only be grown on very gritty/gravelly soils in climates where their hardiness is marginal. D. baronii may be the next hardiest species, and is much easier to grow, since it originates in rainforest and is not xeric.
The verdict is still out on the hardiness of these Dypsis species, but they are certainly very much a gamble and should only be tried in very sheltered places, with some winter protection for young plants.