I've been searching the forum for hard information on this subject -- ideally from gardeners speaking from their own hands-on experience in colder climate zones -- and find that the data pool is limited, to say the least. One finds assertions to the effect that "XXX is not hardy beyond zone 6," but these seem to come mostly from books or from gardeners in warmer climates. (And I mean no disrespect to these kind gardeners who are trying to be as helpful as they can.) I wonder, though, given the hundreds of cultivars out there, how many have actually been tested in zone 5 (or colder) conditions? Of those that have been tested (mostly ubiquitous types like 'Bloodgood'), many do turn out to be hardy. Which gives us reason to hope that many others will turn out likewise. This thread, for instance, contains fascinating information from Maple Society member Daniel Otis, who gardens in Ithaca, New York: This is the kind of thing we need more of. (Not just the post, but the gardeners intrepid or insane enough to plunk things in the chilly northern soil, work their personal magic on them, and see what happens.) Accordingly, I'll be reporting next year on my experience attempting to grow the following outdoors in zone 5b, coastal Maine -- listed roughly in declining order of reliable information I could find to suggest that the plant should succeed here: - Acer triflorum - A. shirasawanum 'Autumn Moon' - A. s. 'Moonrise' - A. p. 'Katsura' - A. x 'Johin' (a palmatum/shirasawanum hybrid) - A. japonica 'Aconitifolium' - A. palmatum 'Yezo nishiki' - A. p. 'Purple Ghost' - A. p. 'Ariadne' (a variety originating in Belgium, I think) I'm hoping other cold-climate gardeners will report their own successes (and failures, though fewer of these, we trust) as time goes by. As a general thought, I'd add that from my experience in growing other kinds of plants (including bamboos) in Maine for 20 years, I've found two things: - Hardiness ratings in reference books and online sources tend to be reliable most of the time, but not always -- and they are often contradictory. - Other factors beyond temperature alone -- for instance drainage, exposure to winter wind and sun, what kind of summer the plant just experienced, and of course the skill of the gardener -- strongly affect survival in challenging environments.