Rightly or wrongly, Japanese maples often listed for USDA 5 by American sources. Northern European conditions with cooler summers apparently tend to make maples one zone less hardy, because Van Gelderens give the same split zone designations between Europe and elsewhere throughout Maples for Gardens (for all maples, not just Japanese).

Commercial sources may also be optimistic in the hope of selling more units. And less experienced gardeners may base conclusions on shorter terms rather than longer. Both can also misinterpret the zoning system and therefore assign the wrong zone to their location (and their surviving plants). The common mistake is to place themselves in a zone based on the lowest temperatures they have seen there rather than the average range of yearly lows for their site or vicinity.

Be carefull in using those maps and temperature as the only thing that will determine hardiness. An acer palmatum on the side of my house protected from wind and sun in the winter will do much better in very cold weather than the same plant fully exposed in a windy area lacking snow cover. David

However, all plants do have a minimum temperature below which they die.