Would you be willing to quantify what "mild" means to you, maybe in terms of a USDA zone? There are maps for both US and Europe:
https://planthardiness.ars.usda.gov/pages/map-downloads
https://commons.wikimedia.org/wiki/File:USDA_hardiness_zones...
https://planthardiness.ars.usda.gov/pages/map-downloads
https://commons.wikimedia.org/wiki/File:USDA_hardiness_zones...