I think it is interesting that the winters can be so mild so far inland. Here in Scandinavia the coastal areas can have mild winters but inland areas normally have harsh winters. In Germany there are many mountains that can create favourable local micro climates suitable for wine growing. Denmark and southern Sweden are quite flat and do not really have any mountains large enough to create good micro climates. Norway has more mountains and may have such areas. The most important factor that gives cities like Frankfurt am Main mild winters is the urban heat island effect. The Scandinavian cities are generally smaller but we can see the same effect in Copenhagen, where many exotic plants can grow.