I would have thought there would be a difference too, I lived on the south coast of England and that strip along the coast was a zone warmer than further inland so I was on the border between 9a and 9b. Sooo, I found this hardiness zone map of the Netherlands, and it says The Hague is 9a
http://www.plantmaps.com/inter...
Oleanders are surprisingly tough, I have one here but of course it comes in for the winter - not until it starts getting below about -5C at night though.
