Hydrangea were first introduced to Europe around 1862 by the German naturalist Siebold (フォン・シーボルト 1796-1866).
It is said he selected the flower's scientific name 'Hydrangea Otaksa' in memory of his Japanese mistress Kusumoto Taki (楠本滝/'Otaki-san' お滝).
On a side note...Siebold and Otaki-san's daughter, Kusumoto Ine (楠本イネ 1827-1903), was the first female doctor of Western medicine in Japan.