As you know Cherry blossoms represent Japanese spring, Hydrangea is also a very popular flower in the rainy season of Japan. The rainy season is in June and beginning of July here, and it is so popular to see Hydrangeas blooming beautifully in gardens. The language of flowers of it is "fickleness" since the colour of the flower changes while it is blooming. The parts that look like petals are actually sepals, and real petals are in the centre and don't stand. The origin species of Hydrangea is from Japan but it looks different from what is popular these days. Hydrangeas' colours depend on the pH of the ground they are. If the pH is acid, the blue colour is strong, but if it is alkalinity, the colour becomes much more purple.
I don't like the rainy season here because it is so dump and not comfortable to go out, however, the season is so important for us because it is necessary to grow rice. And however I feel so uncomfortable about this season, not everything is bad since I can see pretty Hydrangea flowers. If you come to Japan in this season, you should definitely see Hydrangeas. It is not as popular as cherry blossoms for foreigner, but I believe that it is also one of the most beautiful flowers that you can see generally in Japan.