Each year from around early June through to early July, the grounds of Ibaraki's Nihonmatsu-ji Temple are decorated with around 10,000 blooming hydrangeas. In total there are approximately 70 different species of the flower to enjoy over an area of 40,000 square meters - it's no wonder that the temple precinct is referred to as the Nihonmatsu-ji Hydrangea Forest!
As the flowering situation can differ from year to year, it's advisable to check the temple's official social media accounts before your visit. They are updated regularly with the blooming conditions, so you can schedule a trip when the flowers are looking their best. You can find their Facebook here and their Instagram here.
Entrance to the temple grounds during hydrangea season is priced at 300 yen.
My other favorite hydrangea spot is also in Ibaraki Prefecture - https://en.japantravel.com/ibaraki/amabiki-kannon-temple/43754