May - My Favourite Month
When people think about flowers in Japan they almost always think about cherry blossoms in very early spring. While these are quite beautiful, I think the middle of spring offers a much larger variety of beautiful flowers to see. May is especially wonderful, and the weather is more comfortable than April as well!
At the beginning of May you can see westeria flowers, or "Fuji" in Japanese. The most famous place to see them is Ashikaga Flower Park in Tochigi, but Kasukabe in Saitama is another fantastic area to see them! Kasukabe has a 1200-year-old westeria tree, and the city holds a westeria festival every year with a lot of dances in the main street. I saw Westeria at Oyakuen in Aizu-Wakamatsu this year, though.
If you don't have the chance to leave Tokyo, you can see many pretty flowers here! There is a big rose garden in Shinjuku Gyoen, and a smaller one at the Kyu-Furukawa Gardens near Komagome station. Those places cost money to though, though, so if you would like to see roses for free you should visit Otsuka station. There are many roses along the Toden-Arakawa line heading toward Waseda University! You can also see many hydrangea, or "Ajisai" in Japanese, all around Tokyo. They are everywhere! Just take a walk and I'm sure you will find some in no time. ^^