It’s beautiful all the time because they grow different flowers each season which suits the climate. The park is especially gorgeous when it’s in autumn. They have unique flower/tree which other places don’t have.
It's an all day affair, and well worth the admission. Try going in the fall when the plants are at their peak reddish color. The colors are very surreal. I'll return this year. Depending on where you live in Tokyo, it will take you about a 1 to 2 hour drive.