I watched Kokia for the first time in my life when I visited Hitachinaka National Garden in Ibaraki. Kokia is called Houkigusa, its color changes from green to red at Autumn. Its red color is very beautiful and fantasy. I could not believe it, although I hard it was look like a red carpet.
About Hitachinaka National Garden: http://www.hitachikaihin.jp/
There were Cosmos garden at Hitachinaka National Garden, I could enjoy to watch many Cosmos.