Manjushage 曼珠沙華, or Spider Lilies, bloom in late September to early October. They can be spotted in small numbers in gardens and green areas around Tokyo, and are quite beautiful, but are spectacular en masse.
Kinchakuda 巾着田 is about one hour out from Ikebukuro on the Seibu Ikebukuro line and the Seibu Chichibu line, and is accessible from Koma Station 高麗駅. It is a flower field on the inside of a river that almost curves around 360 degrees, and has over a million blooms in red and a few in white. It also has a small field of Cosmos コスモス that bloom around the same time, and a number of hikes and walks in the area. Entrance to the park is 200yen. More details about Kinchakuda 巾着田 can be found on the official website (Japanese only).
A guide to hiking the nearby Mt Hiwata and Koma Pass is also coming soon.
Here are a few photos from my visit to the field on October 2nd 2010.
For more photos of Manjushage check out my set at Flickr!