The gardens have 274 different types of 5100 roses, bloom in spring and autumn. As this festival is so popular, it should be better to visit in the morning so you can avoid crowds. During the festival, the garden place opens at 8:30AM (8:00 on Sundays)

Autumn Rose Festival at Jindai Botanical Gardens
神代植物園「秋のバラフェスタ」

October 7th(Sat) - 31st(Sun) 2017
Location: Jindai Botanical Gardens, Jindaiji, Tokyo

Information by Tokyo Metropolitan Government (Japanese website)
http://www.metro.tokyo.jp/tosei/hodohappyo/press/2017/07/19/03_01.html

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