Katsushika-ku Ward has iris gardens that includes Horikiri Iris Garden, the one with a history of hundred years, has depicted in Ukiyoe woodblock prints by renown artists such as Utagawa Hiroshige and Suzuki Harunobu.
Japanese Iris Festival (Katushika Shoubu Matsuri)
1st(Wed) to 20th(Mon) June 2016
This flower festival is held at Horikiri Shobuen (堀切菖蒲園) and Mizumoto-koen Park, both is a public garden and the admission is free.
The Horikiri Garden has 6000 of iris flowers and the garden in Mizumoto Park has 14000 of iris flowers
Official web page
Getting to Horikiri Shobuen Garden
15 min. Walk from Horikiri-shobuen Station 堀切菖蒲園駅 on the Keisei Line
Google Maps - Horikiri Shoubuen https://goo.gl/maps/jePst
Related article in digi-joho TOKYO