The Koishikawa Botanical Gardens(小石川植物園) are botanical gardens operated by the University of Tokyo, Graduate School of Science (coincidentally, where I work).
They are located in Bunkyo ward in central Tokyo, and are open daily except Mondays. Entrance is ¥330 (£2.15), and one should note that the admission fee is paid in the small convenience store across the road, in exchange for a lovely coupon.
The gardens date to 1684, when the 5th Tokugawa shogun, Tsunayoshi, established the Koishikawa Medicinal Herb Garden. In 1877, after the Meiji Restoration, they became a part of the university and the birthplace of Japanese botanical research.
The gardens' collections contain some 4,000 plant species, including 1,400 hardy woody species, 1,500 hardy herbaceous species, and 1,100 tropical and subtropical species. The main attraction at this time of year is the numerous species of rhododendron - or tsutsuji in Japanese.
... and it wasn't just us tourists interested in them!