Under trees on the path from the honmaru of the East Gardens of the Imperial Palace toward Shiomi-zaka, small white flowers bloom in numbers. This is shaga. Flowers open at most about 40 cm high—not named shaga because you squat to see them—but look closely and white petals show cute blue and yellow markings. Written 射干 or 著莪 in kanji, also called butterfly flower.
Native to China, yet the scientific name is Iris japonica—"Japanese." It produces no seed and no bulbs; roots spread to multiply. Long ago it may have arrived from China potted with soil and spread from there. It grows in shade and propagates vigorously.
Yamabuki introduced on the 13th is Kerria japonica—also "japonica." Whether whoever planted East Gardens trees intended it is unknown, but "japonica" plants in the Imperial Palace at Japan's center may be natural enough.
Shaga also blooms in plantings at Sumitomo Corporation Takebashi Building just north of Palaceside Building.