White Japanese Roof Iris gets its name from its presence on thatched roofs in Japan. Iris tectorum var. album has large white flowers with yellow throats, appearing in late April to early June. All the Japanese Roof Irises spread to make wide patches eventually, and do best with adequate moisture but good drainage. It is the Asian equivalent of Iris cristata 'Alba', but much bigger and not ephemeral. A hard-to-find Iris.