This plant is one of the crested (or Evansia) types of Iris - so-called because they have a 'crest' or ridge on their three larger outer petals (or 'falls') instead of a 'beard'. Many of these crested types come from Japan and China, and in general, they grow very well here. Instead of wanting baking sun in summer and cold winters like the Mediterranean types of Iris, they are denizens of a semi-shaded woodland setting and are generally not as frost hardy as other Iris species. Rhizomatous Iris wattii has large frilled flowers in August and September, coloured a lovely lavender blue and carried on bamboo-like canes that can be 1-2 m tall. The foliage grows in fans, which multiply quite quickly to form a good clump.
The plants will cope with quite dry, neglected conditions; however, they will do better if they are given some moisture, a yearly feed and a blanket of mulch every so often. They are said to dislike lime and poor drainage. They can be divided every few years. They grow well under trees or shrubs. They would probably grow well in a pot. I have one clump growing nearby a pale pink miniature Camellia and bright blue-spired Plectranthus barbatus; another clump has companions of the pale green saucer flowers of Helleborus argutifolius and the dainty lime-green bells of Nicotiana langsdorffii. A third clump grows near cream-coloured Clivia miniata and shrubby Eranthemum pulchellum, which has flowers of the same soft blue as this Iris at the same time of year, and also enjoys a shaded spot in the garden.