Large Wild Iris
The Large Wild Iris is native to Africa and is found frequently in the US. It has large white blossoms marked with yellow nectar guides and outer tepals (outer part of the flowers) and violet central segments. It is a perennial plant that grows up to 5 feet in large clumps and it blooms abundantly during the summer. (Thanks to the Chicago Botanic Garden.)
From the PlantzAfrica website,
The flowers are borne in mass at certain periods – often after rain in summer. The individual flowers do not last more than a couple of days (so are of no use in a vase) however, the plant bears so many flowers during the peak period that the plant looks most striking. The flowers attract lots of bees and other pollinators.
The four Dietes
The Wild Iris (Dietes grandiflora), is very similar to the African Iris (Dietes iridioides) except the African Iris is a smaller plant with smaller blooms that only last one day. The grandiflora has dark spots at the base of the outer tepals, and the blooms last for three days. A third plant, the Butterfly Iris (Dietes bicolor) is also very similar but has cream or yellow flowers where the first two have white flowers. (Thanks to In The Zone.) Unfortunately, I do not know how to distinguish the Dietes robinsoniana.
For more information
The websites linked above have an abundance of information about these plants. The Pacific Bulb Society has some additional information.
(Click to enlarge)