The choice of Irises by O'Keeffe was no accident. This particular flower has symbolic use in religion in a similar way to the Tree of Life, as used by artists like Gustav Klimt.
It is not believed likely that she was duplicating the symbolism from her Christian background, where it can be used to represent suffering and misfortune. Instead, it is probably just being used along the same lines as her other famous flower paintings such as Petunias and Oriental Poppies.
It's inclusion could therefore be purely due to it's bright inherent colours and beautiful detail which the artist would always place a great emphasis upon.
Female metaphors have long since been attributed to her work, but O'Keeffe always maintained that this was not the reason for her use of these items in her work. Symbolism can often be created by a viewer without the initial intention of the artist.
Irises, particularly black Irises were inspiring to Georgia over several years and are repeated across several paintings. She would be able to pick them up locally in New York at certain times of the year and then make use of them in her popular high detail artworks.