I believed Hemerocallis citrina thunbergii to be another subspecies of Hemerocallis citrina. Hemerocallis citrina thunbergii looks very similar to Hemerocallis citrina vespertina but blooms much later. Because of this and other information I have seen it is possible that Hemerocallis citrina thunbergii is just a later variant of Hemerocallis citrina vespertina.