The opening paragraph of the book describes how small flowers spring up in April in Osage territory, only to be eclipsed by larger plants in May, when a large moon appears in the sky:

[T]aller plants, such as spiderworts and black-eyed Susans, begin to creep over the tinier blooms, stealing their light and water. The necks of the smaller flowers break and their petals flutter away, and before long they are buried underground. This is why the Osage Indians refer to May as the time of the flower-killing moon (5).

This ominous-sounding natural phenomenon becomes a symbol for Grann. Its meaning is sometimes literal—Anna Burkhart was killed in May, so the term gives Grann a way of keeping time based on Osage traditions rather than white ones. At the same time, he uses the flower replacement process as a metaphor for what happened to the Osage people at the hands of their white neighbors: Under the light of the flower-killing moon, Osage resources were stolen by the more powerful white citizens of Oklahoma, who killed the first residents of the continent to get access to their wealth.