What does the title Killers of the Flower Moon mean?

On the first page of Chapter 1, "The Vanishing," in Killers of the Flower Moon: The Osage Murders and the Birth of the FBI, author David Gann explains the story behind the title of the book.

In April, millions of tiny flowers spread over the blackjack hills and vast prairies in the Osage territory of Oklahoma. There are Johnny-jump-ups and spring beauties and little bluets. The Osage writer John Joseph Mathews observed that the galaxy of petals makes it look as if the "gods had left confetti." In May, when coyotes howl beneath an unnervingly large moon, taller 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.

In the late 1800s, the Osage Indians were driven from their ancestral home in Kansas to land in northeastern Oklahoma—land that was considered essentially worthless.
The Osage negotiated with the United States government for the oil, gas, coal and mineral rights to the land, and in the early 1920s, one of the largest oil deposits in the United States was discovered under the Osage land. This meant that oil companies and oil prospectors had to pay the Osage for drilling rights, and as a result, the Osage became the wealthiest people per capita in the world at the time.
By 1925, when the FBI stepped in to investigate corruption associated with the mismanagement of the Osage oil rights, over 20 Osage had died under questionable circumstances.
The story of the taller plants overgrowing and choking off the light and water of the smaller spring flowers and eventually killing them at the time of the flower-killing moon is a metaphor for how the Osage Indians were overrun, victimized, and killed by people who swarmed over their land in ruthless pursuit of power and wealth.