What was Georgia O'Keeffe's first painting?

Here are five that I consider to be among her most well-known:

Cow's Skull with Calico Roses, 1931. Art Institute of Chicago.

City Night, 1926. Minneapolis Institute of Art.

Radiator Building, 1927. Fish University, Nashville.

Black Iris III, 1926. Metropolitan Museum of Art.

Black and White, 1930. Whitney Museum of Art.