He is best known for his work as the chief ceramics decorator at Rookwood Pottery, and for his watercolor paintings of botanical subjects.

In 1908, he accepted a commission from philanthropist Ellen Browning Scripps to illustrate the botanical diversity of California.

Over the next ten years, he produced approximately 1200 watercolor "plant portraits" of native California wildflowers, grasses, ferns, and trees.

[2][4] In 1903, the Valentiens visited Southern California, staying several months with Anna's brother in Dulzura, a small community southeast of San Diego.

Valentien worked on the project for ten years, and the scope of botanical subjects grew to encompass native grasses, ferns, and trees.