The lines use personification and help readers visualize the shape and color of the flowers.
Personification is giving non-human things human-like traits. A simile is a comparison between two things using like or as. A metaphor is a comparison between two things without using like or as. Since nothing is being compared, and there's no like or as, the options which include simile and metaphor should be eliminated. The purpose of the personification is not just to explain which flowers boom in the summer. It's to help the readers visualize the flowers. The lilies are described as bowing to show how they bend over. The poppies flaming help the reader to picture the brilliant red color of the flowers.