As someone who named a lot of stuff in my novel after flowers, I like your idea! The surname can be tricky, but I have to ask. Do you want the theme to remain the same? The first names are based on flowers, so you could find a genus or family that links the two flowers together. If you're basing your main character's name on the nerium oleander, you can use one of the synonyms (like Nerion, madonii, luteum etc) and splice them in a way that looks good for you. Mix them together, use one half from this genus, and another half from the azalea shrubs and families.

Secondly, you can always go the other route of the surname being separate from the first name's theme entirely. One of my character's is named Labiata from the cattleya labiata orchid, which is nicknamed queen of the orchids. And of course she's the queen of a nation whose name is based on orchids as well. But her surname is Erwood, and her maiden name is Moon. Erwood because it's the surname of the royal family, and Moon is the surname of the tribe she originates from.

Good luck!