Gaikotsu Ou to Migawari no Oujo - Luna to Okubyou na Ousama

Luna is the only girl who can communicate with flowers in the kingdom that benefits from the special power that flowers possess. One day, however, a turning point comes and she is able to work for the "royal florist" whom she has always dreamed of!　However, she is unlucky enough to get caught up in a kingdom dispute and is ordered to take the place of the princess, and is sent as a "hostage" to the dreaded king of the land of the dead, where flowers do not bloom.

Note: Synopsis from Mangadex.