From MangaHelpers:
Sawamura Soushi is a soldier in a war-torn future. He meets a cute, innocent young girl named Nadeko, with whom he starts up a friendship. Nadeko is a flower girl, one of the girls who has a flower growing from her heart. One day, when Sawamura is trying to protect her from danger, she confesses her love to him and protects him with the plant growth that suddenly comes forth from her body.
Sawamura soon learns that there is a special squad of flower girls and their partners, and that their abilities are awakened when they fall in love. But does he really love a girl that is a decade his junior, and will he be willing to be a good partner for her? What's more, he learns that the awakening also starts the growing of the flowers in their bodies, so that the flower girl soldiers have only a short while longer to live. How can Sawamura and Nadeko make the best of such a terrible situation?