Could be time related. Going by appearances she may have been aging stuff to make it like the flowers wilting, or taking time away from it in the case of returning it to seeds. I may have been more directly related to plant control too, we never saw her power actually used on another person or even animal. The villagers could have been jumping to conclusions thinking it would even be capable of harming them.
@Fuwasagi@Glomoro Shiro's power is explained in the afterword (which is on page 31):
Shiro, a girl about 10 years old. In the story her power was said to be "taking life," but "speeding up or stopping the progression of time" is what her power is actually doing. Spring never came because the shock from her sister's death caused her to unconsciously stop it.
Classic OneeLoli from Hachi, love it. We're left with tons of questions and need of more development with these two as a couple, but I enjoyed it anyways. 18 years gap is really something, tho, even for Itou Hachi-