The word fugacious deals with emotions and interestingly also to the idea of withering leaves in botany. Fugacious derives from the Latin verb fugere or flee. Some derivative words include fugitive, refuge, and subterfuge.
A beautiful word to use in literature:
“Love is a fugacious word. Rounded and comfortable, it lifts the tongue and fills the back of the throat, before slipping beyond reach as the sound is exhaled from the mouth. Yet the word eludes meaning. Love teeters on the edge of the unknown beyond which it becomes almost impossible to speak. It moves us beyond words. We speak about love when we define our longing and desire and yet we fall into silence when we attempt to speak about it in the present.”
― Jonathan Rutherford, I Am No Longer Myself Without You: An Anatomy of Love