The main theme could be isolation. The geranium might symbolize something that the character is attached to in a lonely world.
Well, it might be about the passage of time. The geranium could be used to show how time affects the character and their surroundings. For example, as the geranium grows or withers, it reflects the character's own journey through time. They might be reminiscing about the past or fearing the future, and the geranium is a constant presence in their changing world.