Much like the red rose bush outside the foreboding, great grey door of the prison in the Puritan community of Nathaniel Hawthorne's "The Scarlet Letter," the red geraniums bloom in the corner of the junk-filled dismal yard of the Ewells. When little Pearl of "The Scarlet Letter" responds to the Reverend Wilson's question of who made her by telling him that she came from the rose bush outside the prison, the implication is that beauty and goodness can issue from prison/sin as Pearl has been born of Hester Prynne. Likewise, when the red geraniums planted in slop jars along the remains of a Model-T Ford, a discarded dentist car, an ancient icebox, and other junk, these flowers symbolize that beauty of soul, the predisposition to goodness, exists in everyone. And, the red color indicates that there is also passion. Like Mayella, these plants are trapped in a junk yard and exist in a corrupted environment; however, the passion and inward beauty are still present.
While Harper Lee seems to indicate that beauty and goodness can exist anywhere, another interpretation of the geraniums is that they symbolize that goodness can exist even in a corrupt society. So, the geraniums are symbolic of Atticus, who retains his integrity in the midst of a corrupt society.
In the book "To Kill a Mockingbird" Harper Lee used several sets of flowers as symbols. Mayelle Ewell lived in a dump. She had no friends and her environment was a pathetic view. The Ewell's were considered to be a bad lot. They were poor and their father drank. They had no money and did not attend school.
There are two things that the flowers could symbolize;
They could represent Mayella's attempt to find and have some beauty in her life. They also serve to show that no matter how bad a person or their environment is, there is some good there. Atticus tried to teach this lesson to his children throughout the book.