The flowers are a symbol of esteem and gentility. This is very telling because by keeping these flowers it shows that Mayella wants a more genteel, more pleasing lifestyle. The geraniums repersent that dream, her dream for something better in life, her dream of wanting something better for herself. This shows a lot about her character because it is showing that she strives to be better and she wants something in her life and now all that she has to do is go out and find what she is looking for. Make that dream become a reality and use those red geraniums as motivation; as a reminder to keep going and maybe everything will work out.
Hope this helped :)