Throughout A Tree Grows in Brooklyn, trees emerge as a prominent allegory for not only Francie's life, but the lives of all those around her living in Williamsburg's tenements. The story of the tree is one of strife and struggle, growing in an inhospitable land. Francie, too, must grow under inhospitable circumstances, as Katie emphasizes in the following excerpt from Chapter 10:
It's not better to die. Who wants to die? Everything struggles to live. Look at that tree growing up there out of that grating. It gets no sun, and water only when it rains. It's growing out of sour earth. And it's strong because its hard struggle to live is making it strong. My children will be strong that way.
In this passage, Katie talks to her mother about Francie, who is a frail child during the first few years of her life. The story of the tree growing in the tenement is an allegory for Francie's life. She, too, continues to grow in spite of her circumstances, in spite of setbacks and loss and disillusionment.
Like the tree, Francie also grows where she is not wanted. Many people consider both the tree and Francie a weed: Miss Briggs, the tenement doctor, etc. Such people do not want Francie to endure, but she does so regardless of their ignorance.