The theme of this book is wherever you go, where you've been shows who you are. This becomes apparent because Sumiko travels and leaves a "home" two times over the course of the story. Both times Sumiko brought the lessons she learned with her to the places she went. These brought her joy and helped her to navigate through hard times.
In the beginning of the story, Weed flower, Sumiko is forced to burn the only item she has left of her parents, a photograph. She is devastated at the loss of this believing that the memories of her parents will disappear. However, Sumiko later realizes that her parents have been preserved in her mind as if, "they were in stained in panes of glass." So her past places reflected her ability to remember her parents.
" Dear Sumiko, We don't deserve real letters?.. You must be very busy...... He hopes you have started a garden that would make him proud as you've always wished. "
After Sumkio arrived at the imprisiment camp, she became depressed at the predicament she faced. After several letters from her grandfather she reliazed that just becasue she had went somewhere new didn't mean she had to forget her dreams and wishes from back home.
At the end of Weed Flower, Sumiko promises herself that she woludn't forget the people she left behind. They had made her braver and stronger so she would take those virtues with her. For example, " 'I have to write to Frank, Bull, Ichiro, Jiichan, and Uncle'..... This is what it felt like to leave camp.... Like losing a friend. Like you might...maybe be able to own a flower shop one day. "
So, you can clearly see that the themem of this book is wherever you go where you've been shows who you are. With all Sumiko's adapting to new places she kept the old lessons and memories she learned at old places with her. They helped her to survuve in her changing landscape.