The prevailing symbol of the wisteria vine and the information concerning this plant forms the forever appearing metaphor of the Bean Trees. The wisteria vine grows in seemingly infertile soil due to its obligated mutualism with rhizobia; a bacteria that lives within the plant roots and fertilizes the soil with nitrogen. Barbara Kingsolver relates this to the various symbiotic relationships throughout the novel. There are certainly many examples of this: Virgie Mae Parsons and Edna Poppy, Lou and Taylor, and Estevan and Esperanza. The effect of these symbiotic relationships is that it instills a sense of community through the novel by creating a network of need and fulfillment among the many characters, which sustain and promote one another.
A symbiotic relationship is clearly seen with Virgie Mae Parsons and Edna Poppy. These two elderly and possibly widowed women live in the home next to Lou Ann and Taylor. In their relationship, Virgie Mae provides guidance and assistance to Edna, who is blind. While at the same time, Edna Poppy serves as a buffer against Virgie Mae's often-abrasive manner. The commentary following Edna's response to Lou Ann is a prime example: "Were fine," "Edna said, obviously accustomed to being Virgie Mae's public-relations department" (107). Another example occurs when Taylor realizes that Edna is blind and it was truly Virgie Mae who found the Edna's red bobby pins at the drugstore for her companion (145). This two women living together mutualistically both provide one another with something that they do not have and cannot live without.
Most of the relationships of the novel are symbiotic yet to some degree reciprocal. This is true for Lou Ann, who provides Taylor with a perspective on her abilities, while Taylor calms Lou Ann's neuroses and instills her with a sense of confidence. When Taylor and Esteban go swimming, Taylor's confidence in Lou Ann is portrayed, "I know I can depend on you, Lou Ann, if we sink, you"ll pull us out" (94).