The large, fully-grown tree is a durable, strong, and amazing feat that many do not accomplish. Sethe is a strong woman, which is represented by the size her tree has grown. But there is also a vulnerability to it, as the tree has a large gap from which sap emerges. Sethe's past is like the sap; it is her one weakness in her steely exterior. Her suffering, the experiences her race had to endure, and her child's blood make up the redness within her.
Or perhaps the tree represents a woman broken by the institution of slavery. The tree shows effects of having been beaten to the point where one can no longer feel physically or mentally, driving a woman so far past sanity that she tries to kill her young. The seemingly innocent white blossoms filled with good intentions turn into bloody acts of evil coming from an unsound mind.