Zinnias, elephant ears, sticks, and trees—these are all forms of vegetation. While they are here partly as decoration (they are details that make the poem seem like an accurate description of an actual place), each one also represents something different in its way. The tree in the final stanza, for example, serves as a crutch for the woman and reminds us that in many ways she cannot stand on our own (she is still haunted by her past). The tree also reminds us of the stick the woman uses to whip the boy and is thus also a reminder of the violence in the poem. And you thought all these bushes were just hanging around for no reason.
Line 5: The boy crashes through elephant ears, a plant with huge leaves that you can see right here. A huge plant like this in a house? It sure makes the house seem more like a jungle, a primitive, wild place where a hunter (the woman) pursues her prey (the boy).
Line 6: At first, it sounds like dusty zinnias are coming out of the boy's mouth as he pleads. What these lines really mean is that the boy is hiding behind the zinnias and begging the woman not to hit him. The zinnias symbolize beauty and life in a poem that is full of violence, pain, and metaphorical killing
Lines 9-10: Sticks are made of wood, and wood comes from trees, and trees are vegetation. The natural world (the wood of the tree) has been recruited to be a part of the woman's violent campaign against the boy.
Lines 21-22: The woman leans against a tree, an image that tells us a lot about her. The woman is haunted by her past, and is not in control of her anger and emotions. This image symbolizes the fact that she cannot stand on her own.