The Poisonwood Bible has numerous symbolic elements. To describe them all would take many pages. I will list and describe some of the more noteworthy ones below.
The Poisonwood Tree
This noxious plant lends its name to the novel itself. We first encounter this tree when Nathan is attempting to plant his garden. Although he is cautioned not to touch it, he ignores these warnings and suffered a painful rash for his stubbornness. In this sense, the poisonwood tree represents the Prices' ignorance of their new land.
Poisonwood also has linguistic symbolism. Depending on the particular inflection that the speaker uses, bangala means "beloved" or "poisonwood." In his sermons, Nathan repeatedly calls Jesus bangala thinking he is calling his god beloved. However, his inability and unwillingness to master the local language causes him to call Jesus poison. This turns out to be unintentionally appropriate though. To Nathan, the stubborn adherence to his old ways turns out to be poison and leads to the ruin of him and his family.
Methuselah is the African gray parrot that the Price family inherits when they move into their house on Kilanga. While living in a cage, this intelligent bird has learned to mimic the voices of the people around him. Unfortunately for Methusualh, he has lived in a cage for so long that when he is finally set free he is unable to fly. It is not long after his release before he is caught and killed by a predator. Methuselah is a symbol of the Congo. During its time as a colony of Belgium, the people of the Congo have learned to imitate their colonizers to a certain degree. However, when finally set free and given independence, they are unable to fend for themselves because they have been kept under the control of others for so long. Eventually, like the old bird, the Congo succumbs to the predatory nature of those that would exploit its weakness.
The Demonstration Garden
One of the first things that Nathan does in the Congo is plant a garden. He has brought with him seeds from America that he hopes will thrive in Africa. He wants his garden to show the people of Kilanga a better way to grow food. This garden is a symbol for Nathan's hopes. Just like the Prices themselves, the plants in the garden are unable to thrive. They are so out of place in Africa that the insects do not recognize their flowers and do not pollinate them. The plants that Nathan tends are unable to bear fruit, just like his hopes. This garden is a symbol of how merely transplanting one's notions to a new environment will not lead to success.
When the village is overrun by biting ants, everyone flees in a panic to the river. The ants, while dangerous and painful, are not evil. Anatole explains to Leah that the ants just are what they are. They are a force of nature, but they are not malevolent.
However, they bring out the true nature of the Prices. When the ants arrive, Nathan takes to fire and brimstone preaching. Rachel knocks others out of the way to get herself to safety. Orleanna desperately tries to save her youngest child to the neglect of the others. Adah cries out in helplessness and blames others. In this way, the swarm of ants is a symbol of how people get swept up in events much larger than themselves. As such, they are symbolic of the greater turmoil of the Congo as it becomes an independent nation.