The parrot left by Brother Fowles serves as a symbol for the doomed Republic of Congo. Methuselah is denied freedom for most of his life, and while he is kept in a cage and fed by his masters, he loses the ability to fend for himself. Even after Nathan liberates him, Methuselah continues to stay close to the house he has always known, dependent on humans for his food. He even sleeps in their latrine at night, for fear of predators. Inevitably, the vulnerable Methuselah is ultimately caught by a civet cat, meeting his doom on the same day that the Republic of Congo begins its own short-lived independence. Within a few months the equally vulnerable nation will also be set upon by a predator, the United States, and killed.
Nathan's demonstration garden serves as a wonderful symbol on three levels. First the garden itself can be seen as a stand-in for the attitudes and beliefs that the Prices carry with them into Africa. Like those attitudes, the plants are wildly inappropriate in this environment. The plants become unrecognizable, almost grotesque in their hugeness. More significantly, though, they are rendered inert and useless, unable to fruit. The indigenous North American plants cannot vegetate under the conditions in Africa.
The very act of planting the garden rests upon one of the wildly inappropriate attitudes that the Prices carry with them. The venture reveals Nathan's blind arrogance, the belief that the Congolese are so backward that they have no idea how to grow their own food. It is beyond Nathan's capacity to reason that, if the climate permitted this sort of garden, Africans would have planted it themselves long ago. It does not occur to him to consider whether there is some reason, other than their utter stupidity and backwardness, which might account for the fact that there is little agriculture in Kilanga.
Finally, the garden is symbolic because of its biblical resonance. Gardens, in particular the Garden of Eden, play a prominent role in Christian tradition. It is in the Garden of Eden that Adam and Eve, the original man and woman, ate from the Tree of Knowledge thereby imbuing all future generations of human being with original sin. There is a clear irony in drawing a connection between Nathan's undertaking and Adam and Eve's. Adam and Eve sin by grasping for truth and knowledge that is not intended for them. Nathan, by contrast, sins through his willful ignorance, through his refusal to learn anything about the culture around him, and to enlarge and deepen his understanding of the world.
Nathan first encounters the Poisonwood tree while planting his demonstration garden. Mama Tataba warns him not to touch the dangerous plant, but he contemptuously ignores her and ends up with painfully swollen arms and hands. The Poisonwood's primary role in the story, though, is in the form of a linguistic accident. In the native language the word "bangala" can mean "dearly beloved" if spoken slowly, or else "Poisonwood Tree" if spoken quickly. Unable to grasp this subtle linguistic distinction Nathan preaches week after week that Jesus is the local tree that can cause intense pain and even death. His thickheaded mistake is a result of his general inability or unwillingness to learn anything about the culture around him, a symptom of his general cultural arrogance. The mistaken phrase, however, is actually quite apt as a description of Jesus, at least in the hands of men like Nathan. That is, men who are so thickheaded and culturally arrogant that they could consistently make a mistake like this one for months on end. In such a context, Jesus really is a dangerous poison that can and did cause intense pain and even death.