One primary underlining theme in Petals of Blood is the failure of the ruling Kenyan elite to adequately meet the needs of the people. After the new postcolonial governments come to power, the leaders maintain their connections with the outgoing colonizers, thus marginalizing the everyman. In the novel, the elite are portrayed as both government officials and businessmen who violate the villagers of Ilmorog in both passive and aggressive ways. The corrupt system acts like a chain—in the novel, when the government’s lawyers declare that they have solved the murder cases, the people of Ilmorog realize that as long as the corrupt system stays in place and continues churning out corrupt individuals, there will be no change.
Ngugi makes the dichotomy between the villagers (the honest working class) and the elite (corruption) most visible in the speech that Nyakinyua gives before the villagers, which motivates them to make the trip to Nairobi. She says, “I think we should go. It is our turn to make things happen. There was a time when things happened the way we in Ilmorog wanted them to happen. We had power over the movement of our limbs. We made up our own words and sang them and we danced to them. But there came a time when this power was taken from us.... We must surround the city and demand back our share” (pp. 115–116). However, along their way, they are unjustly detained by Kimeria the businessman, who reveals that he is colluding with the MP, and who afterwards rapes Wanja.
Capitalism is decried in Petals of Blood, with the new Kenyan elite portrayed as controlled by the 'faceless system of capitalism'. The everyman loses out to capitalist endeavours, and is essentially exploited by the new Kenyan elite. Farmers are forced to mark out their lands and mortgage them with loans linked to the success of their harvest; as the quality of the harvests waver, many are forced to sell their land, unable to match their loan repayments. Thang'eta is another symbol of capitalism. Taken from a drink that Nyakinyua brews in a traditional ceremony, it is soon marketed, and becomes extremely popular. Wanja, who introduces the drink to Abdulla's bar, is then exploited by big business who forces her to stop her Thang'eta operation. Neither she nor Munira, who creates the slogan, receive the fruits of their labour. Originally a drink used to help people relax and escape their current problems, it becomes 'a drink of strife'.
Cities are portrayed as places where capitalism flourishs and are contrasted strongly with the village of Ilmorog. In its pursuit for the modern, Kenya adopts capitalism at the expense of tradition as the city begins 'to encroach upon and finally swallow the traditional and the rural.' As time progresses, Ilmorog changes vastly, as do the people that inhabit it. With its modernization, influenced greatly by capitalism and the chance to increase trade, Munira reflects on these changes and how they link with capitalism, saying that 'it was New Kenya. It was New Ilmorog. Nothing was free.'
Agriculture is an important theme in Petals of Blood, most notably in the town of Ilmorog, an isolated, pastoral community. After modernization, the farmers lands are fenced off and ultimately seized when they cannot repay their loans. Although none of the main characters lose their land in this way (Wanja, however, sells her family's plot), it is significant in that Kenya recreates what happened during colonial rule: the loss of land and subsequent desire to reclaim it was "the central claim" for those who rebelled against the settlers.
The notion of land and fertilisation is often linked to Wanja, who is seen as the embodiment of these concepts. As she is portrayed as "the symbol of the nation", the loss of her land to the new Kenyan elite is an important parallel with Ngugi's depiction of Kenya. Land is also linked to Kenya itself, with Ngugi suggesting that anyone who sells their land is a traitor.
Education is often depicted cynically in Petals of Blood. Munira is a teacher, but lacks strong abilities to guide his pupils, instead preferring to stand back and not to assert any of his own beliefs. He rejects the claims of others that the children should be taught more about being African, instead preferring that they be taught politics, and things which are "fact". Two of the three "betrayers of the people", those who are ultimately murdered, are also educators; they are untrustworthy, and depict the education system as a "problematic institution" in independent Kenya.
Although there is a brief suggestion that education does provide hope, as Joseph succeeds academically at Siriana, it the education system as a whole which is criticized. The notion of education as self-liberating is critiqued, as Joseph's success is still within the Siriana school, previously a bastion of "European" education. In a more political sense, Karega's self-education causes him to doubt his initial belief that education was a tool to gain liberation; originally taken in by the lawyer's socialist rhetoric, Karega's dealings with education ultimately leave him disillusioned.