No; the Flood can only infect organisms with enough calcium in their body to support the mutation from a normal member of their species to a Flood form. Usually, sentient (intelligent) creatures are the only organisms that can be infected, because these typically have higher amounts of calcium than less intelligent organisms, as a result of the larger body size and additional requirements of nutrients in a sentient creature. For instance, the Flood could infect Humans, Elites, and Brutes and use them as Combat Forms immediately after mutation, but the smaller creatures, like Jackals, Grunts, and possibly Prophets, would have to be turned into Carrier Forms due to their lesser body mass and calcium amount. Plants do not use calcium as a nutrient, so the Flood cannot infect them. But, honestly, who wouldn't kill to see a Flood-infected sunflower or dandelion?