Symbolism, Imagery, Allegory
No, don't worry. We're not talking about freakish mutant carnivorous plants here.
On one of her treks from hospital to hospital, Miss Sasaki got a glimpse of some crazy fast-growing greenery that had spread all over the city in the wake of the bombing:
Even though the wreckage had been described to her, and though she was still in pain, the sight horrified and amazed her, and there was something she noticed about it that particularly gave her the creeps. Over everything—up through the wreckage of the city, in gutters, along the riverbanks, tangled among tiles and tin roofing, climbing on charred tree trunks—was a blanket of fresh, vivid, lush, optimistic green; the verdancy rose even from the foundations of ruined houses. Weeds already hid the ashes, and wild flowers were in bloom among the city's bones. The bomb had not only left the underground organs of plants intact; it had stimulated them. Everywhere were bluets and Spanish bayonets, goosefoot, morning glories and day lilies, the hairy-fruited bean, purslane and clotbur and sesame and panic grass and feverfew. (4.7)
Um, okay. So this is actually creepier than a mean green mother from outer space.
Even though you might initially think fast growing flowers and plants would be pretty (and potentially offset some of the gloom and doom), there's something pretty disturbing about the notion that the bomb actually spurred this crazy fast, almost lurid greenery. The presence of this bizarre and unnatural growth indicates—and becomes symbolic of—the similarly unnatural and toxic nature of the bomb itself.