One month after Hiroshima was bombed, it was coverered with vegetation

The following is from the book Hiroshima by John Hersey:

"The hospitals and aid stations around Hiroshima were so crowded in the first weeks after the bombing, and their staffs were so variable. depending on their health and on the unpredictable arrival of outside help, that patients had to be constantly shifted from place to place. Miss Sasaki, who had already been moved three times, twice by ship, was taken at the end of August to an engineering school, also at Hatsukaichi. Because her leg did not improve but swelled more and more, the doctors at the school bound it with crude splints and took her by car, on September 9th, to the Red Cross Hospital in Hiroshima. This was the first chance she had had to look at the ruins of Hiroshima; the last time she had been carried through the city's streets, she had been hovering on the edge of unconsciousness. 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. Especially in a circle at the center, sickle senna grew in extraordinary regeneration, not only standing among the charred remanants of the same plant but pushing up in new places, among bricks and through cracks in the asphalt. It actually seemed as if a load of sickle-senna seed had been dropped along with the bomb." 

Copyright by John Hersey, 1946

Originally appeared in The New Yorker magazine.

(Pages. 90-92)

How did these plants, the seeds and the roots, survive a temperature estimated to be 3000 to 6000 C? Why no mention of mutated plants?