IIRC it was (is?) an agricultural method to help crops grow better. They would burn fields in addition to crop rotation.
Actually I might have found something on the matter: http://files.dnr.state.mn.us/assistance/backyard/privatelandhabitat/benefits_prescribed_burning.pdf%C2%A0
"In two brief hours, most of the planet was reduced to cinders."
http://fallout.wikia.com/wiki/Fallout_2_intro
"Continents were swallowed in flames and fell beneath the boiling oceans."
Also, there is plenty of plant life in D.C., grass is fairly abundant, as are bushes, sickly, but abundant.
I don't think there was nuclear waste dumps in Shady Sands.
Zion wasn't hit though, its totally different from basically everywhere else.
http://www.gamebanshee.com/interviews/105836-fallout-new-vegas-and-dlc-post-mortem-interview-part-one-v15-105836/page-2.html
If so, then why did Chris A feel the need to explain where the plants did come from?
Your argument would be correct if Fallout used real world radiation, but it doesn't, never did. It was always based off of the 50's view of radiation, as well as that of 1950's scifi B-movies, which always depicted the world as basically being largely screwed forever.
Hell, even in NV its brought up at the sharecropper farms that the radiation from the nearby leaking Vault is preventing the crops from growing.