Well, after the nuclear winter that must have proceeded the Great War, all plants and animals would have evolved in a similar way (divergent evolution - two similar species in similar environments will ultimately evolve the same way). A root vegetable, especially an onion, would have been grown all over the country - they're fairly hardy vegetables.
As for Broc Flower, we could semi-safely assume that the Brotherhood of Steel (who already aren't fond of adaptation) would be likely to bring along whatever they used back West. And given the amount of extra radioactive waste in DC, the temperature's should be assumed to be higher than they are now (without going into loads of detail, radioactive decay gives off energy as well as radioactive particles - and that energy has to go to somewhere, usually in the form of heat). Maybe not mojave heat levels, but remember that in the Mojave Broc Flower is primarily found in shady areas, such as near caves, or near hills. (http://fallout.wikia.com/wiki/Broc_flower_(Fallout:_New_Vegas). I'm not an expert on plant life or anything, but I presume that means it likes colder areas, or at least areas with less sunlight (due to the fact it's found in hilly/rocky areas, it also means it's used to bad soil, and plants especially have done quite well in Chernobyl.)
I can't think of anything else, but these reasons make sense in my head. But then again, I am trying to find excuses to justify it, so maybe I'm just trying to fit evidence around the conclusion rather than trying to come to a conclusion from the evidence.