He said where did all the plants come from, not the spore plants, or the fungus zombies, ALL plants.Wut? I don't see him explicitly stating that. From what I see it sounds more like he's talking about dlc/main game connection with the fungus zombies from Vault 22 rather than vegetation as a whole. As to Sharecroppers, it wouldn't have to be because of the radiation yknow. Look at the vault and the state it is in. It is very possible that some other shit's gotten into the water that is messing up the Sharecroppers farm. I mean have you seen the water in Vault 34? It looks like all kinds of shit is in it. It has a really disgusting tint of green to it.
I have seen someone on 4chan actually attempt to map out how far Vault 22 and Big MT's plant experiments have spread. Given that spore plants appear as far as Arroyo in southern Oregon(as seen in Fallout 2) they used some radius drawing tool on some internet site, and showed that it would cover all of California, Nevada, Utah, Arizona, the majority of Colorado and New Mexico, and large part of Wyoming and southern Idaho. Which is basically all of the post-war western U.S. we have seen or heard about in Fallout 1, 2, and New Vegas.
It would also explain why the Midwest was a big radioactive dust bowl, and the east cost is in a nearly equally shitty state, whereas the west coast isn't.
Also, as part of that quest you have to go to the nearby "east pump station" to check for contaminants, and it directly lists radioactive contaminants in the water, and says nothing else about any other contaminants. Also yes, the water is green.... because of all the radiation.