Hey East Coasters: sorry to burst your bubble, but California = Eden. The streets are pretty much paved with produce, and you can't leave any seeds lying around because they might just burst into fruitful trees in, like, four seconds. Meanwhile, you guys can't seem to grow anything but rocks. So there.
Really, though, Steinbeck uses California and its fruitfulness as a stand-in for Eden. It's a land full of promises and potential for someone like (cough) Adam—think paradise and the like. Just look at how Adam is first introduced to the concept of the Salinas valley:
Adam had seen and studied a fine color broadside which set forth the valley as the region which heaven unsuccessfully imitated. (13.2.9)
You notice the word heaven tucked in there? Eh? Eh? Salinas is so ridiculously great that even heaven is trying to be as cool.
Adam takes this whole Eden thing very seriously. When he first arrives in Salinas, he keeps talking about how he's going to build a place that will last through all future Trask generations:
Here was a place in which to plant his dynasty. (15.1.2)
He assumes a lot of things about how great the future will be, like the fact that, for one, there will even be future Trask generations. It's called counting your chickens before they've hatched, Adam, and it's a bad idea.
And then, of course, there is the moment when Adam outright makes the Eden connection (we mean, how could he not with a name like Adam?):
"Look, Samuel, I mean to make a garden of my land. Remember my name is Adam. So far I've had no Eden, let alone been driven out." (15.3.42)
But our Adam doesn't get to stay in Eden for very long, just like his biblical predecessor. Before it's even finished, Cathy/Eve decides to bail. Womp womp.