Nitpick: The area affected by a single casting of Plant Growth is about 0.7854 square miles, which is about 502.7 acres. In fairness, that still only lets a single caster keep about 1,800 hectares of land covered, if he's limited to one casting per day. That's a little less than 10,000 5th-level Druids (or Archivists) to handle an England's-worth of farmland.

But the trick is, you don't have 10,000 Druids (or Archivists) going around casting Plant Growth. You have 1st-level Commoners (or whatever) going around using 10,000 Hands of the Oak Father. Those are 5,000-gp items that can cast Plant Growth once per day.

But where do you get 10,000 Hands of the Oak Father? You have a bunch of 3rd-level Adepts (or whatever) with the Craft Wonderous Item feat make them. Each one takes 5 man-days to complete, so factor in however-many Adepts (or whatever) with Craft Wonderous Item you can lay your hands on, and you know how long this will take to get up and running.

You may notice that Adepts don't natively gain access to the spells that serve as prerequisites for crafting a Hand of the Oak Father. But that's okay, because those spells can be supplied from an item. And the item perfectly suited to supply those spells is … an Hand of the Oak Father. So you just need one 5th-level Druid (or Archivist) to craft a few, then you can hand them off to your 3rd-level Adepts (or whatever) for duplication.

So why did I recommend making the cohort an Archivist? Why not just get some random 5th-level Druid to help you kick this off? Well, this sounds like a pretty low-level setting, so 5th-level characters would be hard to come by. And a lot of Druids are kind of reclusive, and not very fond of agriculture. Finding an NPC both willing and able to help you get this started could be tough. Plus, the Archivist cohort has lots of other benefits.