There are some spells in the late game that allow you to spread fertile land growth... 'Bloom of Spring' and 'Bloom of Fall' I think.
They are only available under certain conditions. I think it has to do with how you set yourself up (character or nation). It's a nice perk when you do have it, but it's so late in the magic tree that it doesn't help much early on. Maybe it should be allowed earlier in the game, and not be quite so hard to get set up.
My biggest problem, however, is honestly finding areas without strong monsters blocking me.