Mykan is recalling correctly. Near the start, you want to blanket your city with as many mushroom farms as you squeeze in, but don't level most of them. Don't be in a rush to make more than one rally point when you first get access. Accumulate as many mushrooms as you can. As you move through the chapter you can tear down some of the non-leveled mushroom farms to replace with Rally-points. Near the end of the chapter, they are much more needed.
And I'll repeat that blanket comment. Stretch your orc space as much as you can squeeze, and put down as many mushrooms as you can fit in. You'll need lots unless you are very patient. I recall having around 40 at one point.