The way vertical farming works is that you use some sort of plant that has a massive surface to volume ratio and then just spread out the growing light on as big an area as possible. This strategy will get you very expensive "leafy greens" and inexpensive cannabis and no grains at all because the "growing lights are more efficient than natural sunlight" calculation no longer works because grains actually grow better if they are exposed to intense (sun)light which is expensive to produce in a vertical farm.

(from my own memory and it also happens to be a lazy summary of the paper)