Iris Propagation (en masse)

I love propagating plants...just for the fun of it. Iris are very easy to divide, but you only get a few new plants each season. I'm wondering how large-scale nurseries propagate TBI's in large quantities. I figured that this forum would be a good place to ask.

From what I have seen of large commercial gardens, they are able to get more than a few new plants a year from one 'mother' plant. I have seen clumps at Schreiners and Cooleys where there are a couple dozen new increase in a single clump. I don't know how they do it, but it has something to do with growing them under ideal conditions. I am surprised that you only get a few increase per year, as you live in an area well known for ideal iris growing conditions.
Laetitia