I have a patch of Iris Germanica that has performed well for me for years. Last year the bloom was so-so; this year, nary a blossom. The sword leaves are dense, I am guessing I need to divide them. So here is my question: how do I do this? How much room do I give each fresh division? The soil is ok but the bed lies next to a hot blacktop driveway. Should I amend the soil & if so, with what?

Thanks for your help.