HI there,

I am not sure what "screenings" means, but can I assume it is like the gravel you also mentioned? I have heard mentioned on this forum that if you have heavy clay soil the remedy is to plant your iris in raised beds. However, I am not sure if 4" of compost is going to qualify as a raised bed plunked on top of gravel and clay. When I dig my iris the roots are very long and deep, and I assume that all the root needs the good drainage, not just the top couple of inches. Plants also like to have their roots in mineral soil, not just compost. Sometimes the nitrogen is bound up by soil organisms decomposing the compost, and if you have plants in it too, they might lose out in the nitrogen contest. Still, I think you are on to something, and maybe if you really raised things up your plan could work. How deep are the screenings? Could you get access to a tiller for a day to kind of mix things up? Till first, then add the compost and till again?

It would probably depend on the depth of the gravel stuff and its size. If it's sandy stuff, in other words, quite fine, and it's deep, you can mix in some compost and plant irises. Beware that if you really have dense clay, the water may build up on top of it and drown your irises. Good luck!
Renee