I am in west central Wisconsin zn 4/5. I have an east facing terrace/boulevard under a Norway maple (separated by the sidewalk). I would like to plant daffs and tulips there then use hardy geranium/cranesbill as a ground cover so I don't have to mow that area. My concern is that the geranium will starve the bulbs of light and nutrients. It gets quite a bit of morning sun. Has anyone tried this?
You'll find that bulbs and regular perennials including perennial geraniums get along just fine and there is no starving of light and nutrients. Plants are generally all adapted to live in complex groups of various species with various habits... ephemeral plants live with plants that start to grow up while the bulbs are in bloom and then take over later while the bulb plant dies back. I haven't found any perennials that I would not plant bulbs amongst.