According to my plant records, Allium 'Purple Sensation' starts blooming in early May. My Dutch iris usually don't start blooming until mid-May or later. Allium 'GlobeMaster' starts a bit earlier (in mid-late April) than A. 'Purple Sensation'. This year, my A. 'Purple Sensation' started blooming about the same time that the roses and catmints started blooming. In my pictures, I also see Euphorbia 'Red Wing' in bloom. Hope that helps you out a little.

If your started in early May, then it will probably bloom for me a bit early since I have full morning east and all day south-facing sun. My front yard is a bit of a micro-climate!

Thanks for the info! I think I'll try to mix them with my Dutch irises so that all the foliage will get covered over with nepeta.