Well, since no one else is responding, I'll take a stab at it: I've always planted my alliums in the fall - around October, so I would not recommend waiting until after your dahlias die back. I've had dahlias around Thanksgiving but I don't know how long they persist in your zone. I'm guessing that you will want to dig up your dahlias for the winter. We can often get away with a heavy mulch here. You could pot up the alliums and give them time to establish roots until you remove your dahlias, then plant them out at that time. Maybe plant the bulbs fairly shallow - just a couple inches below the soil to give more room in the pot for root growth, then plant them deeper when you put them in the ground. That's what I would do if I had to lift the dahlias for the winter. Here, I would just slip the alliums in between the dahlias as gently as I could. Hope this helps!

No thanks, thank you though. Best wishes for good luck on your project!

This may not be the plant, but at least a place to start. Not quite like wheat. Could it be northern sea oats, Chasmanthium latifolium? hortster Here is a link that might be useful:

I think you have a clump of nut grass there.... I would get rid of it asap since it's pretty high on the annoying weed scale. If you dig it up there will probably be a bunch of white roots that are spreading out and starting new plants.... it also leaves little brown bulbs (nuts) that will sprout new plants whenever they get a chance!