Not quackgrass! That's the judgment of the Iowa State University Extension Service who took a look at the pictures I sent and responded in a couple of hours. Crab grass and barnyard grass. Both annuals that will die off at the first hard frost.
I just learned about the folks who dispense answers and guidance at firstname.lastname@example.org And now they're my new best friends.
This pronouncement carries a good news/bad news aura about it. The good news you already know. The grass that has now completely overrun my prairie garden will die off with the frost. The bad news is that both of these grasses are prolific seed producers. If I let them go, they'll reseed and the problem will show up again next year. Plus the grass is so thick, I find it hard to imagine the tiny prairie seedlings competing against it and winning.
Glyphosate is still an answer. I call my afternoon garden time: 'Fun with Roundup.' Container in one hand, half-inch paint brush in the other, I head for the prairie garden and kill off the grass one blade at a time.
Perhaps I am certifiably crazy. The garden is 2,400 sq. ft., after all. Viewed in another light, I may be a great artist. I take my inspiration from Michelangelo who spent four years completing the Sistine Chapel. Native prairie takes three years to establish.
Discouraged? Who, me? No way!