Found on 46th Floor, downtown Chicago green roof

It's tough to identify grasses from photographs, but it looks like this could be Sorghastrum nutans. That species can have quite glaucous leaves.

Hi, GreenRoofer.  Welcome to the site.  

Sorghastrum nutans is 4 feet tall and has a panicle flower, this is 4 inches tall and has a spike flower. Main axis of seed head doesn't branch, and the spikelets are stalkless. I transplanted one of these, it's growing on my windowsill. Still quite green and it's 13F today in Chicago. Makes me think it must be an alpine plant and not from the prairie.

Thanks for the welcome.

Wow.  With regard to the size of the grass, the two pics almost seem antithetical.  The left photo seemed so easy to discern that at a glance, I assumed your gloved finger in the right photo was a shoe!!!  I wonder if Janet had the same misconception...

I wish I could help with the identity. But, I am impressed that you took the time to really examine the plant.  I see you already know that an answer can only be as good as the question asked. :)

Welcome to the forum!  You'll find a lot of good things here, as well as the general nargs.org site.

Here's a pic of the same species on my windowsill. There were three of the species, and I relocated one to my home for further study and hopefully propagation. It's a fascinating little species and I'd love to know what it is. Either way, I'll try to breed it. On the skyscrapers, there aren't that many plant volunteers, but it's possible this was planted from a commercial nursery and only a handful survived (likely in fact). The grass to the right was also a skyscraper green roof survivor, but I'm less curious about that one.

Here's the environment it came from

Cool little grass for sure- hope you can find an id.- grasses that small are hard to find- does 4 inches include the height of seed heads? I wonder if it would stay as small in less extreme conditions...

Here's the seed head, I havent stratified the seed yet.

Alopecurus lanatus from Western Turkey looks very similar, and there's only ~30 species in the genus, so that's something to work with, thanks.

This is another green roof volunteer, found in two places on a 5th floor green roof, near Bolingbrook. I believe it is Sagina procumbens (Pearlwort), relocated to my windowsill for further study. Charming, little, moss-like plant. For scale, the Sedums in the photo are S. dasphyllum and S. hispanicum purpureum. This is perhaps not a flashy species, but I find it very interesting, and I'm incredibly enthused about any green roof volunteer that can behave itself.

I believe my grass species to be Koeleria glauca. It is either a cultivar, or the growth response to being in a harsh environment. If it is a cultivar, then someone planted it intentionally, if it isn't, then it's habit is from being in such a harsh environment. Like I said, there's one on my windowsill now, so we'll see how it does. The species is non-invasive.

From crocus.co.uk
"A tuft-forming evergreen grass with pretty, grey-green leaves, and dense silver green flower spikelets that fade to buff in the autumn. It prefers dry soils and will tolerate an exposed spot, but it is short lived, so may need to be replaced in a few years. It makes a striking edging to a path or border, and looks great in a gravel garden"

Sagina can be terrible menace amongst more choice alpine plants, but might bind together plants on a Green Roof where conditions are harsh. I'd be a bit wary of it though!