Thanks for the responses so far.
Poa pratensis is "Kentucky bluegrass", a perennial species of the Poa plant. It can be found on many golf courses, especially used as a hearty variety of rough.
A discussion of Poa annua evolution, from a stalky annual plant to a smooth, leafy perennial plant is presented in
"Developing annual bluegrass cultivars for putting greens", by David R. Huff. The plant evolves and adapts to a low mowing environment over a period of 30-50 years. The USGA article is found here:
http://www2.gcsaa.org/gcm/2006/dec/pdfs/073-088_dec06.pdfHere in the northwest U.S. (west of the Cascade Mountains), there are no examples of courses which have successfully repelled Poa annua colonization. And once it establish a foothold, it quickly takes over. My home course, Pumpkin Ridge GC, saw the following progression:
Year 7 or 8: Poa is easily visible in greens, perhaps 1-2% of grass. Club physically cuts all Poa plants from greens.
One year later: Poa now comprises 4-6% of grasses in greens. Club gives up practice of physically removing, and begins process of encouraging Poa to colonize further.
The next 4-6 years: Greens were bumpy and inconsistent. Greens are rarely maintained at a Stimpmeter speed above 10.5 feet, though it was possible to host successful USGA events (Women's Open, Junior Amateur, etc.) when required. Some greens are better than others; Ghost Creek #4 green in in a shady hollow, and transitioned to Poa years before the others. Some were very florid with poa flowers in spring and summer, to the point of appearing yellowish at times (Ghost Creek #13, Witch Hollow #8). By the end of this six year period, greens were 75-100% Poa.
Ten years later: Greens are 99-100% Poa annua, much of it with small blades and very tight turf. During cooler weeks in summer and fall, greens can be rolled 2-3 times per week to yield fast greens conditions (11-12 feet).
Fairways are 90-95% Poa (originally bentgrass). Roughs are 70-80% Poa, plus some ryegrass (mostly because some rough areas were resodded) and perhaps 1-2% creeping bentgrass. Bentgrass rough is too difficult, and undesirable.
Overall, the course plays pretty fast in the summertime. A low hook with overspin will roll a long way. Grass browns out a bit in July and August, and some Poa rough seems to die off and go into a dormant state.
That's what I see.