Yes and no. Poa annua is a winter annual which means the seeds germinate in fall when soil temps fall below 70 degrees. The plant stays fairly small, and spends the winter in a vegetative state. Once spring rolls around the plant explodes in growth and produces tons of seeds that will be dropped for germination in the fall. Then poa, being an annual, dies in late spring/summer. (typically this is the case, but their are hundreds of variations and perennial forms exist). There are some people having pretty good results using the herbicide Tenacity on it, but generally poa control is established with a strict regiment of preemergents like dimension. A drop in late summer, and again in early spring over time should result in a lessening of poa annua. There is also a new herbicide that I just learned about that might give hope to people battling poa annua. It isn't out yet, but it probably will be available in 2013-14.

Late Summer as in.. end of August?

Whenever your soil temps start falling below 70 degrees. For some people this doesn't happen, so late August would be a good time unless you plan on overseeding/renovating.

>"Late Summer as in.. end of August?"

Maybe - if we knew where you lived.