Why do I have these weeds coming up?!? Poa Annua is actually NOT a weed, this is your Annual Bluegrass “re-seeding” itself. This occurs in spring when temperatures are between 70 and 80 degrees. This process lasts only a couple of weeks, and then your lawn will return to its natural appearance. In fact, this is very healthy and beneficial for your lawn. You will also see this again in the late fall.