Poa Annua, or annual bluegrass appears in small clumps during the Spring. Because Poa Annua is a grassy weed, it is very difficult to control after it has emerged. It is rather difficult to control because the plant will produce several hundred seeds in one season, and the seeds can lay dormant for several years before sprouting.
A WAY TO MINIMIZE IT IS THROUGH THE USE OF PREEMERGENT TREATMENTS IN THE FALL.
For an effective Poa Annua control, apply a pre-emergent herbicide in early fall and again in early spring. This will keep the Poa Annua seeds from sprouting. But keep in mind that Poa Annua seeds are tough and can survive many seasons without germinating. This method will work towards reducing Poa Annua in the lawn over time. You will need to treat your lawn for many seasons in order to rid it completely of this weed. Poa Annua will die out naturally with the increased temperatures in late spring.