Poa annua (or annual bluegrass) is a common winter lawn weed that can leave homeowners frustrated about control. Poa annua is very prolific and one plant can produce hundreds of viable seeds. It germinates in late summer or early fall – when soil temperatures fall below 70oF — and it produces seeds in spring.
There are some cultural practices that can help control poa annua if they are employed throughout the growing season, but inevitably, some always rears its ugly seedhead. Preemergent herbicides may be applied in early fall, but for maximum efficacy, they must be applied before germination and sufficient irrigation or rainfall must occur.
If some annual bluegrass still manages to grow despite best cultural practices year-round and preemergent herbicide application, a post emergent herbicide can be applied. There are many formulations available  — be sure to read all labels and follow package instructions, including choosing a post emergent herbicide that is labeled for your specific turfgrass variety.
Poa annua is one of the most troublesome lawn weeds, but you can minimize it in your lawn with diligent preparation and care.