Poa annua, also known as annual Bluegrass, is an invasive weed that is difficult to kill or control. One reason is that it produces many of seeds that, even when mowed low, can remain viable for many seasons and sprout years after you thought the problem was fixed. Another challenge is that there is not yet an herbicide designed specifically to get rid of this grass. Because poa annua is a winter grass it germinates in late summer or early fall and produces seeds in late spring.
Rake the lawn thoroughly in early fall (early September in most regions) with a thatch rake. Bag and remove all thatch.
Mow the lawn very short: 1 to 2 inches.
Apply a pre-emergent herbicide in early September to kill the poa annua seeds and sprouts as they emerge.
Apply a second application of pre-emergent herbicide in early spring to control the spring germination of the weed.
Call in a lawn expert to spray a post-emergent herbicide with ethofumesate. Ethofumesate can only be applied by a licensed professional. Three applications at four-week intervals between September and December will be required.
Apply a non-specific herbicide with glyphosate as a last resort. This will kill the entire lawn. Spray with glyphosate on a non-windy day with no rain predicted for at least 24 hours. Be careful not to get glyphosate on plants that you wish to keep, as glyphosate will kill everything. Wait seven days and respray with glyphosate.
Rake off all dead vegetation once the glyphosate has done its work, rototill the lawn, add at least 1 inch of organic manure and reseed or sod with new grass.
Apply a pre-emergent herbicide in late fall and again in early spring to control new poa annua growth after reseeding or laying sod.