There are several types of weeds and grasses that are unwanted varieties in many lawns, including tall fescue. Of these the two most common are Poa annua and Poa trivialis. They are easy to identify by the size, shape and color of their leaves. Both varieties have thin stems with short broad leaves and have a light green color. The only way to tell the difference between the two is the production of seed heads. The Poa annua will produce seed heads in both the spring and the fall, while the Poa trivialis will only produce seed heads in the fall. The two grasses not only look alike, but they are also controlled in identical ways.
Change the watering cycle for your tall fescue lawn. Shallow and frequent watering will encourage the growth and production of Poa grasses. Change the watering cycle of the lawn to deep watering on an infrequent schedule. Rotate the days watered every two weeks so the Poa grasses will not get used to the watering schedule. Constantly moist soil encourage the spread of the grass.
Change the mowing height of the fescue lawn. The shorter the lawn is mowed, the more likely the Poa grasses are to spread. Grass heights below 2 inches will encourage the growth of Poa grasses. Raise the mowing height of the lawn to 3 inches or above. The taller grass will shade the Poa grasses and discourage growth.
Aerate the lawn on a regular basis. Tightly compacted soil will encourage the growth and spread of Poa grasses. Aerating the soil allows air to enter the root zone and loosens the soil compaction. Loose soil does not allow the sparse roots of the Poa grasses to gain a hold.
Kill the entire lawn and reseed. This should be done in extreme cases only. The other processes will reduce the spread of Poa grasses and, over time, rid the lawn of them.