That bright green, grassy weed in your lawn this month could be annual blue grass (Poa annua). It is a cool season annual weed that grows in moist, compacted, even shady locations. It is a prolific seed producer, with each plant producing over 350 seeds even when mowed at low heights. The first step to getting this weed under control in your landscape is to apply deep, but infrequent irrigation to your lawn. This will encourage your lawn to have a deeper more robust root system and help it compete with the annual bluegrass for moisture. Wait until your lawn begins to show symptoms of drought stress before irrigating, especially in the early fall. Annual bluegrass seeds germinate in autumn when soil temperatures drop below 70 °F. Limiting moisture in the soil will reduce the germination rate of annual bluegrass in your lawn.
Another cultural control for annual bluegrass is to reduce compaction in the lawn. Core aeration will help to control compaction. Aeration should be performed early enough in the season so that grasses recover completely before annual blue grass begins to germinate in the fall. For more information on aeration, see HGIC 1200, Aerating Lawns.
Raising the mowing height will also help during peak annual bluegrass germination. Low mowing heights and scalping can cause stress to your desired grass and limit growth, which allows annual blue grass to gain a strong foothold. Returning your grass clippings to the soil is an important practice that returns nutrients to the soil and reduces the need for fertilizer, but it is recommended to bag your clippings when annual bluegrass is in flower. Bagging will help to limit the spread of seeds that could be problematic next year. For more information on the correct mowing height for your lawn, see HGIC 1205, Mowing Lawns.
There are many pre-emergent herbicides that can be applied to the lawn before annual bluegrass seeds germinate. These products need be applied once we receive four consecutive days of daytime high temperatures at or below 75 °F in the early fall. Once the weeds have germinated, it is important to use a post-emergent herbicide that is safe for your lawn while weeds are small. For herbicide recommendations, please see HGIC 2325, Annual Bluegrass Control.