That bright green, grassy weed in your lawn this month could be annual bluegrass. It is a cool-season, annual weed that grows in moist and compacted soils and even in shady locations. It is a prolific seed producer even when mowed at low heights, with each plant producing over 350 seeds. The first step to getting this weed under control in home lawns is to irrigate the lawn properly. Always apply deep but infrequent irrigation to lawns during periods of inadequate rainfall. This will encourage the turfgrass to have a deeper, more robust root system and help it compete with the annual bluegrass for moisture. Wait until the 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 soil moisture near the soil surface reduces the germination rate of annual bluegrass seeds in the lawn.
Another cultural method for annual bluegrass control is to reduce soil compaction by core aeration. Aeration should be performed early enough in the season that the turfgrass has completely recovered before annual bluegrass seeds begin to germinate in the fall. Core aeration is best performed in early summer after the lawn has become fully green. For more information on aeration, see HGIC 1200, Aerating Lawns.
Raising the mowing height of the lawn during the fall will reduce annual bluegrass seed germination. Scalping, as well as low mowing heights, stress the turfgrass and limit its growth, which allows the annual bluegrass to gain a strong foothold. Normally, returning grass clippings to the soil when mowing is an important practice that returns nutrients to the soil and reduces the fertilizer requirement, but it is recommended to the 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 each lawn species, see HGIC 1205, Mowing Lawns.
There are several different preemergent herbicides that can be applied to the lawn in the fall before annual bluegrass seeds germinate. The grassy weed preventers are best applied once we receive four consecutive days of daytime high temperatures at or below 75 °F in the early fall. A second application must be made 8 weeks later for extended control. For annual bluegrass seeds that have germinated, use a post-emergent herbicide spray that is safe for the turfgrass while weeds are small. For herbicide recommendations, please see HGIC 2325, Annual Bluegrass Control.