2009-10-21 / Farm & Home

Ag Line: Annual Bluegrass Control

By Wade Parker County Extension Coordinator

It is hard to believe that it is already the middle of fall. As I write this article, it is very cool outside and wet. It seems like we are going to have a long drawn-out harvest season ahead of us.

With the onset of fall, comes annual bluegrass! Annual bluegrass is the most troublesome weed we have to deal with in the yard. Compared to most turfgrasses, annual bluegrass has a lighter green color, coarser leaf texture and produces unsightly seed heads.

Annual bluegrass seed germinates in late summer/early fall once soil temperatures fall below 70 degrees. Seedlings mature in fall, overwinter in a vegetative state, and produce seed in late spring and early summer. Annual bluegrass is a prolific seed producer as individual plants may produce over 360 viable seeds even when closely mowed. Seed may lie dormant in soil for many years before germinating. Annual bluegrass flowers and produces viable seed in spring and at virtually any mowing height. Annual bluegrass grows well under short day lengths and cool conditions, and may out-compete other turf species during late fall and early spring. Annual bluegrass often dies from summer stresses but may survive if irrigated and pests are adequately controlled, especially perennial biotypes.

Several cultural practices improve annual bluegrass control.

• Deep and infrequent irrigation encourages turfgrass root development, which improves the ability of desired grasses to compete with annual bluegrass.

• Withhold water until desirable turfgrass species exhibit initial drought-stress symptoms. Overwatering, especially in shady areas, will pre-dispose turfgrass to annual bluegrass invasion.

• Avoid practices that promote soil compaction. Relieve compaction with regular aerifications in spring and fall.

• Voids left in turf with exposed soil, following aerifications, may permit annual bluegrass invasion during periods of peak germination. Time aerfications in early fall to allow turf to recover before annual bluegrass germinates.

• Reduce nitrogen fertilization during peak annual bluegrass germination and during periods of vigorous growth (cool weather). High nitrogen at these times encourages annual bluegrass spread and survival into winter and spring. Fertilizing dormant turfgrasses when annual bluegrass is actively growing will exacerbate infestations.

• Lower mowing heights encourage annual bluegrass invasion. Height of cut for lawns should be no less than 2 inches.

• Mow lawns at least once per week during periods of vigorous growth to prevent scalping. Scalping thins out turf, enabling weeds such as annual bluegrass to establish. While returning clippings is recommended to recycle nutrients to the soil, removal of clippings may be useful when annual bluegrass is present and producing seed heads. Removing clippings at this time will reduce the spread of viable seed through the lawn.

Chemical Control

Preemergence herbicides may prevent annual bluegrass infestation via seed and limit current infestations from further spreading. However, preemergence herbicides will not eradicate established plants and will not effectively control perennial biotypes of annual bluegrass from spreading vegetatively. Application timing of preemeergence herbicides for annual bluegrass control is very important, and thus herbicides must be applied in late summer/ early fall before annual bluegrass germination. A second application can be applied in spring to control germinating plants. Fall applied preemergence herbicides cannot be used if reseeding or resodding is needed to repair areas of damaged turf within several months after herbicide applications.

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