What are the Best Methods to Prevent Annual Bluegrass in Turf

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Annual bluegrass is an annual winter weed, which can become problematic on turf. Compared to other turfgrasses, yearly bluegrass has a lighter green color, coarser leaf texture, and produces unsightly seedheads.  Contrary to its name, both annual and perennial biotypes of annual bluegrass may be found in turf.  While these two biotypes may not be easily distinguished, annual types are more upright in growth and produce more seeds than lower-growing perennial types. 

Annual bluegrass seed terminates in late summer/early fall, once the soil temperatures fall below 70 degrees.  Seedlings grow and mature in fall, over winter in a vegetative state and produce seed in spring.  Annual bluegrass is a prolific seed producer , and individual plants may produce hundreds of viable seed, even when closely mowed.  Annual bluegrass flowers over several months in spring and produces seed that may remain dormant in soil for years before germinating.  It grows best under short day lengths and cool conditions, and can out-compete other turf species during late fall and early spring. 

There are several methods to prevent annual bluegrass in turf.  The best method may depend on individual circumstances.  The best methods to prevent annual bluegrass in turf include cultural control and chemical control (preemergence control and postemergence  control).

Cultural Control:

Several cultural practices can be utilized to control annual bluegrass in lawns.  These include:

• Deep and infrequent irrigation:  This encourages turfgrass root development and may improve the ability of desired grasses to compete with annual bluegrass.
• Withhold water until desirable turfgrass species exhibit initial drought stress symptoms:  This can help reduce soil moisture for potential annual bluegrass infestations.  Overwatering, especially in shady areas, may predispose the area to annual bluegrass invasion.
• Nitrogen fertilization should be reduced during peak annual bluegrass germination and periods of vigorous growth:  High nitrogen at these times encourages annual bluegrass spread and survival into winter and spring.
• Raising the mowing height:  If done during peak annual bluegrass germination, this may encourage turf competition to reduce potential infestations.
• Mow turfgrass frequently during periods of vigorous growth:  This prevents scalping, which things out turf and may enable weeds, such as annual bluegrass.  Removal of clippings may also be useful when annual bluegrass is present and producing seed heads to reduce the spread of viable seed.

Chemical Control

Preemergence herbicides may prevent annual bluegrass seed germination.  They will not, however, effectively control perennial biotypes of annual bluegrass from spreading vegetatively.  Timing the application of preemergence herbicides for annual bluegrass control is very important.  Herbicides must be applied in late summer/early fall before annual bluegrass germination. A second application can be applied in winter to control later germinating plants.  It is not recommended to apply preemergence herbicides in the fall if reseeding or resodding is needed to repair areas of damaged turf within several months after herbicide applications.

For herbicides, rates and application information, refer to the current edition of the Georgia Pest Management Handbook. See the table below for the efficacy of preemergence herbicides for annual bluegrass control in residential turfgrass:

Efficacy of preemergence herbicides for annual bluegrass control in turfgrasses:

Bensulide, dithiopyr, pendimethalin, and prodiamine are used for summer weed control, but can be used in fall/winter effectively as well.  Atrazine and simazine have excellent activity on annual bluegrass; however, soil residual is typically shorter than some other herbicides (4 – 6 weeks).  Check labels for further information before use.

Postemergence herbicides may be used selectively to control annual bluegrass.  More options are available in managing warm-season grasses for selective postemergence annual bluegrass control than cool-season grasses.  Selective annual bluegrass control options in cool-season lawns are limited.  Spot treatments of nonselective herbicides are generally the most effective treatment regimen for annual bluegrass control in cool-season grasses.

Efficacy of postemergence herbicides for annual bluegrass control in turfgrasses:

Managing Herbicide Resistance:

Annual bluegrass is a genetically diverse species and various biotypes present in turf may have differential responses to herbicides.  Repeated use of one herbicide chemistry may effectively control annual bluegrass, but resistance may develop in local populations if herbicides with different modes of action are not incorporated in to management regimens.  Resistance occurs from repeated use of the same herbicide or mode of action over years and may be a concern with problematic annual weeds, such as annual bluegrass. As you can see there are various methods and products to prevent annual bluegrass in turf.  It is important to follow recommended mowing requirements for the type of turfgrass and use products correctly for the best effect.  Since this can become a complicated process, you may want to reach out to your local expert for recommendations or seek a professional who knows the best method to prevent annual bluegrass in turf.