How Can I Control Annual Bluegrass?

Do you have an annual bluegrass invasion in ornamental plantings?

Do you want to control annual bluegrass with mulches?

Do you want to prevent annual bluegrass seeds from germinating?

Do you want to control established annual bluegrass with herbicides?

Do you have a severe infestation of annual bluegrass in your turfgrass?

Do you want to control annual bluegrass in your turfgrass chemically?

Mulching with landscape fabrics can be effective if the fabric is overlapped so no light is allowed to reach the soil. Use a polypropylene or polyester fabric or use a black polyethylene (plastic tarp) to block all plant growth. Organic mulches or rock can be used over the top of synthetic fabrics. When used alone, plant-derived products should be 2 to 3 inches thick, depending on the coarseness of the mulch. Finer materials can be less thick than coarser ones.

Preemergent herbicides such as dithiopyr, oryzalin, oxadiazon, pendimethalin, prodiamine, and trifluralin can be used to limit seedling germination in sites where the use of these materials is permitted. Some of these products may be available only to commercial applicators. Make the application before seeds germinate in fall when soil temperatures go below 70°F. Preemergent herbicides will be of little benefit if established annual bluegrass plants or the perennial type of bluegrass is already present.

Few postemergent herbicides are registered for use in established ornamental plantings. Clethodim is selective for grass weeds and safe on broadleaf ornamentals, but it has only a partial effect on annual bluegrass. Spot treatment with diquat, glufosinate, glyphosate, nonanoic acid, or other nonselective herbicides can reduce annual bluegrass populations in ornamental beds, but extreme care is needed to prevent herbicide spray or drift onto desirable plants, or the herbicides will injure the plants.

Annual bluegrass can be controlled by various methods in ornamental plantings. Preventing germination and seeding is very important. Hand removal or spot spraying of solitary plants will save time and money in the long run. Cultivation or hand hoeing, although possible under some circumstances, generally isn’t useful unless continued throughout the germination period because the seed that is buried in the soil is brought to the surface where it germinates.

Annual bluegrass infestations often become so severe in commercial turfgrass that complete renovation is necessary. This can be done by spraying the entire area with a nonselective herbicide like glyphosate followed by replanting with a desirable turf species. Planting and establishment of the new turfgrass should take place during late spring and summer so that a solid cover of new turf can be obtained before the annual bluegrass germination period. Choose a species and variety that will compete well with bluegrass.

Preemergent herbicides such as benefin, bensulide, dithiopyr, oryzalin, oxadiazon, pendimethalin, and prodiamine have been very successful in limiting germination of annual bluegrass. They should be applied a few weeks before weed seeds germinate to be most effective, as they do not affect emerged plants. Although most of these products may be used on residential lawns, some of the products aren’t available for homeowner purchase and can be applied only by commercial applicators.

There are a few relatively new postemergent herbicides that control annual bluegrass, and none of them can be used in all turf species. They are usually applied to warm-season turfgrass species. These herbicides can be used on residential lawns by licensed or certified applicators, but they have been of little benefit when used as the sole method of control.

Early removal of solitary infestations has been successful when practiced diligently. Open spots should be overseeded to establish a vigorous turfgrass. Removal of grass clippings might help reduce the number of seeds that reach the soil. Overwatering, especially in shady areas, will predispose turfgrass to annual bluegrass invasion. Use deep and infrequent irrigation to discourage the development of shallow-rooted annual bluegrass. Try withholding water until the desirable turf is beginning to show drought stress, rather than keeping the surface moist.