A primary method of control is preventing new infestations. Maintenance gardeners frequently spread weeds from site to site when weed seeds contaminate mowers, string trimmers, and aerifiers. Cleaning landscape equipment after use in infested sites can help prevent annual bluegrass from spreading to uninfested areas.
If solitary plants of annual bluegrass are found, they should be removed before seed production starts. Isolate small areas of infestation until control can be accomplished. Hand pulling or hoeing to remove annual bluegrass can be effective as long as it is done frequently. Because dense seedling infestations are common, open areas where old plants have been removed will often have new flushes of seedling plants, hence the need for frequent attention. Controlling annual bluegrass infestations manually is very expensive in commercially maintained landscapes and usually not practical or successful. However, it can be very appropriate for home gardeners.
Maintaining turf and ornamental areas properly assures their maximum vigor, which helps these plantings become as competitive as possible and slows invasion of the weed. A dense sward of turf and closely spaced ornamentals shade the soil surface and make the establishment of annual bluegrass seedlings more difficult.
Turfgrass
No single control procedure has been successful in controlling annual bluegrass in turfgrass. 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. Avoid fertilization and don’t aerate turf during the peak of annual bluegrass germination. Also, avoid cultural practices as well as use patterns that tend to promote soil compaction.
Preemergent herbicides such as benefin, bensulide, dithiopyr, oryzalin, oxadiazon, pendimethalin, and prodiamine and their combinations such as benefin/oryzalin 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 have no effect on emerged plants (Table 1). Ethofumesate and pronamide are also available for preemergent use and have some postemergent activity on both varieties of Poa annua, but these herbicides can’t be used in all turf species. 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. To limit bluegrass germination during winter, apply preemergent herbicides in late summer or early fall when soil temperatures drop below 70°F. Where the perennial type is a large component of the bluegrass population, preemergent herbicides will be of little or no benefit.
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. Foramsulfuron, sulfosulfuron, and trifloxysulfuron can be used only on warm-season turfgrass species. Ethofumesate can be used in dormant bermudagrass, creeping bentgrass, Kentucky bluegrass, tall fescue, perennial ryegrass, and St. Augustine to reduce annual bluegrass infestations. Pronamide can be used in warm-season turfgrass for established annual bluegrass, but it is slow acting (15 to 21 days).
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 such as 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. Then preemergent herbicides can be used in late summer or fall to further limit annual bluegrass from establishing.
| Herbicide | Trade name | Applied to lawns? | Applied on ornamentals? | Available to home gardeners? |
|---|---|---|---|---|
| Preemergents—apply before weed seeds germinate | ||||
| benefin | Balan 2.5 G | yes | yes | no |
| benefin/oryzalin | Amaze Grass & Weed Preventer, XL 2G, Primera One OB-2G | yes | yes | yes |
| benefin/trifluralin | Team | no | yes | no |
| bensulide | Bensumec, Pre-San | yes | no | yes |
| dithiopyr | Dimension, Preen Southern Weed Preventer | yes | yes | yes |
| ethofumesate | Poa Constrictor | yes | no | no |
| oryzalin | Surflan, Weed Impede | yes | yes | yes |
| oxadiazon | Ronstar | yes | yes | no |
| pendimethalin | Pendulum, Pre M, Scotts Halts | yes | yes | turf only |
| pendimethalin/dimethenamid | Freehand | no | yes | no |
| prodiamine | Barricade | yes | yes | yes |
| pronamide | Kerb | yes | no | no |
| trifluralin | Vegetable & Ornamental Weeder, Preen Garden Weed Preventer, Treflan | no | yes | yes |
| Postemergents—apply to young weeds | ||||
| clethodim | Envoy | no | yes | no |
| ethofumesate | Poa Constrictor | yes | no | no |
| foramsulfuron | Revolver | yes | no | no |
| pronamide | Kerb | yes | no | no |
| sulfosulfuron | Certainty | yes | no | no |
| trifloxysulfuron | Monument | yes | no | no |
| Nonselective postemergents—apply to weeds (will kill turf and ornamentals) | ||||
| diquat | Diquat, Reward | n/a | n/a | yes |
| glufosinate | Finale | n/a | n/a | no |
| glyphosate | Roundup, Kleenup, Remuda, etc. | n/a | n/a | yes |
| nonanoic acid | Scythe | n/a | n/a | yes |
Ornamentals
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 seed that is buried in the soil is brought to the surface where it germinates.
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. Plant-derived products (i.e., organic mulches) or rock can be used over the top of the 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. If seeds of annual bluegrass get into the mulch, they can germinate and establish, just as if they were in soil. In these cases annual bluegrass plants can be easily removed by hand or with a hoe. Mulch thickness will need to be replenished periodically to maintain cover and eliminate light penetration to the soil.
Preemergent herbicides such as dithiopyr, oryzalin, oxadiazon, pendimethalin, prodiamine, and trifluralin or a combination such as benefin/oryzalin, benefin/trifluralin, or pendimethalin/dimethenamid can be used to limit seedling germination in sites where use of these materials is permitted (Table 1). 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. However, if the existing bluegrass is removed, a preemergent herbicide can be applied to control seedlings that germinate later.
Few postemergent herbicides are registered for use in established ornamental plantings. Clethodim, fluazifop, and sethoxydim are selective for grass weeds and safe on broadleaf ornamentals, but only clethodim has any 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. Clethodim and glufosinate are available only to licensed applicators at this time.