Annual Bluegrass Control

Prepared by Millie Davenport, HGIC Horticulture Extension Agent, Clemson University. (New 5/10. Pesticides updated 09/10.)

HGIC 2325

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Annual bluegrass (Poa annua) is a cool-season, annual grass that spreads by seed. Annual bluegrass has a tufted habit with a bright green leaf color and fine texture. It is native to Europe and is found worldwide.

Annual bluegrass clumping growth habit
Annual bluegrass clumping growth habit.
Millie Davenport, © 2010, HGIC, Clemson Extension

Annual bluegrass has smooth leaves with a boat-shaped tip. It produces greenish white seed heads throughout its life cycle with the majority appearing during the spring months. It can be found growing in a wide variety of conditions, but prefers areas with moist and/or compacted soil.

Before starting a weed control program homeowners should realize that complete eradication of annual bluegrass (or any weed) from the landscape is not practical. A more realistic approach is to manage (not eradicate) the weed by reducing the infestation to a tolerable level.

Smooth leaf blade of annual bluegrass with boat shaped tip
Smooth leaf blade of annual bluegrass with boat shaped tip.
Joseph M. DiTomaso, University of California - Davis, Bugwood.org

Annual bluegrass seed heads
Annual bluegrass seed heads.
Millie Davenport, © 2010, HGIC, Clemson Extension

Control in Lawns

Maintaining the health and density of your lawn is the best method for preventing a weed problem. Proper mowing height, irrigation and fertilization of the turfgrass is the best defense against weeds. For more information on these topics, see the following fact sheets: HGIC 1201, Fertilizing Lawns, HGIC 1205, Mowing Lawns and HGIC 1207, Watering Lawns.

If annual bluegrass does become a problem in a turfgrass area, it can be dug up easily before it is well established. Large patches may be difficult to dig up, and an herbicide may be used. If an herbicide treatment is chosen, it is best to start treatments in the fall before seeds germinate.

Pre-emergence herbicides should be applied to well-established lawns in late summer or early fall when temperatures drop to a daytime high of 75 °F for 4 consecutive days. A pre-emergence herbicide containing both the active ingredients benefin and trifluralin (such as Hi-Yield Crabgrass Control) has performed well in trials for preventing annual bluegrass development. A second application can be applied 10 to 12 weeks later. Apply at the label rate for Poa annua control and water in the product after application. CAUTION: Pre-emergence herbicides are not selective and can prevent the development of turfgrass seeds. Read the label for time to wait before seeding a treated area.

Atrazine (such as Hi-Yield Atrazine Weed Killer, Image for St. Augustine & Centipedegrass, Southern Ag Atrazine St. Augustine Weed Killer, and Spectracide Weed Stop for Lawns Concentrate for St. Augustine & Centipede Lawns) can be applied to St. Augustinegrass and centipedegrass. For post-emergence control of annual bluegrass, apply atrazine in November and again in early January. Atrazine can be applied up to two times per year. It should NOT be applied to newly seeded lawns due to the detrimental effect it has on seed development. Delay atrazine applications to newly sodded and sprigged lawns until they are well established and actively growing. CAUTION: Atrazine can travel through soil and enter ground water. Please read the label for all environmental precautions.

Non-selective herbicides such as glyphosate can be used for spot treatments; however, nearby desirable grasses and plants can be severely injured. Examples of products containing glyphosate are Roundup Original, Eraser Systemic Weed & Grass Killer, Quick Kill Grass & Weed Killer, Bonide Kleenup Grass & Weed Killer, Hi-Yield Super Concentrate Killzall Weed & Grass Killer, Maxide Super Concentrate 41% Weed & Grass Killer, and Southern States Grass & Weed Killer Concentrate.  Glyphosate is most effective when applied to young weeds in November. Annual bluegrass plants that are found growing in April and May will dieback as temperatures rise, so it is not necessary to treat them at that time.

Note: Once annual bluegrass has been eliminated in turfgrass, bare spots will be left behind. To prevent the invasion of new weeds in these bare spots, plan to fill them in with plugs or sprigs of the desired turfgrass at the appropriate planting time. For more information on renovating a lawn see HGIC 1204, Lawn Renovation.

Control in Vegetable Gardens

When planning a vegetable garden, it is best to attempt to treat weeds before tilling the soil. Tilling can break up and spread weed seed throughout the garden plot. Some methods used to reduce weeds in the vegetable garden include hand pulling, hoeing, mulching and applying post-emergence herbicides.

Hand pulling or hoeing weeds is only a practical choice for small garden plots. If hand pulling is chosen, be sure to work when the soil is moist so roots can be removed easily. When cultivating between the rows to control weeds, use care not to damage the roots of crops.

Organic mulch (such as pine needles, old hay or grass clippings) can be used in the garden to help suppress annual bluegrass development. Before laying the mulch, apply a layer of six to eight wet newspaper sheets to act as a weed barrier. The newspaper layer prevents weed development by blocking light to the weeds underneath, preventing their growth. Best of all, the newspaper should decompose before the following spring. To prevent low oxygen levels in the root zone, keep organic mulch levels at a maximum of 3 inches. For more information on mulching a vegetable garden, see HGIC 1253, Controlling Weeds by Cultivating & Mulching.

Pre-emergence herbicides can be used in the fall garden to prevent weed germination. Trifluralin (Hi-Yield Herbicide Granules, Green Thumb Garden Weed Stopper, and Preen Garden Weed Preventer) can be used on many vegetable crops. It should be applied to prepared soil and incorporated 2 inches deep before planting. NOTE: Trifluralin is not safe to use on every garden plant. See the herbicide label for application timing and safety for each crop species, the proper application rate, and watering-in instructions.

Lastly, a non-selective, post-emergence herbicide can be used to treat the garden plot before planting. Glyphosate (such as in Roundup Original, Eraser Systemic Weed & Grass Killer, Quick Kill Grass & Weed Killer, Bonide Kleenup Grass & Weed Killer, Hi-Yield Super Concentrate Killzall Weed & Grass Killer, Maxide Super Concentrate 41% Weed & Grass Killer, and Southern States Grass & Weed Killer Concentrate) can be applied to the garden plot 3 days prior to planting. Check the label for precautions for individual crops. Glyphosate is most effective when weeds are actively growing, so do not apply during extreme heat, cold or drought conditions.

Control in Landscape Beds

In landscape beds, annual bluegrass can be hand dug or controlled with an herbicide. As mentioned previously, it is best to prevent the invasion of annual bluegrass by maintaining optimum cultural conditions and using a 3-inch mulch layer to block weed development. Once annual bluegrass has made its way into the landscape bed, an herbicide may be necessary if hand pulling or hoeing is not practical.

Pre-emergence herbicides can be used to prevent weed germination in the fall. Trifluralin (Hi-Yield Herbicide Granules, Green Thumb Garden Weed Stopper, and Preen Garden Weed Preventer) is safe to apply around certain landscape plants. See the herbicide label for a full list of plant species, the proper application rate, and watering-in instructions.

Glyphosate (such as in Roundup Original, Eraser Systemic Weed & Grass Killer, Quick Kill Grass & Weed Killer, Bonide Kleenup Grass & Weed Killer, Hi-Yield Super Concentrate Killzall Weed & Grass Killer, Maxide Super Concentrate 41% Weed & Grass Killer, and Southern States Grass & Weed Killer Concentrate) can be used for spot treatments around ornamental plants. Glyphosate is a non-selective herbicide that can harm any plant and should be used with caution. Do not allow glyphosate spray mist to contact ornamental foliage or stems as injury will occur.

As with all pesticides, read and follow all label instructions and precautions.

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This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.