How to Manage Pests
Pests in Gardens and Landscapes
Annual bluegrass, Poa annua, is one of the most common weeds of residential and commercial turfgrass, ornamental plantings, and gardens in the United States. It is native to Europe but is distributed worldwide. Commonly referred to as “Poa,” it is a particular problem in golf course greens and fairways, but it can also be a troublesome weed in vegetable and agronomic crops grown in cool climates. Though present in tree and vine crops in California, it usually isn’t a significant problem. The genus Poa consists of about 200 species worldwide. Their typical boat-shaped leaf tips, which curve up like the bow of a boat, are a distinguishing characteristic of the genus.
Three members of the genus Poa are commonly found in turfgrass sites in California. Kentucky bluegrass, P. pratensis, is a common cool-season turf species that is well adapted to cool, well-watered sites such as coastal and intermountain areas. Rough bluegrass, P. trivialis, is a less desirable turf species that does well in moist, shaded areas but lacks heat and drought tolerance, so it is short-lived and is generally considered a weed. Annual bluegrass is a weed species that, unlike Kentucky and rough bluegrass, is able to survive low mowing heights of less than 1 inch and still reseed. A fourth species, bulbose bluegrass, P. bulbosus, is sometimes found as a weed in Northern California turfgrass.
Annual bluegrass is a misnomer because there are two plant types of annual bluegrass—a true annual, P. annua var. annua, and a perennial type, P. annua var. reptans. While the two types aren’t easy to distinguish from each other, the annual type is more upright in its growth habit and produces more seed than the lower-growing perennial type. The annual type also tends to produce a higher percentage of dormant seed. The perennial type produces seed that germinates readily under optimum conditions. Depending on the site, there might be a predominance of one type or a mixture of both. The perennial type is common in such sites as golf course greens, while the annual type tends to be more common in lawns and parkways, although both types can be found in either of these situations.
Annual bluegrass is a cool-season grass weed that starts germinating in late summer or fall as soil temperatures fall below 70°F. It continues to germinate throughout winter, allowing several flushes of germination at any one site throughout the season. Annual bluegrass grows 6 to 8 inches high when left unmowed. It has light green flattened stems that are bent at the base and often rooted at the lower stem joint. Leaf blades are often crinkled part way down and vary from 1 to 3 inches long with typical Poa boat-shaped leaf tips. The inflorescence (flowering structure) has branched seed clusters (panicles) that are 1 to 4 inches long. Seed clusters, also called seed heads, can form as soon as plants are six weeks old; although this can occur from early fall through early summer, most seed heads are formed in spring.
The annual form of annual bluegrass is a rapid and prolific seeder. Each small plant can produce about 100 seeds in as few as eight weeks. Viable seed can be produced just a few days after pollination, which allows the plant to reseed even in frequently mowed turf. The small seed is amber colored and about 1/16 inch long.
Annual bluegrass has a fairly weak and shallow root system and needs frequent rainfall or irrigation to survive. It grows well in moist areas in partial shade to full sun and tolerates compacted soil conditions. In coastal regions or in moderate temperature areas where turf is frequently irrigated, annual bluegrass can persist all year. In warmer areas, it usually dies in summer.
Annual bluegrass can be a major weed problem in home lawns and is a continual nuisance for turf and landscape managers. Because it is a grass weed growing in turfgrass, selectively removing it is very challenging. In turf it forms a weak sod that provides poor footing for athletic fields and golf courses. In addition, unsightly seed heads of annual bluegrass reduce the aesthetic quality of the turf and disrupt the smooth rolling surface of putting greens.
Because of its winter growth habit, it is more competitive than warm-season turf cultivars (e.g., common and hybrid bermudagrass, buffalograss, St. Augustinegrass, and zoysiagrass) during the cool season. This accounts for the severity of annual bluegrass invasions during winter. Although annual bluegrass can be a problem in all turf species, it visually blends into many cool-season turf species (e.g., Kentucky bluegrass, tall fescue, and perennial ryegrass) and is most obvious in closely mowed species, such as bentgrass and bermudagrass, especially the dwarf hybrids. In cool seasons, annual bluegrass grows faster than warm-season turf cultivars, which gives infested turf an undulating or irregular surface in as little as two days after mowing.
When annual bluegrass infests ornamental plantings, it forms a dense mat that lowers the vigor of desirable landscape plants by reducing available nutrients in the soil surface. In established woody shrubs and trees, annual bluegrass probably has little detrimental effect but can be aesthetically distracting.
Once a few annual bluegrass plants become established in turf or ornamental areas, spread can be rapid because of its prolific and rapid seed production. Mowing, foot traffic, birds, and cultivation all spread seed.
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.
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.
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.
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Authors: M. LeStrange, UC Cooperative Extension, Tulare Co.; P. M. Geisel, Statewide Master Gardener Coordinator, UC Cooperative Extension, Glenn Co.; D. W. Cudney, Botany and Plant Sciences emeritus, UC Riverside; C. L. Elmore, Plant Sciences emeritus, UC Davis; and V. A. Gibeault, Botany and Plant Sciences emeritus, UC Riverside.
Produced by UC Statewide IPM Program, University of California, Davis, CA 95616
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