Annual BluegrassAnnual 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.
IDENTIFICATION AND LIFE CYCLE
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.
IMPACT
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.