Weed Technology. 1998. Volume 12:414-416 Intriguing World of Weeds iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii Annual Bluegrass (Poa annua L.) 1 LARRY W. MITICH2 "A meadow of this very common variety of grass, with its feathery flower panicles shimmering in waves of shaded colour, is balm to the soul." The Wild Garden. Lyes de Bray, 1978 INTRODUCTION AND ETYMOLOGY at all, but green, and like our common natural grass" (Simpson and Weiner 1989). The grasses or Poaceae (Gramineae) comprise some Annual bluegrass is a weed of cultivated and waste 9,000 species grouped into about 650 taxa. Although not area habitats; it is noted for its ability to infest numerous the largest, the family is ecologically the most dominant crops. Amazingly, both annual and perennial types exist and economically the most important in the world (Hey within the species. It has a worldwide importance far in wood 1993). excess of its size, and is ubiquitously distributed and The genus Poa L. has about 500 widely adapted to diverse habitats. Annual bluegrass sur­ temperate and cold-climate spe­ vives as a weed because of its high genotypic and phe­ cies; 44 plus four stabilized hy­ notypic variability, rapid germination, survival when up­ brids are found in Europe. Annual rooted, and tolerance to compacted soils. It is a major bluegrass (Poa annua L.) is a tet­ weed in turfgrass and is found on most soil types. Seed raploid hybrid between P. infirma production and seed viability are generally high, which Kunth and P. supina Schrader and makes it a very successful colonizer. Its small, light another species (Mabberley 1989; seeds are readily spread by birds and transported on Totin 1957). The bluegrasses of the U.S. and the coun­ shoes, tools, mowers, and other equipment (Holm et al. terpart meadow grasses of England are important forage 1997; Warwick 1979). and turf grasses. From its postulated center of origin in the Mediter­ ranean, annual bluegrass has spread throughout the tem­ DESCRIPTION perate and alpine regions of the world, no doubt through Poa annua is an annual or a short-lived perennial, human activity (King 1966). Naturalized very early in loosely or compactly tufted, and from 2 to 40 cm high. the Northern Hemisphere, seeds of annual bluegrass The culms, 5 to 25 cm long and with two to four nodes, have been recovered from adobe bricks in California are erect, spreading or prostrate, and sometimes nodally missions dating to about 1797 (Warwick 1979). rooting, especially the perennial types. The branches are Poa is an ancient Greek name for grass or fodder and mostly paired and are spreading and glabrous. Annual annua, of course, refers to its annual nature (Gledhill bluegrass forms small mats with sometimes basal 1985; Hyam and Pankhurst 1995). branching and has fibrous roots. The word bluegrass was first used in print by Chris­ Its bright to dark green leaves are glabrous; the leaf topher Gist (d. 1759) in Christopher Gist's J?umals in sheaths are compressed, keeled, smooth, and surround 1751 where he wrote, "It is ... full of beautiful mead- the culm loosely. The ligules are thinly membranous and ' " I ows covered with wild Rye, blue grass and Clover. n from'2 to 5 mm long. Leaf blades are smooth, v-shaped, 1879, Sir George Campbell (1824-1892) publis?ed from 3 to 10 cm long and 2 to 4 mm wide, with boat­ White and Black; the Outcome of a Visit to the United shaped tips, which is characteristic of Poa species. States. On page 47 he gave his view of bluegrass: "The The inflorescence, 2.5 to 10 cm long, is borne in a blue-grass of Kentucky is famous; though it is not blue terminal panicle, which ranges from 1 to 11 cm long, is ovate or pyramidal, and in color is pale to bright green, 1 No. 61 of the series "Intriguing World of Weeds." . but sometimes reddish or purplish. The spikelets are 2 Extension Weed Scientist Emeritus. Department of Vegetable Crops, Uni­ versity of California, Davis, CA 95616. three- to six-flowered. The fruit is an elliptical and bright 414 WEED TECHNOLOGY of strongly acid soils, high pollution levels, and extremes of temperature and moisture (Beard 1970; Gibeault 1966). The annual type predominates in cultivated areas, and the perennial types flourish in closely mowed areas such as golf courses and pastures (Totin 1957). Annual bluegrass is a weed of 38 crops in 80 coun­ tries. It is found most commonly in vegetables, cereals, turf, orchards, sugar beets, and potatoes (Holm et al. 1997). It is a serious competitor with newly seeded al­ falfa, especially in the fall, and it can endanger success­ ful legume establishment if not controlled. In England's cultivated land, annual bluegrass seed is reported to be the sixth most common weed seed (Gi­ beault 1966) .. The species is an important hay fever plant Annual bluegrass (Poa annua L.). in Colorado (Newmark 1978). amber caryopsis from 1.5 to 2.5 mm long (Holm et al. GROWTH CHARACTERISTICS 1997; Warwick 1979). Numerous characteristics contribute to the success of Sometimes P. annua is confused with two other Poa annual bluegrass as a weed (Totin 1957). These encom­ species: P. pratensis L. (Kentucky bluegrass) and P. pass huge phenotypic and genotypic variability, rapid compressa L. (Canada bluegrass). Poa annua may be germination, a short life cycle, small size ( escapes being distinguished from· these species primarily by its growth noticed), resistance to being uprooted and high degree habit. It acts either as an annual or a short•lived perennial of survival when uprooted, and development of a shal­ that roots at the nodes, whereas P. pratensis and P. com­ low root system in response to soil compaction (Sprague pressa are rhizomatous perennials (Warwick 1979). and Burton 1937). Man is probably the main carrier of seeds through transport by mower and other implements. Certainly, mowing is an important factor in influencing DISTRIBUTION AND WEEDINESS the distribution pattern of the species (Gibeault 1966). Annual bluegrass was introduced to the U.S. from its Annual bluegrass flowering begins very early in the ancestral home in Europe from where it has spread to spring, with maximum seedhead production occurring in over 80 countries in North Africa, North Asia, Australia, May and June; it continues at a low level through the North and South America, and Antarctica. It favors tem­ rest of the season (Wells 1974). Young seedlings grow ·perate to alpine zones, which explains why it is confined rapidly during autumn and during winter in warmer cli­ to mountainous regions in tropical countries (Wells mates (Younger 1959). 1974). Even though it is a very adaptable species, annual In the Northern Hemisphere, annual bluegrass is day­ bluegrass is not found in semiarid regions because of its neutral; flowering is initiated in early spring. Seed pro­ intolerance to drought. It also lacks tolerance to extremes duction reaches its peak in May and June, then decreases of temperature, particularly moisture and soil tempera­ until senescence. Not all tillers on a plant mature uni­ ture (Beard 1970). formly (Wells 1974). When grown at low density, Law Annual bluegrass invades lawns, golf courses, gar­ et al. (1977) estimated annual bluegrass seed production dens, paths, flower beds, cracks in sidewalks and pave­ per inflorescence at 80 viable seeds. Plants germinating ment, wasteland, and roadsides. It occurs in pastures, in the fall in northern latitudes may die during winter agronomic and horticultural crops, on coastal sand, cliff (Beard 1964). faces or rocky outcrops, and at the margins of ponds and Individual annual bluegrass plants are capable of pro­ streams (Walton 1975; Warwick 1979). Annual bluegrass ducing copious amounts of seed; estimates usually range grows in widely diverse soils from sands to clays. Be­ from 1,050 to 2,250 seeds per plant (Holm et al. 1997). cause it tolerates trampling, it is a frequent weed in areas Even plants mowed to 0.5 cm are capable of producing heavily trafficked by livestock or humans; under such 360 seeds each (Beard 1973). The perennial types pro­ conditions, its root system is quite shallow. It exhibits a duce approximately 13,000 seeds per plant (Cordukes high tolerance to poorly aerated soils and an intolerance 1977). Annual bluegrass is not an invader that displaces Volume 12. Issue 2 (April-June) I 998 415 MITICH: ANNUAL BLUEGRASS other plants, but rather a highly opportunistic colonizer Clifford, H. 1956. Seed dispersal on footwear. Proc. Bot. Sci. Br. Isles. pp. 30-35. (Wells and Haggar 1974). Cordukes, W. E. 1977. Growth habit and heat tolerance of a collection of Poa Human activity, water, animal consumption (especial­ annua plants. Can. J. Plant Sci. 57:1201-1203. de Bray, L. 1978. The Wild Garden. New York: Mayflower Books. 191 p. ly livestock and birds), and wind are the chief forms of Gibeault, V. A. 1966. Annual meadow-grass-a major weed of fine turf. J. seed dispersal. Its small, light seeds are delectable to Sports Turf Res. Inst. 41 :48-52. Gibeault, V. A. and N. R. Goetz. 1973. Annual meadow-grass. J. Sports Turf birds (Warwick 1979). Not surprisingly, annual blue­ Res. Inst. 48:48-52. grass seeds were the most abundant of all species found Gledhill, D. 1985. The Names of Plants. 2nd ed. Cambridge, UK: Cambridge in mud on footwear (Clifford 1956); they are also a com­ University Press. 202 p. Goss, R. 1964. Preemergence control of annual blue grass (Poa amzua L.). mon contaminant of forage crop seeds (Tonkin 1967). Agron. J. 56:479-481. Viable annual bluegrass seeds have been found in cattle Heywood, V. H. 1993. Flowering Plants of the World.