Annual bluegrass

Poa annua L.

annual bluegrass panicle
annual bluegrass seedling
annual bluegrass stand

Images above: Upper left: Annual bluegrass panicle (Scott Morris, Cornell University). Upper right: Annual bluegrass seedling (Scott Morris, Cornell University). Bottom: Clumps of mature plants (Randall Prostak, Universiity of Massachusetts).

Identification

Other common names: low speargrass, six-weeks grass, annual meadowgrass, annual spear grass, dwarf spear grass, dwarf meadow grass, causeway grass, speargrass, poanna

Family: Grass family, Poaceae

Habit:  Short winter or summer annual bunchgrass, sometimes surviving through a second growing season.  Two major variants have been identified, P. annua var. annua, an annual with erect habit, and P. annua var. reptans, a short-lived perennial with prostrate growth habit (Hutchinson and Seymour 1982).  The perennial variants are more common in turfgrasnnual spear-grass, dwarf spear-grass, dwarf meadow grass, causeway grass, speargrass, poannas than are the annual variants (Shem-Tov and Fennimore 2003).  Perennial variants survive by growing roots from stem nodes in contrast with Poa turfgrasses which are rhizomatous perennials (Mitich 1998).

Description:  The linear seedling leaf 0.2-0.35” (0.51-0.89 cm) long by 0.04-0.1” (0.10-0.25 cm) wide emerges perpendicular to the ground.  Young true leaves unfold from the bud into a V with a boat-prow shaped tip; they lack auricles and have 0.1-0.2” (0.25-0.51 cm) long, translucent, slightly pointed ligules.  Collar regions are green and hairless.  Light green blades are 0.75-2” (1.9-5.1 cm) long by less than 0.1” (0.25 cm) wide, crinkled or wavy on the surface, and smooth-edged.  Blades may be hairless or have scattered soft hairs.  Hairless, flattened sheaths open down the stem.  Mature plants grow into 1-12” (2.5-30.5 cm) tall, highly tillering, upright or prostrate clumps.  Stems are bright green to yellow-green.  Sheaths open nearly to the base, and easily pull away from the stem.  Sheaths are transparent and pale, hairless, V-shaped, and without auricles; mature leaves have a ligule similar to that of seedling leaves.  Blades are light green, linear, 0.5-5.5” (1.3-14 cm) long by 0.1” (0.25 cm) wide, nearly hairless, with the distinctive boat-prow shaped tip.  The root system is shallow, fibrous and matting.  Spontaneous rooting occurs at tiller bases.  Numerous 1-3” (2.5-7.6 cm) tall, pyramid-shaped inflorescences develop.  White-green immature seedheads are open or airy, with several small flower clusters (spikelets) loosely grouped at the ends of branches within the pyramid.  Individual spikelets are 0.2” (0.5 cm) long and contain 2-6 flowers.  As with other grasses, the apparent seed includes a thin tightly adhering layer of fruit tissue.  Yellow-gray seeds are 0.1” (0.25 cm) long, elliptical, with one blunt end.

Similar species:  Three other perennial bluegrass species resemble annual bluegrass.  Canada bluegrass (Poa compressa L.) is 1-2 ft (30-61 cm) tall, has flat, wiry stems, and narrow, blue-green blades.  Kentucky bluegrass (Poa pratensis L.) is 1-3.25 ft (30-99 cm) tall with dark green leaves 2-30” (5-76 cm) long, and is a coarser looking plant whose seedheads consistently have a whorl of 5 small spikes at the base.  Roughstalk bluegrass (Poa trivialis L.) is 1-3 ft (30-91 cm) tall, with yellow-green leaves 1-8” (2.5-20 cm) long.  Leaves are rough and folded in the bud, with a large tapering membranous ligule.  Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] resembles annual bluegrass, but can be distinguished by its long, narrow auricles.

Management

Annual bluegrass can be a severe problem in turfgrass (Vargas and Turgeon 2004).  In agriculture it is primarily a problem in vegetables and small grain crops.  Close mowing shifts the population in grassy areas to the creeping perennial form that is more easily controlled by tillage when it invades tilled fields.  If annual bluegrass is abundant at the time of spring tillage, use a moldboard plow to invert the soil, which will bury the plants and kill them.  Subsequent shallow secondary tillage should rely on disks or harrows to avoid bringing clumps back to the soil surface.  Rotary tillage is not recommended as it will leave many clumps only partially buried.  These will have a good head start on your crop, and may cause problems if the crop is not highly competitive. 

Severe infestations are usually the result of leaving soil undisturbed from late summer/early fall through the spring.  In this situation, a few plants can produce many seeds before late spring tillage, and the large clumps are likely to survive tillage.  To prevent problems with annual bluegrass the following year, till seedlings under late in the fall after most have germinated and then plant a winter cereal cover crop at high density to compete with late emerging individuals. When cultivating to destroy young annual bluegrass, burial is more effective than uprooting since this weed is very good at re-rooting.  Cultivating annual bluegrass after it forms clumps is not effective unless performed in very hot and dry conditions.

In pastures, annual bluegrass usually indicates over grazing or excessive trampling (Hutchinson and Seymour 1982).  It tends to invade where other grasses have died or been damaged rather than displacing them by competition.

Ecology

Origin and distribution:  Annual bluegrass originated in southern Europe, and has been introduced into more than 80 countries in North Africa, North Asia, Australia, North and South America, and even Antarctica.  It is a weed of temperate and subarctic climates and occurs in the tropics only at high elevations.  (Mitich 1998)

Seed weight:  0.19-0.48 mg (EFBI, Hutchinson and Seymour 1982), 0.21 mg (Milberg et al. 2000), 0.37 mg (Scherner et al. 2017), 0.27-0.36 mg for field grown plants with some unfilled seeds compared to 0.54-0.59 mg for greenhouse grown plants with completely filled seeds (Lush 1989).

Dormancy and germination:  Annual bluegrass produces both dormant and non-dormant seeds (Lush 1989, Standifer and Wilson 1988a, Wu 1987).  The proportion of dormant seeds varies between populations and is affected by conditions during seed set.  For example, a dormant Louisiana population that required warm temperatures to break dormancy functioned as a winter annual, whereas non-dormant Wisconsin populations functioned as summer annuals (Standifer and Wilson 1988a).  Annual forms generally have a higher percentage of dormant seeds while perennial types have minimal dormancy (McElroy et al. 2004).  Dormancy declines over a period of months (Lush 1989, Standifer and Wilson 1988b).  Thus, non-dormant seeds produced in the spring germinate in the summer, whereas the dormant seeds are ready for germination by fall.  Dormant seeds collected in Louisiana required moist heat to break dormancy (Standifer and Wilson 1988b) whereas seeds from Australia required chilling (Lush 1989).  A few seeds can germinate at temperatures as low as 36 to 41 °F (2 to 5 °C) (Scherner et al. 2017), but high germination percentages require temperatures of 41 to 68 °F (5 to 20 °C) (Standifer and Wilson 1988b).  In ecotypes from Alabama, germination was best at 66/50 °F (19/10 °C) day/night temperatures, but declined at higher temperatures (McElroy et al. 2004).  Temperatures over 86 °F (30 °C) decrease germination of non-dormant seeds and can reduce viability of dormant seeds (Standifer and Wilson 1988b).  Many seeds that get incorporated into the soil before germination (for example, if they are cultivated into dry soil) become dormant and require specific environmental cues to stimulate germination. For such seeds, germination is stimulated by light, fluctuation in day/night temperatures and the presence of nitrate (Hutchinson and Seymour 1982, Ohadi et al. 2010).  Germination is reduced in dry conditions more so than for other winter annual grass species (Scherner et al. 2017).  In the coastal area of central California, it was hypothesized that lengthening days in the spring may contribute to inducing dormancy in spring and early summer, whereas shortening daylength may contribute to decreasing dormancy in fall and early winter (Shem-Tov and Fennimore 2003).

Seed longevity:  Few seeds of annual bluegrass survive in the soil longer than five years.  In soil stirred four times per year, seed density declined by 26% per year, and declined by 22% per year in undisturbed soil (calculated from Roberts and Feast 1972 and Mohler 2001).  In another study, seed mortality was 29-50% per year in frequently disturbed soil and 17-26% per year in undisturbed soil (Roberts and Feast 1973).

Season of emergence:  Most emergence occurs in the late summer and fall, but some emergence occurs during spring and summer (Hutchinson and Seymour 1982, Shem-Tov and Fennimore 2003).

Emergence depth:  This does not appear to have been studied, but given the small seed size, most seeds probably emerge from the top 1” (2.5 cm) of soil.

Photosynthetic pathway:  C3 (EFBI)

Sensitivity to frost:  Annual bluegrass tolerates frost well (EFBI).  Exposed, fully frozen plants will die in about two weeks (Warwick 1979), but the species commonly overwinters under snow or in the shelter of cover crops or crop residue (Mohler personal observation).

Drought tolerance:  Annual bluegrass is intolerant of drought and high soil temperatures (Hutchinson and Seymour 1982, Mitich 1998).  Plant leaf length, tillering, and mass were greater when grown at 72 °F (22 °C) than when grown at 90 °F (32 °C) (Lush 1989).  Plants, however, can tolerate short exposure to high air temperatures, with plants surviving a 3-hour exposure to 117 °F (47 °C) (Hutchinson and Seymour 1982).

Mycorrhiza:  Annual bluegrass is mycorrhizal (Harley and Harley 1987). 

Response to fertility:  Annual bluegrass size responds directly to the amount of N, P or K applied.  Nitrogen favors leaf production, whereas P and K promote seed production (Hutchinson and Seymour 1982).  Herbage growth increased with increasing levels of P and K (Hoveland et al. 1976).  It grows best on soils with pH from 5.5-6.5 and is rarely found on soils with pH < 5.3 (Buchanan et al. 1975).

Soil physical requirements:  Annual bluegrass can grow in different types of soils, from clays to sands.  It tolerates extreme compaction and waterlogged conditions (Mitich 1998).  Fine textured soils that maintain higher water content in winter tend to have higher annual bluegrass populations than sandier, well drained soils (Shem-Tov and Fennimore 2003).

Response to shade:  The species grows well in moderate shade, and often thrives under trees and in pastures and hay meadows dominated by taller grasses (Hutchinson and Seymour 1982).  It can sustain vegetative and reproductive growth at 80 to 92% shade, explaining its common occurrence in fields of highly competitive cereal crops (Yasin et al. 2017).   It is associated with shaded cracks in sidewalks (Fagot et al. 2011).

Sensitivity to disturbance:  Annual bluegrass is difficult to uproot with hoes and cultivators because part of the spreading root system often remains in the ground.  The dense roots hold soil and allow uprooted plants to survive and quickly re-root (Hutchinson and Seymour 1982).  Once the plants have made side shoots (tillers), this species is among the hardest weeds to kill by uprooting.  It is very tolerant of trampling, and it will flower and set seed when kept mowed to a height of 0.25” (0.6 cm) (Hutchinson and Seymour 1982).

Time from emergence to reproduction:  Overwintering plants begin growth in the very early spring and flower almost immediately.  Peak seed production occurs in May to early June (Mitich 1998), but also can occur throughout the year (Hutchinson and Seymour 1982).  Cold vernalization accelerates heading; this occurs at the 11-leaf stage with vernalization and the 15-leaf stage without vernalization (Lush 1989).  Spring germinating plants of the annual form set seeds in 44-55 days, whereas the perennial form required 81-93 days.  Shade may delay time to first flowering by up to 20 days (Yasin et al. 2017).  Cut flowering panicles can still produce viable seed (Hutchinson and Seymour 1982), suggesting that seeds mature quickly.

Pollination:  Annual bluegrass primarily self-pollinates, but occasionally cross-pollinates by wind dispersed pollen (Hutchinson and Seymour 1982).

Reproduction:  Annual bluegrass plants typically produce 1,050 to 2,250 seeds per plant (Mitich 1998).  In a British study, annual bluegrass produced about 80 seeds per stalk when the plants were grown at low density.  Pasture populations dominated by the perennial form averaged 30 seed stalks per plant in the first year and 450 the second.  Populations dominated by the annual form averaged 100 seed stalks per plant the first year and 180 in the second (Law et al. 1977).  Under ideal conditions, seed production could exceed 20,000 seeds per plant (Hutchinson and Seymour 1982).  In the perennial form of the species, the prostrate stems root at the nodes.  The stems then decompose over the winter so that this form of the plant has true vegetative reproduction.

Dispersal:  Annual bluegrass disperses short distances by rain wash and wind (EFBI).  People, however, contribute most to the dispersal between fields and farms.  The seeds stick readily to wet shoes, tires and mowing machinery, and since seeds often reach high density in soil, they probably also frequently disperse in soil clinging to tillage and cultivation machinery.  Mowing can be an important factor dispersing this species.  Seeds also pass through cattle and are spread with the manure.  Birds may occasionally disperse the seeds.  (Hutchinson and Seymour 1982, Mitich 1998)

Common natural enemies:  Bluegrass billbug larvae (Calendra parvulus) eat the rootlets.  Several species of Carambus moth, including the bluegrass webworm (C. teterellus) attack annual bluegrass; many fungal diseases also attack it.  Large earthworms eat annual bluegrass seeds and digest about 70% of what they consume (Hutchinson and Seymour 1982).

Palatability:  People do not eat any part of annual bluegrass.  Livestock, however, find it highly palatable.

Notes:  Annual bluegrass pollen causes hay fever symptoms in many people (Mitich 1998).

References

  • Buchanan, G. A., C. S. Hoveland, and M. C. Harris.  1975.  Response of weeds to soil pH.  Weed Science 23:473-477. 
  • EFBI.  Ecological Flora of the British Isles.  http://ecoflora.org.uk/
  • Fagot, M., B. De Cauwer, A. Beeldens, E. Boonen, R. Bulcke, and D. Reheul.  2011.  Weed flora in paved areas in relation to environment, pavement characteristics and weed control.  Weed Research 51:650–660.
  • Harley, J. L., and E. L. Harley.  1987.  A check-list of mycorrhiza in the British flora.  New Phytologist 105:1-102.
  • Hoveland, C. S., G. A. Buchanan, and M. C. Harris.  1976.  Response of weeds to soil phosphorus and potassium.  Weed Science 24:194-201.
  • Hutchinson, C. S., and G. B. Seymour.  1982.  Ecological flora of the British Isles.  Poa annua L.  Journal of Ecology 70:887-901.
  • Law, R., A. D. Bradshaw, and P. D. Putwain.  1977.  Life history variation in Poa annua.  Evolution 31:233-246.
  • Lush, W. M.  1989.  Adaptation and differentiation of golf course populations of annual bluegrass (Poa annua).  Weed Science 37:54-59.
  • McElroy, J. S., R. H. Walker, G. R. Wehtje, and E. van Santen.  2004.  Annual bluegrass (Poa annua) populations exhibit variation in germination response to temperature, photoperiod, and fenarimol.  Weed Science 52:47-52.
  • Milberg, P., L. Andersson, and K. Thompson.  2000.  Large-seeded species are less dependent on light for germination than small-seeded ones.  Seed Science Research 10:99–104.
  • Mitich, L. W.  1998.  Annual bluegrass (Poa annua L.).  Weed Technology 12:414-416.
  • Mohler, C. L.  2001.  Weed life histories: identifying vulnerabilities.  In Ecological Management of Agricultural Weeds, M. Liebman, C. L. Mohler, and C. P. Staver eds., pp. 40-98.  Cambridge University Press: Cambridge, U.K.
  • Ohadi, S., M. H. Rahimian, R. Tavakkol-Afshari, and M. M. Beheshtian.  2010.  Modelling the effect of light intensity and duration of exposure on seed germination of Phalaris minor and Poa annua.  Weed Research 50:209-217.
  • Roberts, H.A., and P. M. Freast.  1972.  Fate of seeds of some annual weeds in different depths of cultivated and undisturbed soil.  Weed Research 12:316-324.
  • Roberts, H. A., and P. M. Feast.  1973.  Emergence and longevity of seeds of annual weeds in cultivated and undisturbed soil.  Journal of Applied Ecology 10:133-143.
  • Scherner, A., B. Melander, P. K. Jensen, P. Kudsk, and L. A. Avila.  2017.  Germination of winter annual grass weeds under a range of temperatures and water potentials.  Weed Science 65:468-478.
  • Shem-Tov, S., and S. A. Fennimore.  2003.  Seasonal changes in annual bluegrass (Poa annua) germinability and emergence.  Weed Science 51:690-695.
  • Standifer, L. C., and P. W. Wilson.  1988a.  Dormancy studies in three populations of Poa annua L. seeds.  Weed Research 28:359-363.
  • Standifer, L. C., and P. W. Wilson.  1988b.  A high temperature requirement for after ripening of imbibed dormant Poa annua L. seeds.  Weed Research 28:365-371.
  • Vargas, J. M. Jr., and A. J. Turgeon.  2004.  Poa annua. Physiology, Culture, and Control of Annual Bluegrass.  John Wiley & Sons, Inc.  184 pp.
  • Warwick, S. J.   1979.  The biology of Canadian weeds. 37. Poa annua L.  Canadian Journal of Plant Science 59:1053-1066.
  • Wu, L., I. Till-Bottraud, and A. Torres.  1987.  Genetic differentiation in temperature-enforced seed dormancy among golf course populations of Poa annua L.  New Phytologist 107:623-631.
  • Yasin, M., E. Rosenqvist, and C. Andreasen.  2017.  The effect of reduced light intensity on grass weeds.  Weed Science 65:603-613.