A small, soft, hairless, tufted annual grass with boat shaped leaf tips and a finely branched, open, Christmas tree shaped seed head. It can be quite variable in habit depending on where it is growing.
Blade - Light green, flat or folded when young, soft, thin, 5-140 mm long by 1-5 mm wide, smooth or slightly rough to touch, often transversely wrinkled, parallel sided and abruptly pointed into a boat shaped tip. Edges often slightly rough to touch. Hairless
Ligule - Thinly membranous, 1-5 mm long. Flat or pointed tip.
Auricles - None.
Sheath - Flattened, keeled, striped. Hairless.
Erect, spreading or low lying, 20-300 mm long, tufted, weak. Sometimes rooting from lower nodes. Sometimes bent at the lower nodes. 2-4 nodes. Rarely branched at the base.
Triangular to ovate, open panicle, 10-120 mm long, with more or less stiffly spreading, very slender, single or paired, branches that are initially held against the stem. Pale to bright green, whitish, reddish or purplish panicle held just above the leaves. Spikelets towards the ends of the branches. Hairless.
White/green turning red/purple with age.
Spikelets - 2-10 flowered, egg shaped to oblong, 3-10 mm long. Often stalked. Green to purplish or whitish.
Florets - Overlapping (Distinctly separated from each other in P. infirma). No web of hairs at the base.
Glumes - 1.5-4 mm long, unequal, papery, edges translucent. Lower glume 1 nerved, 1.5-3 mm long. Upper glume 3 nerved, 2-4 mm long.
Palea - 1.75-2.75 mm long.
Lemma - Obviously 5 nerved, egg shaped, 2-4 mm long, translucent, keeled, pale edges, obtuse tip. Densely hairy along the nerves or hairless. Awnless.
Anthers - 0.2-1.2 mm long
Breaks above the glumes and between the florets.
Oval to tear shaped with a pointed tip, ridged, brown, hairy.
Enclosed in fruit.
Small annual up to 300 mm tall.
Leaves soft, herbaceous, hairless.
Ligules thinly membranous.
Panicle triangular to ovate with more or less stiffly spreading capillary branches.
Lower panicle branches spreading or reflexed after flowering.
Floret closely imbricate with no web of hairs at the base.
Lemma manifestly 5 nerved.
After Black(1978), Burbidge and Gray (1970) and Marchant et al.(1987).
Annual. Seed germinates from autumn to winter. It grows 3 leaves then produces a tiller in each subsequent leaf, which grows another 3 leaves and so on. Each of these tillers produces a flower head in succession so that flowering is extended over a long period from late winter to early summer. The plant dies with the high temperatures and drought of summer but may continue growing into the second season in mild, moist situations. It can be found at any time of the year but is most abundant in winter and spring. It can complete its life cycle in a few weeks.
Tolerant of repeated mowing.
Winter to spring in western NSW.
Most of the year in SA with a flush in spring.
August to October in Perth.
Seed Biology and Germination:
Population Dynamics and Dispersal:
Spread by seed.
Origin and History:
ACT, NSW, QLD, SA, TAS, VIC, WA.
Many soil types.
Palatable fodder but poorly productive.
Weed of crops, rotation crops, perennial crops, pastures, grass land, lawns, gardens, cultivation, fallows and disturbed areas.
Not recorded as toxic.
Management and Control:
Mowing is ineffective.
Grazing has variable effects.
Pasture manipulation with paraquat plus simazine reduces infestations.
Prevent seed set for 3 years.
Establish a competitive and desirable pasture grass.
Propyzamide provides good control.
Achieve provides selective control in broadleaf crops.
Blue Tussock Grass (Poa poiformis, Poa australis) has a short membranous ligule and may have slightly swollen bases.
Bulbous Poa (Poa bulbosa) is similar but perennial and has swollen pear shaped leaf bases.
Kentucky Bluegrass (Poa pratensis)
Knotted Poa (Poa drummondiana) also has bulbils or 1-3 swellings at the base of each stem but a much looser head
Rough Meadow Grass (Poa trivialis) is similar but perennial, stoloniferous, has a longer ligule at 4-10 mm long and a web of hairs at the base of the lemma.
Scaly Poa (Poa fax, Poa lepida)
Snowgrass (Poa sieberiana)
Sweet Swampgrass (Poa fordeana)
Tussock grass (Poa labillardieri)
Poa infirma is very similar to Annual Poa but smaller with stalkless spikelets and narrower panicle whose branches are not bent back after flowering.
Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P133. Photo.
Hussey, B.M.J., Keighery, G.J., Cousens, R.D., Dodd, J. and Lloyd, S.G. (1997). Western Weeds. A guide to the weeds of Western Australia. (Plant Protection Society of Western Australia, Perth, Western Australia). P66. Photo.
Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).
Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #995.1.
Marchant, N.G., Wheeler, J.R., Rye, B.L., Bennett, E.M., Lander, N.S. and Macfarlane, T.D. (1987). Flora of the Perth Region. (Western Australian Herbarium, Department of Agriculture, Western Australia). P981.
Paterson, J.G. (1977). Grasses in South Western Australia. (Western Australian Department of Agriculture Bulletin 4007). P77. Diagram.