Plant 325

Poa annua L. (Poaceae)

Annual meadow-grass

Among snowdrops and aconites that briefly turn winter borders into swathes of white and yellow, there will be annual meadow-grass blooming with a much less showy display. Annual meadow-grass is easily recognised, even when it lacks its pyramids of tiny, pale flowers. The light green, frequently wrinkled leaves are hairless with flattened sheaths and tips that look like the prow of a boat.

The species flowers throughout the year in the United Kingdom. Individual plants can produce more than 20,000 seeds in a single year, which may germinate immediately or remain as part of the soil seed bank. Consequently, when soil is disturbed this grass colonises rapidly. Annual meadow-grass is mostly a summer annual, that is seed germinates in the spring and can go through several generations in a year. However, it may also behave as a winter annual – germinating in the autumn and then overwintering. In favourable conditions, the period from seed to seed can be fewer than six weeks.

Most annual meadow-grass seed is the product of selfing. This means groups of plants, even those growing close together, can be different to each other. For example, sorts found in regularly mown lawns are different to those growing in adjacent flower beds. In the United Kingdom, flower beds weeded by hand are more likely to contain a purple-brown mutant of annual meadow-grass than beds treated with general herbicides. The reason appears to be that the camouflaged mutant, which escapes gardeners’ notice, is contributing more seeds to the soil seed bank than the usual sort.

Given its variability, biology and close association with humans, it is unsurprising annual meadow-grass has travelled across the planet in the wake of our migrations. A Eurasian species, it is now found throughout temperate and subtropical regions of the planet from sea level to elevations of greater than 2,000 metres; in the tropics it is a species of mountain tops. Since the mid-1980s, when annual meadow-grass was discovered in Antarctica, it is the only non-native flowering plant to have successfully established breeding populations and a seed bank. Moreover, it is spreading throughout the sub-Antarctic, where it competes successfully with the few flowering plants native to such extreme environments.

In the United Kingdom, annual meadow-grass is one of the most abundant weeds of man-made habitats. However, annual meadow-grass is also an economically important species where fine turfs are desired, for example, on bowling and golfing greens.

Further reading

Hutchinson CS and Seymour GB 1982. Biological flora of the British Isles. No. 153. Poa annua L. Journal of Ecology 70: 887-901.

Molina-Montenegro MA et al. 2019. Increasing impacts by Antarctica’s most widespread invasive plant species as result of direct competition with native vascular plants. NeoBiota 51: 19-40.

Warwick SI 1979. The biology of Canadian weeds. 37. Poa annua L. Canadian Journal of Plant Sciences 59: 1053-1066.

Stephen Harris