Poa annua Seeds | Annual Bluegrass
LATIN NAME: Poa annua
COMMON NAME: Annual Bluegrass
Plant Profile
Annual bluegrass is a small, soft, bright green grass of the family Poaceae, native to Europe and now present on every continent including Antarctica. It grows five to thirty centimetres, with folded shoots, a slightly wrinkled leaf tip and a triangular, open panicle of small spikelets, flowering from early spring until the frost.
It is the grass that appears in bare patches of lawn, in the cracks of a path and in the mud of a building site, and it flowers and sets seed at almost any size, even when it is mown to a centimetre. That flexibility is what makes it so successful. The plant is usually annual, but perennial forms persist in golf greens and in irrigated turf.
Science & Biotechnology
Annual bluegrass is an allotetraploid with twenty-eight chromosomes, descended from the hybrid of two annual species, Poa infirma and Poa supina. The hybrid arose in Europe and then doubled its chromosomes, and the resulting plant combined the quick life cycle of one parent with the shade tolerance of the other, which is a large part of why it spreads so widely.
The species has evolved resistance to herbicides more often than almost any other grass. Populations resistant to glyphosate, to the ALS inhibitors and to the dinitroanilines have been found on golf courses and in nurseries, and the resistant plants often show no fitness cost, so they persist after the chemistry stops. A chromosome-level genome published in 2022 has given researchers a way to find the mutations involved.
Uses & Benefits
For most gardeners it is a weed, and a persistent one: it seeds within weeks of germinating, the seed survives in the soil for years, and it dies back in summer leaving brown patches in a lawn. On golf greens, though, it is both the curse and the surface, since it tolerates mowing at three millimetres better than any bentgrass.
It is also a forage grass in wet pastures and a minor component of some lawn mixes, and its pollen is a significant cause of spring hay fever. In a garden it can be kept out of a lawn by mowing high, feeding in autumn and overseeding bare patches, since it is a plant of thin, compacted, infertile ground and loses to a dense sward.
Growing from Seed
It needs no help. If a sowing is wanted, scatter the seed on bare, moist soil in spring or autumn and keep it damp; it comes up in five to seven days at 15-20 °C. The plant will flower within six weeks and set seed within ten.
To control it rather than grow it, raise the mowing height, feed the lawn in early autumn, spike compacted ground and overseed in September, when the desired grasses establish fastest. In a border, pull the plants before the seed heads open, since a single plant can shed several hundred seeds.
Our Take
The most successful weed in the world, and a lesson in how far a small grass can travel. Grow it by accident, or study it on purpose.
Related Reading
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