The ingress of Poa annua into perennial ryegrass swards

The ingress of Poa annua into established swards of perennial ryegrass was recorded in four experiments aimed at determining whether ingress was due to P. annua aggression or to a decrease in ryegrass vigour. Emphasis was placed on severity of defoliation, effect of bare spaces and the time of year most favourable to colonization.
In Experiment 1, P. annua ingress was greatest in the autumn, especially where ryegrass had been sown in rows rather than broadcast sown. In Experiment 2, seedlings of P. annua only established successfully in mature ryegrass swards that had been defoliated at intervals of two weeks. Experiment 3 showed that ingress was inversely related to the vigour of the ryegrass cultivar, being particularly high on frequently defoliated plots. In Experiment 4, which involved a study of the colonization of poached areas, more seedlings established in quadrats surrounded by P. annua -dominant pasture compared with patches surrounded by perennial ryegrass.
It is concluded that P. annua is not an aggressive species; it colonizes only open ryegrass swards that have become weakened by intensive management.