The difference between the preemergent plots and the untreated control indicates that over time the use of pre-emergent herbicides could be expected to reduce the seed bank. A = thatch layer, B = 0-1cm layer, C = 1-2cm layer and D = 2-4cm
reserve in the untreated control compared to the plots treated with pre-emergent herbicide. The difference between the pre-emergent plots and the untreated control indicates that over time the use of pre-emergent herbicides could be expected to reduce the seed bank. These results demonstrate the ongoing regenerative potential of Poa annua and clearly not all seeds germinate in the same time period. This long-term seed reserve needs to be understood if a suitable management programme is to be implemented. From this work, the use of pre-emergent herbicides is critical in the management of Poa annua, particularly in warm-season grasses. Where there is a high population of Poa annua and it is possible to control these established plants, it has to be followed up with a pre-emergent almost immediately if control is to be successful.
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seeds/m2. Most of the 30,000 seeds populating the green from the end of summer until the next spring were assumed to have persisted from preceding pulses of flowering. The persistence of the seeds was not caused by innate dormancy but is enforced or induced dormancy caused by seed burial. Lush (1988b) stated that 78 per cent of the seeds that carried-over to winter were buried in the thatch and soil beneath the region of current growth. In the research that David Nickson and I have undertaken there were two sites used to study the seed bank. The trial sites used for the evaluation of pre-emergent herbicides where sampled by taking 50mm diameter cores to a depth of 5cm from the plots (couchgrass and kikuyu) previously treated with oryzalin as well as the untreated control. Four samples were taken from each plot with a total of 24 cores taken. Each core was separated into; thatch layer, 0-1cm layer, 1-2cm layer and the 2-4cm layer. The sections were broken up and planted into trays and watered. At two and four weeks after planting, the Poa annua seedlings were counted. The results are detailed in Figures 1 and 2. The results indicate that there is a substantial seed bank at each site with most of the seeds in the thatch layer which is consistent with the work of Lush (1988b). The total seed numbers were as high as 18,000 seedlings/m2. The results from the couch site demonstrated a significantly higher seed
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