Despite grower efforts to control weeds, which cost Australia more than $3.5 billion per annum, the ground continues to harbour these floral pests and throw them back up year after year.

Research into how some of Western Australia’s most troublesome weeds hide as seed banks and stagger their emergence to persist in the face of efforts to limit seed set, reveals that a fully integrated, long term strategy is required to rid the soil of these biological time bombs.

Left uncontrolled, one annual ryegrass plant, can produce up to 10,000 seeds per year and while many might appear as plants the next year, others may lay dormant in the soil for several years before sprouting and disturbing future crops.

Wild radish, wild oats and wall fumitory also persist in the soil and can return within two weeks of the break of subsequent seasons to compete with crops and contaminate harvests.

With support from growers and the Federal Government through the Grains Research & Development Corporation (GRDC), Sally Peltzer of the Department of Agriculture is testing just how stubborn some weeds are when it comes to depleting their seed bank.

Replicated across a range of Western Australia soil types, the study examined annual ryegrass, wild radish, wild oats, wall fumitory, brome grass and barley grass persistence in plots isolated to guard against the arrival of new seed.

Emerging seedlings were counted every four to six weeks after the break of season and then killed to ensure no further seed set.

While almost all brome and barley grass seeds germinated in the first year after seed set, annual ryegrass, wild radish and wild oats maintained a subterranean force of invaders for following years.

When new inputs were denied, the seed bank for annual ryegrass decayed by 70 to 80 per cent per year, while wild oat seed bank reserves declined by 80 per cent in the first year and 60 per cent in the second.

The GRDC project also monitored the influence of tillage on emergence, to determine if growers could use a light tickle to promote the emergence of dormant seeds and then treat them with herbicides to help control seed numbers.

In the first spring after seed set, tillage hastened germination of annual ryegrass to increase emergence, but this effect was lost the next year.

The first year cultivation probably helped break ryegrass seed dormancy by moving dark-dormant seeds nearer to the light and light-dormant seeds into the dark.

Tillage also increased the first year seedling emergence of wild oats in some cases and the emergence of wall fumitory every year.

Dr Peltzer believes that tillage in the second year can also help breakdown wild radish seed pods to increase germination and help raid its seed bank.