Grains Research and Development

Section 3: Managing the weed seedbank

Section 3.1 - Introduction Section 3.2 - Burning residues Section 3.3 - Encouraging insect predation of seed Section 3.4 - Inversion ploughing Section 3.5 - Autumn tickle Section 3.6 - Delayed sowing Section 3.7 - Further information

3.1 Introduction

The weed seedbank is defined as the mature seeds that exist in the soil. At any given time, the soil seedbank contains viable weed seeds produced in several previous years (the seedbank).  These seeds (consisting of different ages) will either be able to germinate when the conditions are favourable (suitable temperature, adequate water and enough oxygen) or be dormant.

When new seed is prevented from entering the seed bank, persistence can be determined by measuring the time taken for the number of weed seeds in the soil to diminish to negligible levels. This will vary with weed species due to the differing levels and types of dormancy.

There are two ways to diminish the seedbank:

1)      Weed seed germination and subsequent seedling emergence. Factors including light, soil conditions such as temperature and moisture, the soil’s gaseous environment and nutrient status all affect the ability of the seed’s dormancy and ability to germinate.

  • Tillage can affect seed germination by redistributing the seed to a different profile in terms of moisture, temperature etc. or changing the amount of available light. Autumn tickle (also referred to as an ‘autumn scratch’ or shallow cultivation) stimulates weed seed germination of some weed species by placing seed in a better physical position in the soil. At a shallow depth of 1–3 cm the seed has better contact with moist soil and is better protected from drying than when left on the soil surface. (Note: not applicable to surface germinating weeds). A well-timed autumn tickle will promote earlier and more uniform germination of some weed species for subsequent control. Tickling often needs to be used in conjunction with delayed sowing. Delayed sowing is the technique of planting the crop beyond the optimum time for yield in order to maximise weed emergence. Weeds that emerge in response to the break in season can then be killed using a knockdown herbicide or cultivation prior to crop sowing.

2)  Seed loss other than germination. Most seeds fail to emerge as seedlings.

  • Some are buried at depths too great to permit emergence and a large fraction simply lose viability over time and die of old age. After long-term reduced or no tillage, most weed seed is located at or close to the soil surface. Inversion ploughing can be used to fully invert the soil to ensure that weed seeds that were on or just below the soil surface are placed at a depth from which they cannot germinate. This can be practiced every 10-15 years where no/reduced tillage is used in the intervening years.
  • Some may also be eaten or attacked by pathogens. A study in the Western Australian wheatbelt found that 81% of the original annual ryegrass seed and 46% of wild radish seed had been removed by ants. (Seed predation)
  • Natural mortality rates of weed seed are far higher in no-till systems where weed seed is left on the soil surface than in systems where weed seed is mixed in the top few centimetres. Burying some types of weed seeds can increase seedbank dormancy and slow the rate at which the seedbank is depleted.


3.2 Burning residues

Chaff dumps can be burnt in autumn killing a high proportion of seeds present
Chaff dumps can be burnt in autumn
killing a high proportion of seeds present
Photo: A. Storrie

Fire can be used to kill weed seeds on the soil surface if there is sufficient fuel load and the fire is hot enough. Burning over summer poses an unduly high fire hazard and is illegal in most regions. An autumn burn often poses a lower fire hazard and leaves crop residue in place to protect soil from wind and water erosion for a longer period. Maintaining stubble for longer also benefits soil water capture and retention, provided summer weed growth is controlled. 

To obtain high levels of control of weeds such as annual ryegrass and wild radish, a hot fire is needed. This is obtained by windrow burning, where crop residues from either cereal, canola or pulse crops is concentrated with weed seed in a narrow windrow and then burnt.

Tactic 1.1, Part 4 of the Integrated Weed Management Manual on the key practicalities and benefits of burning residues

Weed Seed Bank Destruction - Narrow Windrow Burning

Michael Walsh, (AHRI) explains the subtlities of burning narrow windrows in canola, pulse and cereal crops. How weather conditions, fire temperature and type of crop stubble all contribute to destruction of the weed seed.

Weed Seed Bank Destruction - Burning Windrows Safely

Michael Walsh, (AHRI) shows the results of an effective narrow windrow burning operation

3.3 Encouraging insect predation of seed

Grass seeds collected by ants
Grass seeds collected by ants
Photo: A. Storrie

The contribution that insects make to seedbank reduction is often overlooked, despite weed seeds comprising a major component of many insect diets. This predation of seed contributes to ‘natural mortality’ and partly explains why less seed germinates than is produced.

Understanding the role that insects play in removing weed seeds could potentially help the development of farming systems that encourage greater removal of seeds from the seedbank. In New South Wales, seed theft by ants has commonly caused failure of pastures, and data from WA shows ants can remove up to 60% or more of annual ryegrass in no-till systems, where weed seed is on the soil surface and accessible. Therefore weed seedbanks could be also decreased by encouraging ant predation.

Tactic 1.2, Part 4 of the Integrated Weed Management Manual on the key practicalities and benefits of insect predation

Weed seed removal by ants in the crop growing areas of Western Australia - 16th Australasian Weeds Conference (2008)

3.4 Inversion ploughing

Inversion ploughing is used to fully invert the soil to ensure that weed seeds that were on or just below the soil surface are placed at a depth from which they cannot germinate. This can be practised every 10 to 15 years where zero or reduced tillage is used in the intervening years. Inversion ploughing is particularly effective at resetting the weed seedbank and can be very useful if herbicide resistant weeds are a problem in a no-till system.

Inversion ploughing has been adopted in Western Australia using commercial two-way machines. Mouldboard ploughs are fitted with skimmers to throw top soil and weed seed to the bottom of the furrow. The technique is used after the break of season, immediately prior to sowing, when the soil profile is wet to a depth of at least 40 cm. Sowing immediately reduces the chances of wind erosion.

Mouldboard plough working near Geraldton showing skimmers in action.
Mouldboard plough working near
Geraldton showing skimmers in action
Photo: P. Newman

The process has been successful on a range of soil types, including duplex sands over clay, loamy clays and deep sands. It should be noted that for self-mulching soils many weed seeds will already be deeply buried in soil cracks and inversion ploughing may not be an effective weed management tactic in these soil types. 

Although whole paddock inversion ploughing is quite expensive (estimated at the time of writing at $70 to $100/ha on deep sands for an owner/operator machine and $125 /ha plus diesel for a contractor), there are long-term benefits for the reduction of the weed seedbank and the amelioration of soil problems such as water repellence in non-wetting sands and subsurface acidity.

Tactic 1.3, Part 4 of the Integrated Weed Management Manual on the key practicalities and benefits of inversion ploughing

Mouldboard Ploughing - Pros & Cons

Mouldboard Ploughs have returned to the landscape but in the modern farming system do they have legitimate place?

3.5 Autumn tickle

Autumn tickling (also referred to as an ‘autumn scratch’ or shallow cultivation) stimulates weed seed germination by improving seed contact with moist soil. At a shallow depth of 1 to 3 cm the seed has better contact with moist soil and is protected from drying. Because weeds that germinate after an autumn tickle can be controlled, such a process will ultimately deplete weed seed reserves.

An autumn tickle can be conducted using a range of equipment including tyned implements, skim ploughs, heavy harrows, pinwheel (stubble) rakes, dump rakes and disc chains.

Tickling can increase the germination of some weed species but has little effect on others.

Tickling needs to be used in conjunction with delayed sowing to allow time for weeds to emerge and be controlled prior to seeding.

Tactic 1.4, Part 4 of the Integrated Weed Management Manual on the key practicalities and benefits of Autumn tickle

3.6 Delayed sowing

Delayed sowing allows use of knockdown herbicides or cultivation to control small weeds prior to sowing and reducing the pressure on selective herbicides
Delayed sowing allows use of knockdown
herbicides or cultivation to control small weeds
prior to sowing and reducing the pressure
on selective in-crop herbicides.
Photo: D. Holding

Delayed sowing (seeding) is the technique of planting the crop beyond the optimum time for yield in order to maximise weed emergence and control prior to sowing. Weeds that emerge in response to the break in season can then be killed using a knockdown herbicide or cultivation prior to crop sowing.

This tactic is most commonly employed for paddocks that are known to have high weed burdens. Paddocks with low weed burdens are given priority in the sowing schedule, leaving weedy paddocks until later. This allows sufficient delay for the tactic to be beneficial on the problem paddock without interrupting the whole-farm sowing operation.

Choosing a crop or cultivar with a later optimum sowing time can reduce yield impact of a later sowing date.

Tactic 1.5, Part 4 of the Integrated Weed Management Manual on key practicalities and benefits of delayed sowing

3.7 Further information

Part 4 of the Integrated Weed Management Manual: Managing weed seedbank depletion

GRDC fact sheets and other publications

GRDC Weed Seed Bank Fact Sheet (2010)

GRDC video links

Mouldboard Ploughing - Pros & Cons

Mouldboard Ploughs have returned to the landscape but in the modern farming system do they have legitimate place?

Weed Seed Bank Destruction - Narrow Windrow Burning

Michael Walsh, (AHRI) explains the subtlities of burning narrow windrows in canola, pulse and cereal crops. How weather conditions, fire temperature and type of crop stubble all contribute to destruction of the weed seed.

Weed Seed Bank Destruction - Burning Windrows Safely

Michael Walsh, (AHRI) shows the results of an effective narrow windrow burning operation

Other information

Specific weeds - information on seedbank ecology is also found under links to specific weeds

RIM (Ryegrass Integrated Management program)
RIM is a computer based decision support tool designed specifically for evaluating various ryegrass management options.  It provides information about the economics and seedbank impacts of different weed management tactics in a farming system.

Weed Seed Bank Wizard
A modelling tool to help predict the impact of multiple weed management tactics on weed seed bank decline. (Department of Agriculture and Food, Western Australia)

Weed trimming - a successful non-chemical seed bank reduction technique - 17th Australasian Weeds Conference (2010)

Manipulating the seed bank to manage herbicide-resistant weeds - 17th Australasian Weeds Conference (2010)

Weed seed removal by ants in the crop growing areas of Western Australia - 16th Australasian Weeds Conference (2008)

Weed seed bank response to 12 years of different fertilisation systems - 17th Australasian Weeds Conference (2010)

Depleting weed seed banks within non-crop phases for the benefit of subsequent crops - 16th Australasian Weeds Conference (2008)

Burning snails and weed seeds - YouTube clip of local farmers and experts speaking about the increase in burning paddocks prior to the commencement of the 2012 season. (SANTFA)

Update papers

Outsmarting the weed seed bank (2016)

Why the obsession with the ryegrass seed bank? (2015)