Weed management on organic farms
|Weeds are often rated as the number one concern for organic farmers and for farmers considering ‘going organic’. A ‘whole system’ approach to weed management can help organic farmers to minimise the negative impact of weeds and reap the benefits of non-weedy volunteer plants, while reducing the energy, time and other costs of weed control.|
The concern regarding weed management on organic farms may relate to the comparable ease with which weeds are usually controlled chemically on conventional farms. In contrast, organic farmers must achieve effective weed control without using synthetic herbicides and ideally without relying on cultivation.
This fact sheet discusses ways for organic farmers to manage the risks that weeds and weed management pose to farm productivity and the environment.
What are weeds?
Plants could be defined as ‘weeds’ if they display any of the following disadvantages:
- compete with crops for water or nutrients
- contaminate crops at harvest
- poison livestock
- interfere with harvest or other farm operations
- increase the pest or disease risk to crops
- have spiky seeds that injure workers or livestock
- interfere with irrigation
- invade neighbouring properties including conservation areas
The actual definition of a ‘weed’ however depends on your weed management priorities and objectives in the context of your own farm. It also depends on what level of disadvantage you will accept, in order to gain any benefits offered by the ‘weed’.
Noxious weeds are an exception to this. They are defined by State Government legislation and must be reported and/or controlled by the landholder.
To develop a weed management program, you should first reconsider your definition of weeds, remembering that it is of no practical or economic benefit to spend resources combating ‘weeds’ that in fact cause no practical or economic harm to the farm or broader environment. As with any pest, weed control is only warranted when weeds cause, or are very likely to cause, economic or other loss.
Weed or volunteer plant?
Volunteer plants are often called ‘weeds’ simply because they were not deliberately planted. That alone, does not justify efforts to control those plants. To determine their real ‘weed’ status, you need to consider the qualities of plants in relation to your objectives as farm manager. Weed management becomes easier if you classify plants as ‘real’ weeds requiring attention, and ‘other plants’ that you can safely ignore.
Before you classify and target plants as weeds, weigh their potential negative impacts against any benefits they may provide, which can include:
- Soil protection and enhancement. Most plants protect otherwise bare soil from the effects of sun, wind and rain; produce organic matter and improve soil biological activity and water infiltration rates.
- Food source for beneficial insects. Many flowering plants produce pollen or nectar that is useful food for natural enemies of crop pests. Others carry alternative prey like aphids that help to maintain populations of natural enemies on the farm.
- Nutrient recycling. Deep-rooted plants absorb nutrients from lower in the soil profile and redistribute them when the plants decompose.
- Weed suppression. Heavy growth of volunteer plants helps to suppress the development of undesirable ‘real’ weeds like Emex (three-cornered jack) and caltrop.
- Soil indicators. Unlike crop plants that grow where we force them to grow, volunteer plants tend to thrive under conditions to which they are naturally adapted. They may therefore be used to some degree as indicators of underlying soil conditions (problems) such as compaction, acidity or salinity. When those problems are corrected, the soil environment becomes less favourable for the ‘weeds’, and the competitiveness of crop or cover crop plants is increased.
A whole system approach
Our approach to weeds (like our approach to pests and diseases) has typically been curative, using herbicides, cultivation or other activity to suppress them. Some disadvantages of this are:
- Many treatments, including cultivation, have undesirable off-target impacts. Cultivation in particular is very disruptive to soil structure.
- Many treatments are relatively short-lived and require repeated use because they are often 'neutralised' by nature, e.g. 'banks' of long-lived seed produce a new flush of weeds after cultivation.
- A focus on tackling weed problems with action distracts busy growers from observing and thinking why the problems arise and how they might be reduced by better design of their farming system.
Rather than focusing on specific weeds, consider your whole farming system: how it naturally contributes to or limits weed problems, and how it could be made less favourable for weeds. Most aspects of a farm and its management need to be combined into an integrated approach to weed management.
Integrated weed management
Integrated weed management (IWM) can help you to achieve the most effective weed management outcomes in the short and long-term. Successful IWM relies upon some basic principles that include:
- correct identification of weeds - What are you are dealing with?
- some understanding of weed ecology - Why do weeds grow where they do? How do they get there?
- appropriate farm design - e.g. choice and layout of irrigation system can influence weed development and weed management approaches
- choice of appropriate management methods - use effective methods that minimise negative impacts
- correct timing - weed management activity needs to be timed effectively, e.g. to prevent seed set
- monitoring - Where and when did weeds become a problem? How effective was the control strategy?
- IWM makes use of any appropriate organic methods to deliver the best outcome. The integration of several methods makes sense. No one method can control all weeds, cost-effectively, all the time, because:
- some weeds are easier to control than others
- some are annual, some perennial
- some spread by cultivation, others by wind or water
- some are avoided by grazing animals
- some are very competitive against cover crops.
Using a combination of methods also helps to reduce the risk of particular weeds dominating the farm.
Useful guides on integrated weed management are available to farmers wanting to develop a more holistic approach to weed management. Some of these are listed under ‘Further information’.
Short-term & long-term approach
You should consider your short- and long-term approaches to weed management carefully, and review them occasionally, to ensure they remain practical and complementary.
Short-term aims are likely to involve reducing the quantity of weed growth during critical periods in the crop cycle, such as times of water stress.
Long-term aims might relate to changing the ‘quality’ of volunteer vegetation by, for example, suppressing grasses and encouraging perennial legumes. This can eventually make short-term management easier.
Weed management objectives need to be kept realistic and achievable. For example, the eradication of a weed in the short to medium term may be unrealistic if its seeds are very long lived. Preventing further seed set may be a more realistic goal.
Whole farm management
IWM needs to consider whole farm management to avoid clean areas being reinfested by weeds from infested areas. Also, because weeds, pests, diseases, soil, water, crops and livestock are all interrelated aspects of the organic farm system, they need to be managed as a whole system for best results.
Pre-conversion weed control
If you are planning to ‘going organic’ you may want to consider whether you should tackle major weed issues with conventional non-persistent herbicides before joining an organic certification scheme. This depends on your philosophy regarding chemical use, but it may have merit, especially where invasive perennial species are well established and would be more difficult to manage under an organic regime.
Pre-planting weed control
Because it is easier to tackle weeds in open ground than amongst established perennial crops, a serious effort should be made to suppress or preferably eradicate, perennial weeds like couch grass (Cynodon spp), kikuyu (Pennisetum clandestinum), Johnson grass (Sorghum halepense) and nutgrass (Cyperus rotundus) before establishing a perennial crop.
Weed management techniques
- Quarantine minimises the introduction and spread of weeds and is cost-effective compared to control of established weeds. Effective quarantine may require:
- restrictions on movement of machinery, livestock, people, soil etc onto and across the farm
- exclusion of animals and traffic from areas infested with high-priority weeds
- buffers including windbreaks and surface water diversion, against wind and water-borne weed seed.
Like quarantine, hygiene reduces the introduction and spread of weeds. You should aim to:
- select weed-free sources of crop seed, mulch, compost ingredients and other materials
- avoid spreading weeds vegetatively, e.g. avoid cultivating through infestations of perennial weeds then into clean areas
- clean equipment of plant material and soil before it is taken onto the farm
- prevent weed seed set on the farm
- destroy seed of noxious weeds by burning, deep burial, etc. Grazing is not reliable as seed of many weeds is viable after passing through livestock.
Animal manures can contain large amounts of weed seed and should be composted thorougly (temps. exceeding 60°C) to reduce or eradicate viable seed.
As mentioned earlier, when soil problems are corrected and conditions improved in favour of the cropping system, some weed issues will fade naturally as the weeds lose their competitive edge over crops.
Monitoring helps to identify weed issues and hotspots, select the best management techniques and timing, and determine the effectiveness of those techniques.
Crop water and nutrition management
The distribution of weed growth on irrigated farms is influenced by the choice of irrigation technique (e.g. flood vs dripper) and so can be manipulated by that choice. Well fed and watered crops will have the best chance of out-competing weed growth.
Many plants adapted to colonise bare soil quickly and effectively are considered weeds. Farmers who try to maintain bare soil find themselves in ongoing conflict with these colonising ‘weeds’. Many weeds are poor competitors and their establishment, growth or seed production is reduced by competition from crops and other plants. Wherever possible, design bare soil out of your farm and replace it with crops, perennial or annual cover crops or mulch. To maximise the competitiveness of your seeded crops or cover crops:
- choose healthy and clean crop seed
- choose vigorous crop species
- consider increasing the seeding rates.
- When selecting cover crops for weed suppression, consider their following characteristics:
- time to develop full soil cover (the sooner the better)
- density of cover (the greater the better)
- duration of cover (the longer the better)
For effective weed suppression, cover crops need to be managed well to cover the soil as quickly as possible to smother emerging weeds. Pay particular attention to irrigation, nutrition and pre- and post-planting weed management.
The planting of fields to very different crop types over time helps to prevent specific weeds from becoming dominant. Rotations are especially useful if they include dense green manure or cover crops that out-compete weeds, or a pasture phase during which weed seed set is prevented through grazing, mowing or cutting of silage. In crop rotations, slow-growing crops which are more susceptible to weed invasion, should follow weed suppressing crops.
Allelopathy is a special form of competition, where seed germination or plant growth is suppressed by chemical substances produced by another plant. These substances are leached from foliage or secreted from roots of the allelopathic plant, or released when the plant dies and decomposes. Two green manure crops - barley and cereal rye - are known to suppress numerous weed species and are considered to have high allelopathic potential.
Slashing is commonly used by organic farmers to manage cover crops and weeds. It is relatively fast and causes minimal soil disturbance.
The timing and height of slashing are important. Slashing before weeds flower will prevent seed production. A high cut (e.g. 20cm above ground) will reduce or prevent seed set in medium to tall weeds while allowing useful species including non-grasses, to regenerate. A low cut favours grasses over many other desirable plants because the growing point of grasses is low. Low cuts are not effective against weeds with low crowns, like capeweed and wireweed.
Weeds, cover crops and green manures may be flattened with a ‘crimping roller’ instead of being mown. Rolling leaves the plants largely intact, so they break down more slowly and create a longer-lasting mulch layer. Rollers can be cheap to manufacture and maintain and can be towed by a light four-wheel ATV.
Grazing can contribute to organic weed management and has the benefit of incorporating livestock into the farm. To use grazing effectively, you need to address stock management issues including:
- control of stock movement and stocking rate to avoid over-grazing
- protection from predators (e.g. foxes & dogs)
- dominance by specific weeds from selective grazing
- damage to perennial crops.
Thermal weeding uses heat (radiant, flame, steam, hot water) to kill weeds. This technique has been around for many years, but is generally economically effective only against small seedlings. It is used most commonly on row-crop seedbeds to kill the new flush of weed seedlings before the crop emerges, although it has also been used in perennial crops.
Thermal weeding does not disturb the soil, but has the disadvantage of high energy cost - it is usually based on fossil fuel such as diesel or bottled gas. It is also relatively slow to achieve a good weed kill and may create a fire risk where flame is used.
Mulch inhibits seed germination and weed growth by blocking sunlight from reaching the soil surface. It is also a physical barrier to weed growth. Almost any organic material can be used as mulch.
- protect shallow crop roots from heat and dryness
- conserve soil moisture by reducing evaporation
- add organic matter to the soil
- provide some nutrients
- enhance soil biology (increase microbial activity)
- are convenient in perennial crops where cultivation and mowing are difficult.
Depth of mulch is an important factor. Weeds will not be suppressed well if the mulch is too thin, while overly thick mulch can shed light rainfall away from crop plants. Experiment to determine the best mulch depth for a particular situation.
Mulches do have some disadvantages:
- they may harbour pests such as snails
- partly decomposed mulch may create an ideal seedbed for weeds
- weeds at the mulch/non-mulched interface can be awkward to manage without disturbing the mulch.
You need to take account of the cost of reapplication of mulches, possibly every one to three years. As most organic mulch material is bulky, the costs of transport and handling need to be considered. On-site production of organic matter for mulch can reduce costs and avoid the risk of introducing new weeds in imported mulch material.
Synthetic woven weed mat suppresses most weeds. The mat must eventually be removed from the soil and not be cultivated in or left on-site to decompose. A temporary mulch of black plastic can also kill difficult weeds including couch grass by starving them of water and light, but organic standards prohibit solid plastic sheeting as permanent mulch for weed control.
Solarisation kills plants, seeds, pests and pathogens with the solar heat trapped when soil is covered by a clear plastic sheet during hot weather. This technique is more commonly used against soil diseases but is also effective against many weeds and their seeds. Deep tubers or rhizomes are less likely to be killed. Solarisation for at least four weeks is likely to be needed for effective weed kill. This varies according to the quality of plastic used, the daily temperatures and weed species. Winter weeds are likely to be less tolerant of high temperatures than summer weeds, and so more strongly affected by solarisation.
Weeds have their own natural enemies that include diseases and root, stem and seed-eating insects and mites. After much official research, natural enemies may be released to help control exotic noxious weeds. Information on these biological control programs can be sought from State agriculture departments.
Cultivation destroys or buries weeds, disrupting their growth and preventing seed set. Most forms of cultivation are very disruptive to soil structure and to soil organisms. Cultivation can also spread seeds, tubers and rhizomes of noxious weeds, is relatively energy-expensive and increases the risk of soil compaction. Cultivating as shallowly as possible will help to reduce negative impacts on the soil.
Many organic farmers do some manual weeding to clear around vegetable crops and perennial plants where cultivation and slashing is risky. This is time-consuming and costly, but is effective and gives you an opportunity to observe your soil and crops closely – a vital aspect of good organic management.
Chemical weed controlOrganic herbicides based on pine oil are available in Australia and work by disrupting the cuticle (‘outer skin’) of plants, causing them to dry out. Although these herbicides are allowed under organic standards, they do involve application of chemicals to the farm ecosystem and should be used with care as the long-term effects of their use are not known. Also, their use to replace synthetic herbicides diverges from the spirit of the organic approach, which involves considering and managing farms differently, rather than simply substituting allowed inputs for prohibited inputs.
Carefully managed fallow may reduce weed pressure prior to cropping, but is not considered a sustainable weed management method because of its negative impacts that include increased soil erosion risks, loss of soil carbon and destruction of soil life.
The value of good timing
Weed management actions should be prioritised and performed at the optimum time in relation to weather conditions or weed growth stage. For example, if the priority is prevention of seed set, action must be taken before flowering. With each new generation of seed, the weed problems will be perpetuated, as will the need for ongoing control. If cultivation is used to disrupt weed seedlings or the rhizomes of perennial weeds, a greater kill will be achieved if the cultivation is followed by hot, dry weather rather than cool, wet weather. Priority should therefore be given to timing cultivation according to prevailing weather conditions.
Integrated weed management: An introductory manual, by the Ministry of Agriculture and Lands, British Columbia, Canada (2008). http://www.agf.gov.bc.ca/cropprot/weedman.htm
Organic farming: Which green manure should I grow? By C Jaeger (2010). Department of Primary Industries Victoria, Information Note AG1122. (Includes details of 50 cover/green manure crops)
Seven steps to managing your weeds, by the Ministry of Agriculture Food and Fisheries, BC Canada (2002). http://www.weedsbc.ca/pdf/7StepsToManagingYourWeeds.pdf
Weed, by T Marshall (2010). ABC Books. A good cover of many weeds and organic management techniques, adaptable to farm-scale use.
New tools for organic no-till, by L Sayre (2003). Rodale Institute, Pennsylvania USA. Introduction to the crimping roller.
This fact sheet was adapted from the Rural Industries Research and Development Corporation report:
Madge D (2009) Organic citrus: A growers’ manual. Rural Industries Research and Development Corporation, Canberra, Pub. No. 09/050, https://rirdc.infoservices.com.au/items/09-050
This information has resulted from the organic extension project funded through the Victorian Department of Primary Industries. The project was managed by Department of Business and Innovation, and initiated and guided by the Victorian Organic Industry Committee (VOICe). VOICe is a committee of volunteers that represents the full diversity of Victoria's organic sector, including farmers, traders, retailers, exporters, certifiers, researchers and consumers. It was established to oversee and drive the development and implementation of the Victorian Organic Action Plan. VOICe is the key contact point for government on organic industry matters. For more information about VOICe visit Victoria Organic www.victoriaorganic.com