Managing weeds manually can be extremely specific, you don’t need to worry about herbicide drift hitting your desirable plants, or damage to soils or animals as with chemical weed sprays. Plants are usually completely dead and not standing to impede further work.
Remember, there are some underlining principles which we should aim for:
- Healthy plant stands are the best means of eradicating weeds – encourage healthy soil conditions
- Know your weeds and their control methods, otherwise things could get worse!
- In general, work from good areas to bad ones
- Work from upstream to downstream (to prevent seeds being washed down to newly cleared areas!)
- If the infestation is too big to eradicate, remove flowers and seeds to stop it from spreading
- If the weeds include wildlife foods and habitats, remove them gradually to allow the development of alternatives.
- Address any nutrient issues – enriched soils promote weed growth
- Consider runoff scenarios and take appropriate measures to address them.
The choice of weeding method and of implement depends in part on management goals or conservation targets at each type of site. Chapter 8 of the West Australia Coastal Planning and Management Manual has an excellent section on weeds and weed management.
Sometimes it is not so much a matter of removing weeds but of developing alternative management to avoid weed growth. Sometimes it is a matter of addressing our desire for cosmetic parks. We do not advise removing annuals or biennials in bush regeneration since they function to protect the soil and feed soil microbiology.
In a paper delivered to the National Conference on Urban Water Management in 2014 Jeremy Winer of Weedtechnics outlines holistic management for urban storm water catchments. He developed a matrix of techniques generally used against such factors as type and frequency of application, surface types, location, human or environmental harm, efficacy, and economic efficiency. The economic elements like purchase price, operating costs and labour requirements should always take into account human and environmental health including long-term chronic effects as well as short-term toxic ones.
Highly recommended reading:
- Bradley, Joan, 1988. Bringing Back the Bush. Reed New Holland, Currently out of stock but likely to be available in libraries. The Bradley sisters were the first to research manual methods for control of environmental weeds. They also set out principles which included: 1) Work outwards from good bush areas towards areas of weed; 2) Make minimal disturbance to the environment: and 3) Do not over clear. In general, these principles still hold.
- Marshall, Tim, 2010. Weed. ABC Books, Sydney, Australia. Tim Marshall has been a leader in the organic movement for some decades. Armed with this experience, his book is packed with information about weeds we are likely to meet and their control. There is a section on environmental weeds including how to deal with some which are listed as Weeds of National Significance.
- Wolff, A., 2012. The Weed Book. New Holland Publishers (Aust.) P/L, Sydney, Australia. This also includes extensive information on the biology of common weeds and a well-illustrated step-by-step guide to their manual removal. This book also has a section on ‘Using Herbicides’ for weeds he considers difficult although he does stress that ‘All herbicides must be used with great care and be considered to be potentially hazardous substances, and as such require careful handling.’
- French, Jackie, 1989. Organic Control of Common Weeds. Aird Books P/L, Flemington, Victoria. This is oriented more to gardening and farming but contains great information on many weeds and their management.
- Romanowski, Nick, 2011. Wetland Weeds: Causes, Cures and Compromises. CSIRO Publishing, Collingwood, Victoria. Excellent cover of wetland weeds and their control.
“The author takes a pragmatic approach to weed control, recognising that some weeds may not be possible to eradicate, and emphasising the need to assess the extent and future potential of any infestation before taking action…. A range of strategies for controlling wetland weeds are considered, from containment actions to prevent the development of a soil seed bank to physical removal, and biological approaches from biocontrol to shading, overplanting and use of turbidity. The widespread use of chemical controls is also discussed, with the warning that these are often only a short-term cure and can cause more harm to aquatic ecosystems than the weeds they are holding at bay.” CSIRO review.
With hand weeding techniques the whole plant is removed, ensuring propagules are not left behind. Knives, loppers, secateurs, pruning and bush saws, and tree poppers are the most common tools used. They have the advantage of being low cost and though labour intensive, this method will usually reduce labour required for follow-up.
Hand Pulling: Pulling or uprooting plants is effective for herbaceous and floating weeds and some shrub and tree saplings. For many herbaceous and grassy weeds, a sharp knife cutting the root mass below the soil surface is effective. Some tree saplings which have few lateral roots initially may be quite large yet still easy to pull (e.g. Coastal Teatree, Leptospermum laevigatum)
Digging out: For plants with bulbs, corms or tubers, a bulb trowel will assist removing the below ground sections. They must be completely removed. For vines, cut off all aerial stems but leave in the canopy as pulling them may damage the natives. If the vine roots at the nodes along the ground, ensure that all sections are removed.
Stabbing: Some plants can be killed or severely weakened by severing or injuring (stabbing) the carbohydrate storage structure at the base of the plant. Depending on the species, this structure may be a bulb, corm, storage rhizome (tuber), or taproot.
Tree Poppers/ Weed Wrench: For saplings too big to be pulled by hand (even several metres tall), Tree Poppers are quicker, more cost effective, and much less destructive than using a herbicide with cut and paint. These grip the weed stem and provide the leverage necessary to pull the roots out. Tools vary in their size and weight, and the size of the weed they can extract. A similarly powerful tool in the US is the Uprooter or Weed Wrench.
Crowning:With many shrubs, removal of the root crown is sufficient to kill the plant (for example, Bitou Bush, Chrysanthemoides monilifera).
Lopping: With some trees (e.g. Coastal Tea-tree, Leptospermum laevigatum) removal of all leafing branches will kill the tree. There is no need for herbicide use on the stump. With other species, follow-up will be required to remove leaf growth. The plant will soon enough run out of energy. If plants are too far away for easy follow-up, covering the stump so that no light will get through will kill the plant.
Mowers, brushcutters and slashers are used to reduce above-ground biomass or grasses or herbaceous weeds. Brushcutters or chain saws are used for removing woody weeds.
Mowing and slashing: These can reduce seed production and restrict weed growth, especially in annuals if cut before they flower and set seed. Done regularly, it can change the composition of a grassy sward towards more prostrate forms. Slashing is often used to manage roadside vegetation along. It is also used to prepare areas for reafforestation.
Lawn Management: “The best defence against weeds is a healthy lawn, because grass can easily out-compete weeds when conditions are maintained in the grasses favour. Nutrient deficiencies, nutrient excesses, soil compaction, acidic soils, excessive shade, wet soils and many other problems can be detected by what plants are successful in infiltrating a lawn.” (The Lawn Institute)
Councils, home gardeners, landscapers, and golf course managers are becoming increasingly interested in organic and least-toxic turf care because it:
- Eliminates pesticides from recreational areas such as lawns, parks, golf courses, and athletic fields to eliminate exposure of people (particularly children) and pets
- Decreases susceptibility of turf to pests, diseases, and drought
- Reduces runoff and leaching of excess nutrients and pesticides into surface and ground waters
Beyond Pesticides has the following 8 points for a healthy lawn (which they expand here:
- Develop healthy soil.
- Plant well-adapted, pest-resistant grass varieties.
- Aerate the lawn to allow moisture and nutrients to reach the roots of the grass
- De-thatch. Thatch is a dense layer of grass stems and roots on the surface of the soil.
- Maintain proper pH.
- Fertilize. Use a slow release fertilizer formulation (or compost, manure, grass clippings) once a year
- Water properly. Too much or too little water can induce pest outbreaks.
- Mow correctly.
Some additional tips:
- Develop a turf management plan to maintain larger green spaces, like sports fields
- Over-seed lawn once a year to make it thick and healthy – this helps to crowd out weeds
- The best height to mow varies with the turf grass species. Some withstand weeds better when cut to 5cm or more. Others with a prostrate habit should not be mowed higher than 2.5cm to avoid excessive accumulation of thatch.
- Sharpen mower blades. Dull blades tear and stress grass blades, increasing the potential for disease and infestation
- Mulching tree wells in lawn both suppresses weeds and maintains healthy trees. The diameter of the mulched area should extend as far as roots are visible. Mulching also helps protect the roots from damage from mower blades
- Reduce the area of grass that needs maintenance by planting natives gardens
- Tolerate small weeds which pose no threat to public health.
For organic lawns and species for Australian conditions, check out.
The following guides have useful information though from America:
Ringbarking: Ringbarking is most effective on trees and shrubs which have a single trunk. It involves cutting away a complete ring of bark from a tree trunk, removing the vascular cambium, the thin layer of living tissue which moves nutrients from the roots to the leaves.
To ringbark a tree, cut parallel lines approximately ten centimetres or more apart around the circumference of the tree. The cuts can be made using a knife, axe, or saw, and should be slightly deeper than the cambium. Strike the trunk sharply between the cuts using the back of an axe or other blunt object. The bark should come off in large pieces and prevent the tree from any further growth. Over time, this will leave a dead standing tree which can provide valuable wildlife habitat, and if left to decay, allows the nutrients of the tree to be returned to the system, rather than being removed and deposited elsewhere.
A few species respond to ringbarking by producing many fast growing root and stem sprouts. For these species, follow-up to remove shoots will need to be regular for a few months. Alternatively, they may be cut down to about 10-20 centimetres and covered with black plastic or other material which will inhibit light getting to the stem and therefore allowing photosynthesis.
Aquatic Weed Harvester: “Mechanical harvesters are large floating machines that have underwater cutting blades that sever the stems of underwater plants, gather the weeds and raise them on conveyor belts, storing the vegetation on board in a hold. Periodically this is discharged to a barge or an onshore facility. The harvested product can be composted… Mechanical harvesters can be effective at clearing aquatic weeds but the machines are expensive and the process may need to be repeated several times in a growing season.” (Wikipedia)
Boiling Water: A kettle full of boiling water to kill weeds is pretty inefficient but if you have an area of weeds right outside your kitchen door, why not dump your leftover boiling water on them?
Saturated Steam Weeding: Saturated steam treatment is commonly used in both urban environments and by organic farmers. It is ideal for hard surfaces and, in public spaces, can also be used to clean playground equipment, toilets, etc.
The system operates by putting water under pressure through a heated chamber onto the weeds. The hot water removes the waxy cuticle coating on plant leaves and stems, breaking down the cellular structure and causing rapid death. One treatment can kill most annuals and some young perennials. The top growth of older perennials is scorched off, but the impact on the roots is minimal unless treatment is repeated frequently. The addition of a surfactant or wetting agent will increase its efficiency.
While Steam Cleaners can be used to cause plant damage, the steam cools very quickly and is unlikely to maximise control. On the other hand, saturated steam weed control uses superheated water (above 100oC). It does not boil due to pressurisation of the water. As it depressurises it explodes into saturated steam which causes thermal shock.
It is safe to use in public places and naturally has no residual effects. In Council areas where it is used, it has been appreciatively received by the public.
In Australia, agents for Saturated Steam Weeders are licensed by weedtechnics . This site also outlines the 8 hidden costs of chemical weed control and the benefits of this non-chemical approach.
Flame Weeders: “Flame and radiant heat tend to be more portable, use LPG /propane but do not penetrate into the crown of the plants efficiently, often requiring more frequent interventions. Exposed flame weeders pose significant fire risk in dry conditions and on mulches, and cannot be used on rubber soft fall, rubber paving, near litter, debris or irrigation lines and fittings.” (Winer, Weedtechnics)
Pros and Cons of these techniques are demonstrated in this video.
Flame weeders can be used for total vegetation removal or for selective removal of unwanted plants. Selectivity may be achieved by timing the application to kill weed seedlings before the crop emerges (pre-emergence flaming). For best effect, flaming requires a level soil surface. In addition, flame weeders have the advantage that they can be used when the soil is too wet for mechanical weeders.
They work directly by damaging the waxy cuticle that protects the cells in plants’ leaves and indirectly by subsequent desiccation. The weed flaming process does not require the weed to be burnt: it raises moisture temperatures to above 100°C, at which time the moisture turns to steam and ruptures the plants’ cells. Small herbs are generally more susceptible to flaming than large ones. Species with upright habit and thin leaves are also more sensitive than species with a low stature and protected growth points. Weed torches will kill beneficial soil micro-organisms that help your plants thrive, so it is not optimal for planted areas.
Flaming equipment has been developed and commonly used overseas in Germany, Holland, Sweden and Denmark, the UK, and the USA. For example, in Sweden flame weeding has been used for many decades, particularly in organic farming situations for pre-emergent weed control in carrots and other slow-germinating row crops, but also for general weed management on hard surfaces in urban areas.
According to Dragonfly Environmental “Flame weeding can be undertaken outside of the fire seasons and in wetlands. Flame weeding allows for the mimicking of a burn in areas where a control burn could not be undertaken… Thermal weeding is particularly useful in situations where conservation or health considerations are high and weed density is low such as waterways where herbicide use is not permitted. Dragonfly Environmental an industry leader in ecological restoration techniques has been trialling this method and has had good results on treating Alligator Weed and Ludwigia peruviana seedlings.
“While flame weeding is not suited to most streetscapes due to the fire hazard nor can it be used on materials such as soft fall and similar playground equipment it is noted that ‘flame’ weeding in waterways allows weed management in areas where herbicides are not permitted.Also for native vegetation areas thermal weeding, with a flame weeder, has been shown to stimulate germination of native plants while killing the seeds of annual weeds such as Devils Pitchfork, Bidens pilosa. Flame weeding is also effective in killing persistent weeds like Mother of Millions.”
“Best results are obtained when follow up weed control is undertaken 4-6 weeks after treatment. In addition, weed control should be conducted periodically after that for example to control weeds over a period of a year it is likely that between 3-5 applications will be necessary, depending on rainfall and the extent of the weed seed bank.”
In an article in NSW Agriculture’s Organic News, Tony Atkinson of Gameco Pty. Ltd outlined applications for their flame weeders:
“Channel banks and supply ditches
The advantages of flaming for irrigation channels naturally, is the speed of application, the speed of seeing the results and NO CHEMICAL RUN OFF into the waterways or water table. Flaming for irrigation channel banks and supply ditches is carried out by a different LP Gas Burner set up to that used for row crop flaming. A single liquid vaporising burner incorporating a high pressure liquid LP Gas spray is used to give a flame that can be up to 2 metres wide by approximately 4 metres long. This type of apparatus can be used at approximately 10 to 15 miles an hour. This equipment can be boom mounted with remote (at the operator’s finger tips) mounted controls to get the job done quickly and efficiently.
Many local Government councils and Shires have discovered the advantages of flame weed control and are now regularly using hand flamers. So much so that one south eastern Queensland shire has set-up a trailer equipped with gas cylinders and safety equipment to make the use of this equipment an ‘everyday’ thing.”
In Sydney, Blue Mountains, and Central Coast regions, the National Trust’s Bushland Management Services team also uses flame weeding to restore natural areas when requested. Their clients include government departments, local councils, State owned authorities and private land owners.
A range of flame weeders is available at here
Infrared radiation: Radiant Flame or Infrared units direct the propane flame (under the protection of a cover) onto a ceramic or metal surface which becomes incandescent and radiates the heat towards the target plants. This method eliminates the potential for fire hazards associated with flame weeders in dry conditions, especially in the presence of mulch or dry residues. While it is similarly effective, it requires several times as much energy as direct flame. Infrared weeders have the disadvantages of needing time to heat up, the IR panels are sensitive to mechanical damage, and they are more expensive than flame weeders.
Units can be hand held, trolley or vehicle mounted. Hand held LPG/ propane flame burners can be used for small or difficult to access areas.
Solarization: Solarization is a method of heating moist soil by covering it for around 6 weeks with plastic sheeting to trap solar radiation. Where soils are typically dry, they must be irrigated first.
Transparent plastic (polyethylene) is more efficient than black plastic since transparent film lets visible light in, allowing more radiation to reach the soil than black films. This causes even greater temperature increases. However, because black films exclude visible light, they stop photosynthesis, which can be enough to kill some young annuals and perennials given sufficient time. Thin films are more effective than thick films (>2mm).
Solarization kills emerging weeds, some soil-borne pathogens, nematodes, and insect pests, and even some weed seeds and vegetative propagules of perennial weeds. The degree of weed suppression achieved with solarization varies with weed species, depth of seed in the soil, and length of solarization. It is best done during long periods of sunshine to heat up the soil under the sheeting and maintain a sufficiently high temperature (> 65oC) for long enough to kill the weed seeds. However, if conditions are too cool or cloudy to support effective solarization, the clear plastic can simply accelerate weed growth under the plastic layer by creating near-optimum temperatures.
Unlike steam sterilisation, solarization does not sterilise the soil and create a biological vacuum, though it can cause significant biological, physical, and chemical changes in the soil that can last up to two years, and deter the growth of desirable native species. The influx of nutrients that results from solarization can be advantageous to restoration efforts, but can promote weeds that typically thrive in nutrient-rich soils.
Mulches are used to hinder the emergence of light-responsive weed seedlings. A mulch may take many forms: a living plant ground cover, loose particles of organic or inorganic matter spread over soil, and sheets of artificial or natural materials laid on the soil surface.
Organic mulches such as hay, straw, tree leaves, and wood chips: About 5-15cm of organic mulch (depending on the size of the mulch particles) can greatly reduce the emergence of broadleaf weed seedlings. Organic mulches are less effective against grassy weeds and they usually do not significantly hinder the emergence of perennial weeds from rootstocks, tubers, rhizomes, or bulbs whose food reserves allow them to grow up through the mulch. However, existing broadleaf weeds can also be slashed and mulched – they will rot under the cover. Weeds in mulch are relatively easy to remove.
Groundcover plants or dense shrubs which will also suppresses weeds, can be planted through the mulch for long term control.
Organic mulches also provide other ecological benefits: they give shelter to ground beetles and other weed seed consumers; they conserve soil moisture and improve water infiltration; they maintain good soil tilth, prevent surface crusting, feed soil life, and sometimes provide slow-release nutrients. Unlike synthetic mulches, organic mulches are biodegradable and don’t contribute to landfill problems.
Avoid contaminated mulch and don’t use plastic under mulch because plastic stops water and air reaching the soil.
High nutrient mulches, such as green waste and grass clippings, should not be used in nutrient-poor environments like coastal sands as they will encourage weeds. Organic mulches high in carbon may temporarily reduce the availability of soil mineral nitrogen as they decompose.
Newspaper mulches: These will also degrade naturally. However, there are toxins involved with the paper and with the inks used. Information on these is available here. This may not be significant for non-food garden beds. Paper mulches used alone are not very effective in the longer term, but used underneath hay or other organic mulch they will enhance weed control over the organic mulch alone.
Black plastic film: This is commonly used as a mulching material. It effectively blocks emergence of most weeds, including perennials. However, these synthetic materials do not enhance soil quality or soil organism habitat, can interfere with infiltration of rainfall, and (unless a biodegradable material is used) will eventually require pickup and disposal. In addition, they are synthesized from petroleum – plastic represents a significant use of non-renewable fossil fuels. In addition, black plastic can be penetrated by weeds with sharp, tough growing points.
Biodegradable plastic mulches: Made from plant starches, these are broken down slowly but completely by soil microbes into carbon dioxide and water. These perform as well as standard black polyethylene film.
Landscape fabrics: Woven black polypropylene mulches (weed barriers) are a durable and effective barrier to weed growth but are readily penetrated by rhizomatous perennials. They are permeable to air, water, and nutrients, and are useful for perennial plantings; their efficiency can be increased by a mulch cover. Woven black plastic is also useful along roadsides, steep banks and cuttings where areas need to be revegetated. This option is viable for small areas and can assist in weed management, bank stabilisation and erosion. They are long-lasting.
Spray-on mulches: Ecoblankets and Ecoberms use recycled organics (green waste) with a ‘tackifier’ to increase the stickiness of the surface for large scale erosion control on newly constructed and existing roadways and construction sites. They use blower trucks with long flexible hoses to put the material in place which can also give access to distant or confined spaces. Large areas can be reclaimed quickly without damage by heavy machinery on site. These materials can be used in steep terrains or as berms for erosion control, dam walls, or bioretention basins. They do not need other reinforcements like non-biodegradable plastic silt fences to inhibit erosion. They establish a buffer between rainfall impact and the soil and allow the absorption of rainwater into soil.
Revegetation is achieved at the same time by incorporating grass seed or native seed blends depending on the site requirements. This technique requires little ongoing maintenance as the thick regeneration cover inhibits weeds. “While each native species has different growth cycles, you can expect a healthy crop of seedlings within the first few months and a relatively thick covering of shrubs by the end of the first year. Within two years the site will barely be recognisable as having once being disturbed by previous construction, mining or other infrastructure work.” (Ecoblanket )
Some of the companies using this technique and providing case studies on their websites are:
- The Hills Bark Blower, NSW.
- Groundworks, Queensland.
- WA Ecoplant, Western Australia.
- WeBlow, Victoria.
- LCS Landscapes, South Australia, in collaboration with Jeffries
“Managed grazing is based on ecological principles that enhance natural systems.” (Eco Goats Qld) Well-managed grazing may eliminate small infestations and reduce severe ones. Grazing animals are cost-effective to reduce large infestations with other methods of control then used to avoid overgrazing. Even where herbicides are still the preferred method of weed management, grazing animals may be used in areas where herbicides should not be applied (e.g., near water).
Many of the weeds we are dealing with are due to the wide-scale clearing of lands for grazing; when the grazing is removed, weeds can proliferate either as successional indicators or because they were kept in check by grazing. Plants like Bugle Lily (Watsonia bulbillifera) are kept under control in their native South Africa by baboons feeding on the corms.
Cattle, goats, sheep, and even geese may be used to manage weeds. Predation problems in many areas may dictate the type of grazing animals that can be used. St John’s Wort causes photosensitisation in sheep, cattle, horses and goats which can lead to weight loss, and in extreme cases, death.
A site/weed/native species-specific Grazing Management Plan should be developed which includes strategies to:
- Assess the appropriate species of animal grazers and stocking rates. Note that different animal species will graze different plant species.
- Assess the appropriate timing, duration and frequency of grazing. This also relates to seasonal palatability of plants as this may change during the growing season.
- Ensure overgrazing does not occur. It is important to monitor that native species are colonising bare areas.
- Keep grazing animals away from waterways or intensely managing them in these areas.
- Ensure soils are not disturbed or native communities impacted
- Ensure weeds are not introduced from elsewhere. Animals that are removed from an infested area should not be transported to weed-free areas until all seeds have passed through their digestive tracts (five to nine days). Weed seeds can also be transported to new areas in animal hair.
- Monitoring and removal of animals if damage is noted
- Ensure the health of the animals. This is best achieved by contracting professionals.
The main reference for this section is the Weed Control Methods Handbook, The Nature Conservancy.
Sheep and Goats: Sheep and goats prefer broadleaf herbs. Both species appear able to neutralize the phytochemicals toxic in many plants. For instance, both species can tolerate Fireweed (Senecio madagascerensis), listed as a Weed of National Significance. Merino sheep are more tolerant than British breeds or crossbred sheep because they restrict their daily intake of the plant whereas other breeds will eat it to excess (DPI, NSW). Sheep and goats are also well adapted for grazing in steep or rocky terrain.
Sheep: Sheep do not graze an area uniformly. Consequently, a method (i.e: herding, fencing, or the placement of salt licks) should be employed to concentrate activities in an area. Sheep often need a period of adaptation before they will start to consume a new forage type. This process can be expedited by using herds as opposed to individual animals because sheep will follow the lead of their peers.
Goats: Goats are most commonly used for weed control as they eat a wide variety of plants and since grass is their least desirable food choice. They are also more tolerant of our wetter coastal areas. Goats can control woody species because they can climb and stand on their hind legs, and will browse on vegetation other animals cannot reach. They will also kill trees and shrubs by ring-barking and structural weakening. Goats additionally, tend to eat a greater variety of plants than sheep. Goats find such weeds as Lantana (Lantana camara), Blackberry (Rubus fruticosa), Sweet Briar (Rosa rubiginosa), Privet (Ligustrum spp) and Scotch Broom (Cytisus scoparius) highly palatable. Less palatable but also eaten: Groundsel (Baccharis halimifolia), and Serrated Tussock (Nasella trichotoma).
Goats can be especially effective for roadside management, along railroad tracks, parks, forests, etc. (Successfully Controlling Noxious Weeds with Goats;
Meat and Livestock Australia, Weed Control Using Goats.
Cattle and horses: Cattle will graze invasive grasses, can trample inedible weed species, and can incorporate native seeds into soil. Horses can also be used to control invasive grasses, but horses tend to be more selective than cattle.
In rangelands, where overgrazing by hard-hoofed animals led to massive detrimental environmental changes, there are some innovative solutions related to grazing management and protection of waterways which allow for ecological regeneration (Soils for Life, Natural Sequence Farming, Alan Savory’s Holistic Management).
Geese:Geese are also useful for the control of invasive grasses, but are more subject to predation than other animals. Geese of any age can be used for weed control. A goose at 6-7 months of age can consume enough grass equal to their weight daily. Provide birds with adequate shade and drinking water and a little prepared feed in the evening. Allow 7–15 geese/ha, and move them according to weed growth.
Organic or non-synthetic pesticides are safer than cosmetic products since they do not persist in the environment and have low toxicity to non-targeted organisms. Organic herbicides break down quickly, leaving no residual effects. However, they are all nonselective contact herbicides useful in spot treatments but will affect all plants sprayed. Since they affect only foliage (they are non-systemic), underground bulbs or tree roots will not be affected. Desirable plants can be protected by shields to prevent splashes or, alternatively you can use a cardboard tube or paper cup set over the spray nozzle – this will catch spray drift inside the cup.
These products are also non-systemic herbicides which means they won’t penetrate woody stems and roots. Generally they won’t be as effective on woody plants or plants with runners, rhizomes or tap roots. So shrubs like lantana, gorse or bamboo, deep rooted plants like thistle, and runners like kikuyu, wandering jew, nutgrass or onion weed will not likely yield results.
Organic herbicides work best on weeds that are less than four inches tall. Mature perennial weeds will likely need multiple applications and may repeatedly re-sprout from the undamaged roots. For the best results, apply organic herbicides to young weeds on a hot, sunny day.
These herbicides fall into the following categories:
- Natural acids (vinegar + citric acids),
- phytotoxic oils (clove, peppermint, pine, citronella),
- Herbicidal soaps,
- Non-toxic chemical compounds,
- corn gluten meal
- Salt-based herbicides, (not recommended for repeated usage)
Combinations of these groups are also made.
Some preparation recipes are available at No Dig Vegetable Garden. Products are available on-line in Australia.
1. Vinegar (acetic acid)
Acetic Acid acts by burning the waxy cuticle off above-ground portions of the plant. Household vinegar has a 5% acetic acid concentration which will kill small plants but is not strong enough for larger ones. Horticultural vinegar is 10-20% acetic acid and formulations are commercially available.
Vinegar is particularly useful for broad-leafed plants but also for many grasses. Pick a hot, dry day to spray weeds until saturated. Within hours, they will wilt and shrivel up. For perennials, it usually requires more than one treatment before the weeds are dead. It can also be injected into taproots or the base of the plant using a syringe.
Efficiency has been increased by the addition of soap which breaks the surface tension of the vinegar so it sticks to the weeds, forcing leaves to absorb it more readily. Recipe: fill a garden sprayer with white vinegar and add 1 teaspoon liquid dishwashing soap.
In some preparations, salt is added but this may accumulate in the soil with frequent use. (Add a cup of common salt to a litre of vinegar. After it dissolves, brush it directly onto weeds.) Vinegar can also be used to clean concrete but repeated use on weeds in these areas may do damage. Repeated applications will acidify the soil so you need to consider the place and frequency of use.
In addition, vinegar can be used as a soil treatment. “Vinegar concentrates that are sprayed onto the soil, on the other hand, work by lowering the soil pH to a level where the plants can’t survive. This acidifying effect lasts from several months to a year, depending on the soil type and the weather. Once the soil pH is lowered, taproots will eventually starve, but before dying they will often have one last flush of growth from nutrients stored in the roots.” Read more.
Effective weed control requires a highly concentrated acetic acid solution, which may be dangerous to handle so use these products with caution. Concentrations of acetic acid greater than 5% can cause skin irritation or eye damage
2. Citric Acid.
In America, Blackberry and Brush Block is Certified Organic. It is 20% citric acid and 80% water. There it is used in spring, sprayed on woody weeds like blackberries with multiple applications. Best success is treatment when the plants are young. It also works on non-woody weeds like dandelions. Brushweeds and Grass also has organic certification. It’s made of 20% citric acid, 8% concentrated vinegar, 72% water. It works by burning off the waxy cuticle that protects plants’ leaves.
Oil works by suffocating plants. Some oils also strip the surface of the leaves which then dehydrates the plant. Cheap vegetable oils, such as canola and sunflower are biodegradable and will be broken down by soil bacteria. Bulbous weeds (e.g. Onion Weed, Oxalis) can be killed by injecting vegetable oil very carefully into the soil around the bulbs so they suffocate, die and rot into the soil.
Gasoline, old engine and diesel oil are toxic to the soil, so don’t use these.
Many herbal and plant oils contain natural pesticide and herbicide properties. They can be mixed with other substances such as vinegar and soap to safely eliminate weeds. These oils are often found in organic preparations in garden shops, and include d-limonene, or citrus oil extract, neem oil, castor oil, pine oil, cinnamon, clove, and thyme oil. Cinnamon for example contains eugenol, a particularly potent herbicide.
1. Pine oil
Oil based Herbicides are based primarily on pine oil. They are organically certified and kill most weeds and their seeds by stripping their outer protective coating and causing dehydration. Success will depend on the type of plant, size, season used and coverage. Soft, young, non-woody weeds will respond best. Spray directly onto foliage, completely covering the plant, during the early hours of a hot sunny day. Safe to use around pets and doesn’t harm earthworms or soil microbes. It works best on soft, non-woody plants – seedlings, grasses, herbaceous plants and seeds.
Application times and methods differ from other common herbicides so this must be taken into considerations when using the product. Here is a comprehensive manual on the uses of Pine Oil.
Pine Oil products are available at:
2. Weed Zap has been approved by the Organic Association of Australia! Active Ingredients: Clove Oil 19%, Cinnamon Oil 19%, Cottonseed Oil 19%, Oleic Acid 19%, Lauric Acid 19%
Herbicidal soaps – Fatty acids
Herbicidal soaps made from naturally occurring fatty acids are highly refined soaps that can penetrate the waxy coating on plant leaves, causing them to dry out. Some of these products also contain essential oils that enhance their herbicidal properties.
Nonanoic acid, also called pelargonic acid, is a clear, oily liquid with an unpleasant, rancid odour. It is a fatty acid which occurs naturally as esters in the oil of pelargonium. It is found in almost all species of animals and plants and at low levels in many of the common foods we eat. The ammonium salt of nonanoic acid, ammonium nonanoate, is used as an herbicide. It is readily broken down in the environment. For food crops, pelargonic acid is allowed to be applied from planting time until 24 hours before harvest. The chemical also controls weeds at sites such as schools, golf courses, walkways, greenhouses, and various indoor sites.
Pelargonic acid is a skin and eye irritant, and product labels describe precautions that users should follow to prevent the products from getting in their eyes or on their skin. Here is the US EPA factsheet for pelargonic acid.
Slasher organic herbicide is made from pelargonic acid and is available from Greenway Enterprises in West Australia.
Certified Organics in Victoria sells BioWeed Control which also kills soil seed and seed on plants. It is made from terpene alcohols and saponified fatty acids. The manufacturers recommend this product for agricultural purpose as well as woody weed control, and use in public spaces.
Non-toxic Chemical Compounds
Sugar works by providing food (carbon) for soil micro-organisms which then use lots of soil nutrients, including nitrogen, to grow and multiply. Nitrogen is one of the most important nutrients for weeds and it is often at elevated levels in degraded sites. Lack of nitrogen allows native Australian species, adapted to low nitrogen levels, to outcompete the weeds. Broadleaf and annual weed species are more susceptible than grasses and perennials.
“The method is simple. Take about a cup full, or even a handful, of sugar and sprinkle it around the base of a weed. Take care to avoid other plants and coat the soil thickly over the offending weed’s root zone. Check the weed in a day or two and recoat if the area was saturated or the weed is not showing signs of decline.
“Killing Lawn Weeds with Sugar: You can use granulated or powdered sugar sprinkled lightly over your lawn, or a molasses spray. (Mix molasses at a rate of 1 ¾ cups to 10 gallons of water in a backpack or manual sprayer.) Evenly coat the lawn and water it in lightly. Don’t over coat or forget to water, as the sugar will attract insects and animals if left on top of the leaf blades. The best time to start sugar weed control is spring when weeds are small and before they go to seed.” (Bonnie Grant in ‘Gardening Knowhow’
2. Ammonium sulphate
Sulphate of ammonia burns out weeds. Water the weeds lightly about half an hour before applying the sulphate of ammonia so there is a little moisture to start the burning action. The advantage of sulphate of ammonia is a short residue period, unlike table salt which is much longer. This is effective for Bindi control.
3. Iron sulphate and ammonium sulphate
Jerry Colbwy-Williams recommends the following treatment, learnt from his grandfather, for a weed-free lawn:
“The ingredients include: dry washed sand, sulphate of potash and iron sulphate. Use a plastic bucket and a plastic measuring container because these can be washed after use. Simply mix the ingredients in equal proportions. So put in one cup of washed sand, one cup of sulphate of potash or sulphate of ammonia and one cup of the iron sulphate, and mix these up immediately. It’s important to use exactly what you make because it doesn’t store well. If it soaks up any humidity, it will become rock solid. Also wear gloves because the iron sulphate can make your hands rusty. Grandad’s lawn sand is fantastic for controlling a variety of weeds in the lawn. It’s especially good for attacking cudweed, bindii, white clover, cat’s ear and plantain.
“Sprinkle by hand aiming directly at the weeds. There’s no need to cover the entire lawn. It’s best to apply just before sunset so that dew fall will activate the mix, or lightly sprinkle the treated weeds to damp them down to start the process. To control all weeds repeat this operation every other week. Remember lawn sand won’t kill narrow leaf weeds like onion weed or winter grass, or waxy leaf weeds like some types of oxalis. If the mix comes in contact with flowers or footpaths, wash it off with water immediately. Keep a watering can nearby when applying. If pets or children want to use the garden, water the lawn sand in the following morning. As soon as it’s dry, it’s safe for them to go into the garden. Iron sulphate can stain clothes and skin, so always wash hands after use. … The objective is to complete the task in mid-winter, particularly in eastern and southern Australia. This prevents the weeds setting seed and starting another generation. The end result to aim for is a lush, thick sward of grass that helps to suppress weeds.”
In New Zealand, gorse, blackberry and other woody similar plants have been eradicated with garden lime. Changing the pH, making the soil more alkaline, works against weeds adapted to an acid environment. Ideally, you cut the gorse or blackberry down to near ground level and dose the surrounding area with a good amount of lime. The lime will prevent regeneration of gorse, etc.
Corn Gluten Meal
This is a by-product of corn starch production. It has been identified and patented as a natural pre-emergence herbicide for turf grass. Time of application is extremely important, as the gluten must be present when weed seeds germinate to inhibit root formation. If conditions are too wet during germination the plants will recover and form roots. It is most effective for many broadleaf but also inhibits germination of many grasses. An additional benefit of corn gluten meal is its high nitrogen content (about 10% by weight).
Weeds affected by corn gluten meal include black nightshade (Solanum nigrum), common lambsquarters (Chenopodium album), yellow dock (Rumex crispus), creeping bentgrass (Agrostis palustris), purslane (Portulaca oleracea), and dandelion (Taraxacum officinale). Corn gluten meal applied to lawns in early spring can help contain Crab Grass problems (Digitaria spp.).
Since established plants are not affected, corn gluten meal can be used in garden beds containing existing plants or transplants. The suggested application rate is 1kg/10m2.
Environmental control or site modification involves manipulating the environment to make conditions unfavourable for weeds.
Reafforestation/supplementary tree planting
Reafforestation is a large-scale, long-term method of weed control. The aim of reafforestation is to form a dense tree canopy that restricts sunlight penetration to weeds on the forest floor. Dense canopy formation may take 5 to 10 years and weed control can be critical to success. Seed should be sourced from local indigenous plant species for use as part of direct seeding or for propagation. For site preparation, standing vegetation should be slashed. Plantings can be mulched with cardboard and wood chips for adjacent weed control. With increasing extremes of high temperatures and floods, leaving surrounding standing weeds to protect seedlings/saplings should be considered.
Vegetated sites may only require competitive weed removal and supplementary or enrichment planting to complement existing indigenous vegetation. Check for the presence and diversity of native species before developing a planting framework.
Primefact 982 from the NSW Department of Industry and Investment, Planting trees for biodiversity gives a good overview of strategies and factors to consider to improve biodiversity outcomes. (Johnson, I., Kavanagh, R., and Coburn, R., 2009)
In grasslands, restoration native grass seeding can decrease weed incidence.
“Fire has been used for many years as a form of vegetation and weed control. Its success depends on the amount of fuel, the speed and intensity of the fire, and the time of year that burning takes place.” NSW Department of Primary Industry, Noxious and environmental weed control handbook: a guide to weed control in non-crop, aquatic and bushland situations.
Controlled burns have a many effects, including the death of trees, increases in shade, sprouting of shrubs and herbaceous perennials, temporary reduction in competition, changes in seed germination, and changes in nutrient availability. The use of fire as a weed control method requires detailed planning and all relevant permits and approvals must be obtained.
Although it unfortunately focuses heavily on herbicide use (like most control manuals), the Weed Control Methods Handbook of the Nature Conservancy (2001) has an informative section on the use of prescribed burns to promote desired vegetation and species. It also provides examples of weeds that have been controlled by prescribed fire, and the variable effects of burning on weeds.
In places where controlled burns are unsafe, during fire seasons, or in wetlands, Dragonfly Environmental uses Flame Weeding to mimic the effects of fire and stimulate the germination of native plants.
Woody weeds have been managed with fire:
- in South Australia (Post-fire woody weed control in bushland, Fact sheet, May 2015, Natural Resources, Adelaide and Mount Lofty Ranges)
- in Queensland Gulf country with responses of a number of species outlined (Fire: Management of native and invasive woody weeds in the Northern and Southern Gulf, http://cdn.futurebeef.com.au/wp-content/uploads/fire_factsheet_web1.pdf )
- For Boneseed and Bitou Bush, a Weed of National Significance which germinates rapidly after fire, outcompeting native seedlings. (Melland, R., and Preston, C., 2008. The role of fire in integrated management of boneseed (Chrysanthemoides monilifera subsp monilifera). Plant Protection Quarterly, V23(1), p32.)
- (with care) for Lantana (http://www.environment.gov.au/biodiversity/invasive/weeds/publications/guidelines/wons/pubs/l-camara.pdf)
- “Fire can play a major role in the management of woody weeds in western regions of NSW and can also be a useful option for the control of lantana and blackberry in certain situations.” (NSW Department of Primary Industry, Noxious and environmental weed control handbook: a guide to weed control in non-crop, aquatic and bushland situations, http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0017/123317/noxious-and-environ-weed-control-handbook.pdf )
Most environments benefit from strategies for the retention and use of water where it falls. In eroding areas where surface runoff is high causing erosion, the focus is on soil surface conditions, either to increase surface roughness through pits or contour furrows, or to add above ground structures like logs, rocks, or woody debris. This will affect both surface and subsurface flow and improve conditions for plant establishment. Erosion control measures may include fencing or man-made erosion control products.
“In wetlands where it is possible to control water levels, drawdown or flooding may be useful techniques to contrl some types of weeds, though the results aren’t always conclusive” (Romanowski, 2011). In situations where the water level of a wetland or riverine system can be manipulated, flooding can be used to control some plant species. Some species, however, have vegetative buds or underground storage organs that can survive several months or more under flooded conditions. (Weed Control Methods Handbook, The Nature Conservancy, 2001)
This ‘requires the area being treated to be saturated at a depth of 15 to 30 cm for a period of 3 to 8 weeks. The saturation of the soil reduces the availability of oxygen to the plant roots thereby killing the weed. This method has been shown to be highly effective in controlling establish perennial weeds and may also suppress annual weeds by reducing the weed seed populations’. (Wikipedia)
Engineered Log Jams
Stream restoration and river engineering projects are employing engineered log jams (ELJs) increasingly for stabilization and in-stream improvements. This has been utilised well in America. In Australia, a recent study in river management which encouraged the reintroduction of large woody debris into streams to rehabilitate stream channels demonstrated that engineered log jams “can provide a successful mechanism to managing active channel destabilisation, though this must be framed within the context of long term riparian rehabilitation”. (James S. Daley, 2012, Taming the Hungry Beast: the effectiveness of engineered log jams in an incising gravel-bed river).
Engineered log jams are also being used in South East Queensland to stabilise creek banks and protect high value farmland in Warrill Creek following the January 2013 floods.