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Weedkillers   Weeds   Weed Removal   Weed Prevention   Weed Seedlings

These herbicides kill both broad and narrow-leaved plants. They can be divided into three main groups, Contact, Systemic and Residual.


Give rapid knock-down of all top growth and are ideal for clearing ground prior to sowing or planting. They do not kill the root system or stems of perennial plants, but repeated application may exhaust their reserves, so such plants may eventually be removed. Sometimes known as post-emergent herbicides as they have to land on growing plant tissue to work.

Ammonium Octanoate and Ammonium Decanoate - naturally occuring fatty acids which work by damaging the cell walls of surface plant cells causing rapid dehydration of soft plant tissues and subsequent death of the topgrowth. Should be effective on plants with a waxy surface. The whole plant must be coated, so the lower leaves of dense foliage may be missed and treatment will be less effective. Perennial weeds can re-grow. This may work on sparce, tiny seedlings, most other weeds recover, even annuals, as it is difficult to cover the foliage completely and there is just some scorching. Sold as Bayer Garden 3 Hour Weedkiller - it works in theory! Ammonium sulphamate (AMS) - kills vegetation on contact, for full details see below. Paraquat - It is one of the most widely used herbicides to control broad-leaved weeds and grasses. A bipyridyl chemical type of herbicide, it is quick acting and non-selective, destroying green plant tissue on contact and by translocation within the plant.
It binds to soils, and when adsorbed it is inactivated.
Ingestion of the active ingredient is fatal, even small quantities, and it is banned (Austria, Finland, Sweden), or restricted (Germany, Hungary, US) in some countries for this reason. Diquat - is closely related to Paraquat and works by the same action. Glufosinate ammonium (Glufosinate) - is a broad-spectrum contact herbicide and is a natural compound isolated from two species of Streptomyces fungi. It inhibits the activity of an enzyme which produces glutamine and reduces ammonia levels in the plant tissues. This causes photosynthesis to stop and the plant dies within a few days. It inhibits the same enzyme in animals and could have effects on the nervous system, but the manufacturers say this is unlikely at normal usage levels. Genetically modified, gluphosinate-resistant soybean and maize crops have been developed. Pelargonic acid - (nonoaic acid) is a natural fatty acid found in small quantities in most living organisms, but at high concentrations it the removes the protective cuticle of the green matter, which dehydrates rapidly and scorches. In theory this should make it effective on plants with waxy foliage such as Liverwort. The whole plant must be coated, so the lower leaves of dense foliage may be missed and treatment will be less effective.
It was first marketed in 1965 by ICI, advertised as "The Chemical Hoe". As soon as it has dried it has done its work, leaving no harmful residues, so sowing and planting can be carried out immediately, with results showing in a few hours. Also when dry the area is safe for children and pets.
With the trend towards using more natural products in the garden, it has been given a new lease of life by the introduction of Weedol MAX in the Scotts range of pesticides in the UK (in the US it is sold as Sythe)
Sodium Chlorate - see below.

Systemic (translocated) Weedkillers

The active ingredient is taken up by the foliage and transported down to the rest of the plant including the perenniating stems and roots which allow some plants to survive adverse conditions. There should be plenty of foliage so that the maximum amount of active ingredient is absorbed and it must be allowed to continue to grow until it is well on the way to extinction (so this is also known as a post-emergent herbicide). It could be two to three weeks before the weeds are completely dead, but they can be removed after about six days when signs of yellowing and wilting may begin, and any remaining parts should not recover. In the meantime they may produce seeds, especially fast maturing ephemeral weeds like Hairy Bittercress, or any weeds which are in flower so removing them before application would be advised - a mower at its highest setting or a line trimmer held high could be used for a large area. Also if the area has been infested for some time there will be a reservoir of seed which will germinate later - turning over the soil and applying a mulch should greatly reduce this occurrence. If you are an impatient gardener or follow the school of the instant 'makeover', then this type of herbicide may not suit unless you plan ahead and treat the weeds a few weeks beforehand.

Ammonium sulphamate (AMS) - (alt. spelling "sulfamate") is an inorganic herbicide, so called as it is a salt and does not contain a carbon-based molecule. It kills any plant tissue it touches an is used to kill tree roots as well. It can persist in the soil for one to three months, depending on conditions, ie. soil type, weather. It can leach to ground water, so care must be taken to avoid this. The ammonium breaks down releasing Nitrogen which acts as a fertilizer. Usually supplied without a trade name in crystalline form for killing tougher weeds and tree stumps. The crystals are inserted into holes drilled in the stump where they delequesce (ie. liquefy by taking moisture from the air or surroundings). Foliage is treated with dissolved crystals and shows an effect after 7 to 10 days. Treated areas can be cultivated after 8 to 12 weeks as by this time the poison will have broken down. Individual weeds can be treated using a hand spray.
Some countries allow it as an Organic herbicide as it breaks down to harmless by-products, but it is not approved in the COMPENDIUM OF UK ORGANIC STANDARDS.
AMS was withdrawn from the UK amateur gardening market in November 2007 and permitted use of stored product ceased in May 2008. This was due to manufacturers not submitting the data required by EU regulations and not because of any safety concerns - carrying out the extensive testing to produce this data is expensive and would not make commercial sense. Products include Bramble Killer, Ivy Killer, Dax Root-Out, Doff Tree Stump Killer, Tough Weed Killer and Growing Success Deep Root which will no longer be available or may be reformulated with a different active ingredient which has approval.
Glyphosate - derived from the amino acid Glycine (with the addition of a Phosphoric acid molecule). It acts on an enzyme (EPSP synthase) which produces an essential amino acid in an essential biochemical mechanism commonly found in plants, but not in animals. This reduces the production of protein in the plant and inhibits plant growth and is why it takes some time to work. In good growing conditions this can be after about 7 to 10 days, but it can take 3 to 4 weeks at cooler temperatures. It is not usually absorbed from the soil by plants so it can be used to clear ground for planting. This makes it less useful by itself on paths and gravel as seeds will germinate and re-infest the area. It has recently become better known because of its involvement in geneticly modified crops, the gene which is responsible for producing the enzyme is modified in a selected plant to resist the action of the herbicide, so it can be applied to kill everything else. This could also have possibilities in the garden, eg. grass or ground cover plants could be made resistant to the herbicide, but would hardly be commercially viable proposition.
A new more concentrated version has been introduced for killing stumps claimed by the manufacturer to be more effective on Japanese Knotweed - probably in response to the withdrawal of AMS. It contains a higher concentration of active ingredient - ready-to-use product for normal weedkilling usually has 7.2g/litre.

The patent protection on Glyphosate ended in 2000, so there is some competition in the market. The best known branded product, Roundup, has been modified slightly to 'improve' it's action probably with added sufractants to enhance absorption and to justify the higher price. The standard formulation works just as well and is found in many of the herbicides currently available. If larger quantities are required, diluting the concentrate is better value than the ready-to-use product so after an initial purchase the useful sprayer can be refilled. 30mls of the 120g/litre concentrate makes a litre of spray for normal treatment, and 10mls of the 360g/litre product does the same. Regardless of the name and description for its use, it is the dilution which is important so there is usually no need to buy different products as most have 7.2g/litre - this information is usually in tiny print hidden deep in the text on the label. For tree stumps, bamboo and Japanese Knotweed where the weedkiller is placed in drilled holes or cut, hollow stems, the dilution is one part 360g/litre glyphosate to six parts water.
One product (Resolva) uses diquat with glyphosate to give a quick acting result - the claim is that the special surfactants included allow the glyphosate to be absorbed faster so it is transferred to the roots within a day while the diquat kills the topgrowth and you don't have to wait for two weeks for the weeds to die back - a small comparative trial with standard product on some Ground-elder seemed to confirm the claim.

Glyphosate has been threatened with a ban by the anti-chemical dominated environment committees in the European Parliament in recent years and was due to be affected in 2012, but decisions have been postponed until 2018. Also in 2015 the International Agency for Research on Cancer (IARC), a research arm of the World Health Organization, classified glyphosate as “probably carcinogenic” to agricultural workers, but it would have to be noted that it is in the same category as grapefruit juice. There is no indication as to the amount of exposure, but on a purely toxicity measure caffeine is over ten times as toxic in LD50 tests (see below). On scientific calculations you would have to consume about 15 litres of glyphosate at normal usage concentrations to kill the average 63kg human being.
The IARC does not do original research it merely evaluates things that could be a hazard to bring them to attention. Out of over a thousand substances considered only one has been categorised as "probably not" a carcinogen. Following the announcement environmental action groups have made claims that several countries have banned products containing glyphosate, but most have been restrictions on use such as not spraying on hard surfaces to prevent run-off as in the Netherlands, not a complete ban. The European Food Safety Agency has declared it not to be a carcinogen as have similar bodies in the USA and New Zealand. On the 15th of December 2017 a 15 year license for use in the European Union runs out and it it being speculated that a new one will be issued with a much shorter duration, possibly 3 years.

Amitrole - prevents carotinoid synthesis, which results in the breakdown of chlorophyll by light, tissues lack green coloration and appear bleached. It is absorbed slowly by the plant and is translocated by the phloem and xylem. It has a short soil residual life, but can be washed down to roots under a path, causing damage to nearby plants.

(LD50 stands for Lethal Dose 50 which is the amount that kills 50% of test mice given the substance being assessed.)

Residual Weedkillers

Sometimes called pre-emergence herbicides, they are non-selective and remain in the soil, killing germinating seeds and shoots from perennial roots. Some can be used with care around established shrubs and trees, but not where bulbs are planted. Usually they are used in combination with other weedkillers on paths and gravel and should keep them relatively weed-free for the growing season if applied in the spring. They can be transported through the soil by percolating water and in surface run-off, so care must be taken near bodies of water and streams. The rate of movement will depend on a number of factors such as soil type and the organic content. Also grass and flowerbeds next to treated areas can be affected (The roots of shrubs which are growing under treated gravel paths, can show signs of damage such as white blotches on the leaves).

Atrazine - acts by inhibiting photosynthesis (carotenoid biosynthesis) in plants it is adsorbed to soil particles and is active in the soil for about 5 to 7 months. Soil microorganisms break it down. Now banned under EU Directive 91/414/EEC except for 'essential uses' Dichlobenil (2,6-dichlorobenzonitrile) - is active in the soil against all types weeds and is absorbed from soil by the roots. Applied to foliage it causes reduced cell growth at the growing tips and inhibition of germination in seeds. It does not dissolve easily in water so is usually applied as granules. For non-selective weed control, apply in early spring just before growth is due to start, treated areas should not be used for about 2 years. It can also be used in still water to control aquatic weeds.
(The EU Standing Committee on the Food Chain and Animal Health voted for non-inclusion of dichlobenil in Annex I to Council Directive 91/414/EEC, resulting in the withdrawal of authorisations for plant protection products containing dichlobenil.
Products affected are: Casoron G, Casoron G4, Midstream GSR, Scotts Dichlo G Micro and Scotts Dichlo G Macro. Sales ceased in March 2009, followed by a further period of 12 months for storage and use. The final date for use was 18 March 2010)
Diflufenican - works by inhibiting carotenoid biosynthesis. Diuron - used to control a wide variety of annual and perennial broadleaf and grassy weeds, as well as mosses. It works by inhibiting photosynthesis and is taken up mainly by the roots. It can remain in the soil for up to a year depending on the soil type and the amount of organic matter present. Flufenacet - a thiadiazole effective at controlling certain grasses and broadleaf weeds. Metosulam - a selective herbicide with residual properties see Selective Herbicides. Oxadiazon - works on seed and early post-emergant shoots to keep ground clear of growth. Simazine - Plants take up simazine mainly through the roots and it acts by inhibiting photosynthesis. Activity remains for 2-7 months after application. Now banned under EU Directive 91/414/EEC except for 'essential uses', so not available to the amateur gardener. Sodium Chlorate* - a non-selective contact herbicide, killing all green plant parts and has a soil-sterilant effect. It may persist in soil for 6 months to 5 years, depending on rate applied, soil type, fertility, organic matter, moisture, and weather conditions. It is highly toxic to animals and humans, breaking down red blood cells. It forms explosive mixtures so products sold as weedkillers have Sodium Chloride added to inhibit flammability.
*In July 2008 the European Commission Pesticides Review Standing Committee voted not to include Sodium Chlorate in Annex 1 of the approved pesticides list and given a last date of sale on 30th September 2009. It was totally banned after 10th May 2010 and any excess had to be disposed of in the approved manner on that date, ie. not down the drain.
Trade Name**TypeActive Ingredient(s)Use
B&Q CompleteSystemicGlyphosategeneral and spot weeder
GramoxoneContactParaquatfast acting ground clearance (agricultural use only), perennial weeds not completely killed
PathclearSystemic and ResidualGlyphosate, Diflufenican and Oxadiazonpaths and gravel
RoundupSystemicGlyphosategeneral to clear for planting and as a spot weeder
TumbleweedSystemicGlyphosategeneral to clear for planting and as a spot weeder
WeedolContactDiquatfast acting ground clearance, perennial weeds not completely killed
Weedol MAXContactPelargonic acidfast acting, perennial weeds not completely killed

** These are some of the products available and not a recommendation for their use.

Health and Safety

Whatever the herbicide care must be taken during use. Some can be absorbed through the skin and contact can lead to rashes, nausea and even death in the case of paraquat, particularly through cuts and abrasions.

Follow the manufacturers instructions carefully. Always wear rubber or vinyl gloves. Keep a separate watering can for herbicide use. Only make up enough to do the job in hand, use several small batches if uncertain to avoid the need to dispose of an excess. NEVER store diluted pesticides in a "pop" bottle. For small treatments use the ready-made sprays now available. Keep the concentrate in a safe place in its original container, well away from children and pets. Wash hands thoroughly with soap and water after handling. Take care near ponds and waterways so that the herbicide does not contaminate.

The flashy containers of herbicides on the shop shelves look innocent and easy to use. However, there is much evidence that the active ingredients are doing great harm to the environment and more especially, to us. MCPA and 2,4-D are used widely to control weeds in grass, recently more evidence has prompted calls for them to be banned. Research by the US Environmental Protection Agency has concluded that there is an increased risk of heart and breathing problems in new-born infants. 2,4-D and MCPA are chemically similar to 2,4,5-T which is only a few chemical steps away from tetrachlorodioxin, and the latter can be an impurity in production if it is not carried out properly. A combination of 2,4,5-T and 2,4-D make up Agent Orange which gained notariety during the Vietnam war in the 1960s. The 2,4,5-T itself is not toxic, but concentrations of 20 - 30 parts-per-million of the dioxin impurity can cause birth defects and cancers.

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