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Weeds Weed Removal Weed Prevention Weed Seedlings
Application
It is important to time the application of weedkillers, or herbicides as they are also called, correctly. The best time is at the seedling stage for all types of weeds and since most spread by seed then to apply before flowering is essential. Biennial weeds are more easily eradicated in their first season as seedlings or small rosettes. For perennials an application of a systemic weedkiller in the autumn when they are passing sugars to the underground parts for winter storage, can be very effective for weeds that are difficult to control. Also for best results allow the weed to develop plenty of leaves to increase the surface area which can absorb the herbicide to be passed down to the roots and leave them intact until the plant shows signs of dying.
Plants under stress such as waterlogging, drought or low temperature are less susceptible to the weedkiller as they are growing poorly. They have a vascular system taking water up through vessels (xylem) and moving the sugars they produce in the leaves to the rest of the plant through others (phloem), so it is best to apply during periods of good growth when the sap is moving around at a higher rate and the chemical will be taken with it.
One application may not be enough to remove the weed completely if it is well established with a good perenniating system and there is insufficient topgrowth to absorb enough chemical. Also if it has been present for some time there will be a reservoir of seeds and these will re-establish it, eg. Clover, Daisy and Creeping Buttercup in a lawn.Most weed killers for domestic use are now available in ready-made sprays which are useful for spot treatment of weeds. A watering-can with a sprinkle-bar fitted is the easiest way to apply weedkiller evenly over a larger area such as a yard or path. The main drawback is that the nozzles can become blocked, this can be reduced by flushing with clean water before and between batches over an untreated path - take the plugs out of the ends of the bars. Sometimes the mixture froths up while diluting due to the inclusion of surfactants which help mixing and improve the wetting of the weeds. Running the water down the inside of the can or filling down the spout, should reduce this; another method is to use an open-ended hose to fill from below the surface. Also make sure powdered concentrate is completely dissolved or suspended, as small lumps will clog the bars; disperse the powder in a small quantity of water first, crushing any lumps, then top up to the required volume.
It is important to apply at the correct rate; too little may be ineffective and could lead to the development of resistance; too much is a waste of money and for selective weedkillers may damage non-target plants. You can calibrate the sprayer or sprinkler using plain water.
If you have to cover a given area with a litre, mark it out where it is easy to see when wet, eg. concrete or flags. Then spray or sprinkle the patch adjusting your pace to ensure the volume is evenly dispersed.
More likely the directions give a quantity per square metre (m²), mark out an area in m² and starting with a full can of water, apply at your pace, note the quantity needed to fill the watering-can up again and divide by the number of m² marked out. Repeat a few times and take an average. This is the volume of liquid applied per m², so add the amount of concentrate needed per m², multiplying up for a larger container.
eg. An area 3m x 5m = 15m²
The can requires 2.1 litres to top it up
So 2.1 divided by 15 = 0.14 or 140 mls per m²
If the application rate is 6g per m²,
add 6g to every 140mls = approx. 43g per litre.
This also shows that one litre covers approximately 7 m² (1 divided by 0.14).Please note these are individual figures which vary with walking pace.
Organic Herbicides
For the organic gardener there are some herbicides which are derived from plant sources.
Vinegar is a biodegradable natural agent which will kill plant material; normal vinegar for putting on your chips (fries) contains about 5% acetic acid and is produced from grapes, apples or grain by fermentation in anerobic conditions. This weaker product works on some young weeds, but higher concentrations produced by distillation (15%) and freeze evaporaton (30%), are needed for a more effective herbicide. The vinegar degrades after a few days so any increase in the pH is only temporary. Best results are obtained if it is used hot and repeated for three days. Concrete and stone can be hosed down after treating to reveal their true colour.
Corn Gluten Meal which is a byproduct of the wet milling of maize. It works by preventing the germinating seed forming roots with no effect on existing plants or seedlings which have developed roots, so it has limited uses. The main one being as a weed and feed treatment for lawns since it also releases about 10% Nitrogen as it breaks down. Of course it is also limited in availability as most maize production is in North America, and it is used in animal feeds as well.
There has been some success in using a biological control on weeds. This is usually a pest which weakens the plant and it dies or is overcome by the non-target plants. Some evaluation is being carried out at present with a weevil which attacks Japanese Knotweed to see if it could be used as a control.
back to top /\Weedkiller types
There are plenty of conventional weedkillers or herbicides available, but the best results are obtained if you use the right one for the job. They can be categorised into groups for the task in hand or the type of plant you want to kill.
The two main categories are Non-selective which damage or kill all plants they touch, and Selective which can be targeted at a specific group of plants. The Non-selective herbicides are further categorised into Contact, Systemic and Residual, but there are overlaps between some of these.
Non-selective Herbicides
These herbicides kill both broad and narrow-leaved plants.
Contact
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.
Gluphosinate-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) 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) - (also spelt "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.
Unfortunately AMS was withdrawn from the UK amateur gardening market in November 2007 and use of stored product should cease 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 - acts on an enzyme (EPSP synthase) which produces an essential amino acid, so reducing the production of protein in the plant and inhibiting plant growth. This is why it takes up to 10 days to work. 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 geneticaly 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.
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.
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 disappear.
Amitrole - prevents carotinoid synthesis, which results in the breakdown of chlorophyll by light, tissues lack green colouration 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.
Residual
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 will cease in March 2009, followed by a further period of 12 months for storage and use. The final date for use is 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 propertiessee below.
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'
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, so from a date yet to be decided, it will be banned from use. When the date is chosen there will be six months to withdraw sales and 12 months to use up any products already in circulation.
Trade Name* Type Active Ingredient(s) Use B&Q Complete Systemic Glyphosate general and spot weeder Gramoxone Contact Paraquat fast acting ground clearance (agricultural use only), perennial weeds not completely killed Pathclear Systemic and Residual Diflufenican and Oxadiazon paths and gravel Roundup Systemic Glyphosate general to clear for planting and as a spot weeder Tumbleweed Systemic Glyphosate general to clear for planting and as a spot weeder Tumbleweed Paths & Patios Systemic and Residual Gluphosinate-ammonium and diuron paths and gravel Weedol Contact Diquat fast acting ground clearance, perennial weeds not completely killed Weedol MAX Contact Pelargonic acid fast acting, perennial weeds not completely killed * These are some of the products available and not a recommendation for their use.
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Selective Herbicides
- As the name suggests these agents can be applied to remove target plants without damaging others which are desired. For the gardener this means a weedkiller which will kill broad-leaved weeds and will not affect grasses. They are usually applied to the lawn as part of a weed-and-feed application, but can be applied on their own. Their action is also systemic since they have to be absorbed into the tissues with usually a hormonal effect on growth rates.
Mono- and dicotyledonous plants (narrow and broad-leaved respectively) are affected differently. This works due to the different growth habits of the plants, monocotyledon leaves grow continuously from the base, eg. grass, and are less susceptible to the chemicals. They are usually used in combinations of two or three to overcome resistant weeds. Do not apply to lawns less than six months old, either seeded or turfed.
Stronger applications are used in brushkiller herbicides and in some spot-weedkiller sprays.
There are selective products known as Graminicides which will kill grasses and cereals growing among broad-leaved crops such as peas or Oilseed Rape, but they are not available to the gardener. They work by inhibiting an enzyme (acetyl-coenzyme A carboxylase) which is involved in the synthesis of fatty acids by the plant cells and greatly reduce its growth, eg. Fluazifop. There are restrictions on the use of such products and they can only be applied once in a season.The herbicide is usually broken down by soil bacteria, but can persist in the tissues of treated plants so any dead material or grass cuttings should not be added to the compost heap unless it is to be left for two or more years. It could be stacked in a separate pile. Sometimes grass clippings are used as a mulch to suppress weeds or retain moisture in beds, but this should also be avoided.
Recently there has been a problem with the selective agent Aminopyralid which has been damaging mainly vegetable crops in gardens where farmyard manure has been applied. Shoots can become elongated and distorted as can the leaves, which can look cupped or fern-like. The effects have been found with vegetables as the manure was incorporated while planting, with potatoes, tomatoes and beans being most susceptible. Some damage has been reported with roses and raspberries where it was applied as a mulch.
The Aminopyralid is an agricultural herbicide and was introduced in 2006 to clear broad-leaved weeds from pastures and fodder crops. It binds to the lignin in plant cell walls and this slows its breakdown - even on passing through the gut of grazing animals. It requires the micro-organisms in soil to break it down. The fodder and bedding straw from fields which had been treated ended up in manure and this was distributed to gardeners for use on their vegetable plots in 2008, although notices with the product warned about the residual effects of the herbicide. As the problem was recognised after the application in spring 2008, fodder from the summer contains the chemical. It is said that ground where the manure was used may take up to three years to be clear of the herbicide when it will have been broken down by soil bacteria and this will be helped by turning over the soil to encourage aerobic decomposition - most should have degraded after about six months with cultivation. Even the manure should be clear when it is well-rotted - as it should have been in the first place.
The levels found in the manure are well below the safe amounts for the product, so any that may be in crops would be less and should be safe to eat if they survived to maturity.
Products containing Aminopyralid have been suspended from sale - including:- Banish, Forefront, Halcyon, Pharaoh, Pro-Banish, Runway, (USA - Cleanwave, Milestone vm, Forefront r&p, and Milestone). As it is a very effective product there is a demand for it to be reintroduced, but the approving authorities will not permit this until adequate controls can be effected.Below are some of the selective agents used to kill broad-leaved weeds - any grass cuttings from treated areas should be left to rot down separately for a couple of years, with plenty of aerating, and not added to the compost heap.:-
2,4-D (2,4-Dichlorophenoxyacetic acid) - a plant-growth regulator that stimulates nucleic acid and protein synthesis and affects enzyme activity, respiration, and cell division. It is taken up by plant leaves, stems, and roots and moves throughout the plant. It accumulates in growing tips.
2,4-D is due to be banned if the proposals of the EU Committee on the Environment, Public Health and Food Safety are approved early in 2009.
Dichlorprop (2,4-dichlorophenoxypropionic acid or 2,4-DP) - similar to 2,4-D. (No longer has approval for use under EU Directive 91/414/EEC)
Dicamba - taken up by leaves and roots, and moves throughout the plant. In some plants, it may accumulate in the tips of leaves. It acts as a growth regulator. Some plants can metabolize or break it down.
Dichlorophen - kills moss and is quick-acting.
Mecoprop-P (MCPP) - absorbed by plant leaves and translocated to the roots. It affects enzyme activity and plant growth, and acts relatively slowly requiring three to four weeks for control. Active against small-leaved creeping weeds like yarrow
MCPA - similar to MCPP
Metosulam - a triazolopyrimidine herbicide developed for pre-emergence control of broadleaf weeds in cereal and maize crops.
Trade Name* Active Ingredient(s) Use Bio Toplawn 2,4-D and Mecoprop-P use instead of a weed-and-feed when fertilizer is not required or as a spot weeder SBK Brushkiller 2,4-D, Mecoprop-P & Dicamba more concentrated selective weedkiller for mature nettles, brambles and tree stumps. Verdone Extra fluroxypyr, clopyralid and MCPA use instead of a weed-and-feed when fertilizer is not required or as a spot weeder * These are some of the products available and not a recommendation for their use.
After 25th July 2003 the formulation of many of the branded products was changed to comply with Directive 91/414/EEC Concerning The Marketing And Use Of Plant Protection Products. The trade names of some products remain in use or are slightly altered, reflecting the change of active ingredients to comply with the regulations.
The effects of the Directive are ongoing with the Pesticides Review Standing Committee continually assessing products, so every now and then more are withdrawn from the market. Sometimes this is due to manufacturers not being prepared to carry out the expensive process of testing and not because a product is considered too dangerous for use.
Directive 91/414/EEC is to be replaced by the Plant Protection Products regulation which is to be voted on by the European Parliament in either December, 2008 or January, 2009 with amendments which have been passed by the Committee on the Environment, Public Health and Food Safety on November 5th, 2008. These include withdrawals and restrictions on use for many pesticides already in use.
For the list of products reviewed by the Pesticides Safety Directorate which indicates the substances which may be removed, released in December 2008, follow this link. It would appear that about 14 to 23% of the substances could be affected, but some have a derogation which may apply, so could have partial use.
back to top /\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.
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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Weeds Weed Removal Weed Prevention Weed Seedlings
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