Down Garden Services

WEEDKILLERS

Application   Organic Weedkillers  Non-selective Weedkillers  Selective Weedkillers   Health and Safety

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. Plants under stress such as waterlogging, drought or low temperature are less susceptible to the weedkiller as they are growing poorly. Plants 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 apply water 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). Remember these are individual figures and 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.

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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.

Non-selective Herbicides 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. 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 a related chemical 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 engineered, gluphosinate resistant soybean and maize crops have been developed. 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. It could be two to three weeks before the weeds are completely dead, but they can be removed after about six days 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. 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. 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 reinfest 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. 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. 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. 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. Oxadiazon - works on seed and early postemergant 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. Trade Name* Active Ingredient(s) Use B&Q Complete Glyphosate general and spot weeder Gramoxone Paraquat fast acting ground clearance (agricultural use only) Pathclear Diflufenican and Oxadiazon paths and gravel Roundup Glyphosate general to clear for planting and as a spot weeder Tumbleweed Glyphosate general to clear for planting and as a spot weeder Tumbleweed Paths & Patios Gluphosinate-ammonium and diuron paths and gravel Weedol Diquat fast acting ground clearance * These are some of the products available and not a recommendation for their use. back to top /\

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 separately. Their action is usually hormonal affecting growth rates, mono- and dicotyledonous plants (narrow and broad-leaved respectively) are effected 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. Below are some of the selective agents used to kill broad-leaved weeds:- 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. 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 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 Directive is ongoing and products are being continually assessed, 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. 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.

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 the ready-made sprays now available are useful.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.

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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