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SELECTIVE WEEDKILLERS

Application   Organic Weedkillers  Non-selective Weedkillers  Health and Safety

Weedkillers   Weeds   Weed Removal   Weed Prevention   Weed Seedlings

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.
The authur, poet and gardening writer Vita Sackville-West who created the gardens at Sissinghurst with her husband Harold Nicolson, was fascinated with selective weedkillers and wrote in her gardening column in the Observer in 1954:

'The weeds in the lawn are all curly. Dandelions, plantains and the daisy leaves have all turned upwards as though they were raising small hands to heaven in one last despairing prayer. ....
Instead of crawling about on all fours in solitary bad temper and incipient lumbago with a trowel or a broken kitchen knife, you may now promenade in a leisurely way, saunter up and down, sprinkling selective death from a watering-can as you converse with the friends who have come to tea.
'

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.
Recently a new selective product (Rescue) has been introduced for use by green-keepers to remove weed grasses on golf courses. It contains pinoxaden which targets a specific plant enzyme to stop cell division causing die-back within about four weeks, eg. in Ryegrass the enzyme is more susceptible, but in Fine Fescues the enzyme is resistant. Some grass species are partially affected, but they usually recover.

Selective herbicides are 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 closely related product Clopyralid which is included in some lawn weedkillers, will cause similar damage if the clippings are composted or used as a mulch. Traces as low as 10 parts per billion can markedly reduce the growth of tomatoes. Some people are calling for a ban of Clopyralid in the UK; it is already banned in some countries and US states for use in lawn treatments. 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. Straw and manure are used in the production of mushroom compost, so using the spent compost in the garden can be another way for contamination to occur.
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, Mileway, Pharaoh, Pro-Banish, Runway, (USA - Cleanwave, Forefront r&p, Milestone vm, 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. The suspension was ended in 2009 and the product returned for use, but in 2010 there are new reports of crop damage so it is still ending up in the manure used in gardens.

Similar problems have been occurring with some peat-free or reduced peat potting compost or grow-bags. The manufacturers use composted green waste as a substitute so this can have some material which has been treated with herbicides, eg. lawn treatments. The green compost is supposed to be tested for the presence of contaminants but this is probably done by sampling, so there could be pockets of material containing herbicide that has not degraded.

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 metabolise 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 Toplawn2,4-D and Mecoprop-Puse instead of a weed-and-feed when fertilizer is not required or as a spot weeder
SBK Brushkiller2,4-D, Mecoprop-P & Dicambamore concentrated selective weedkiller for mature nettles, brambles and tree stumps.
Verdone Extrafluroxypyr, clopyralid and MCPAuse 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; usually by professionals and growers, not the amateur market.




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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