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

Weeds   Weed Removal   Weed Prevention   Weed Seedlings

Weedkillers or Herbicides, come under the general category of Pesticides when they are defined in regulatory terms. In the European Union the Chemicals Regulation Directorate has been reviewing all of the Pesticides in use to remove the ones that could be harmful or where there is duplication. This includes Insecticides, Fungicides and Herbicides, and there has been quite a large cull during the process. Some of the chemicals listed on these pages are no longer available, but are included for information and some may be available in other jurisdictions.


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 the plant should be actively growing with plenty of fresh leaves which can absorb the herbicide to be passed down to the roots and leave them intact until the plant shows signs of dying. 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 weedkillers which are absorbed by the plant as they are growing poorly. They have a vascular system which carries water upwards (xylem) and moving the sugars they produce in the leaves to the rest of the plant through other vessels (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 will germinate in the gaps left by dead plants from the seeds present giving the impression that the selective weedkiller used has not worked.

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.

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 or Translocated and Residual. There are overlaps between some of these and all types can be said to have some systemic action.

Information on Non-selective Weedkillers

Information on Selective Weedkillers

Organic Herbicides

For the organic gardener there are some herbicides which are derived from plant sources.

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 2mm psyllid or plant louse (Aphalara itadori)2mm psyllid or plant louse (Aphalara itadori) which attacks Japanese Knotweed to see if it could be used as a control.

The most recent attempt to produce an "non chemical" method of killing weeds is a hot oil foam. This uses olive, rapeseed or coconut oil in a hot emulsion with water sprayed onto the plants. The heat withers the top-growth so would probably kill seedlings or annual weeds. Perennial weeds would regrow so multiple applications would be needed, the oil will degrade so there should be no residue after a whilw, but there could be staining on paving.

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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Weeds   Weed Removal   Weed Prevention   Weed Seedlings

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