Understanding Weeds And Weed-Killers

Mechanical Destruction: All weeds can be killed in time by digging and hoeing. Frequently these are the best means, though laborious. Deep-rooted weeds such as bindweed, dock, coltsfoot, and ground elder cannot be destroyed readily by chemical means. If their roots are dug out to a depth of 18-24 in. and top growth is prevented for one whole spring and summer by hoeing, no further trouble is likely.

Annual and biennial weeds such as groundsel, chickweed, and common purple thistle can be exterminated quickly by hoeing only, if this is done before they ripen seeds.

Most surface-rooting weeds, e.g. creeping buttercup, nettles, and couch grass, can be killed by burying them 18 in. deep.

Types of Herbicide. Some chemicals kill almost all kinds of plants and are called ‘total herbicides’. Some kill particular types of plant but are more or less harmless to other types and are called ‘selective herbicides’. Some kill plants by being scattered or sprayed over them, some are applied to the soil and some are effective applied in either of these ways. Herbicides which kill only part of the plant they touch are known as ‘contact herbicides’ in contrast to ‘trans-located’ or ‘systemic herbicides’, which enter the plant either by leaves or roots and are carried round in the sap. Herbicides which remain active in the soil for a considerable time, preventing the growth of seedlings or small plants, are known as ‘residual herbicides’. Scientists prefer the term ‘herbicide’, meaning plant killer, to ‘weed-killer’ since no chemical can distinguish between a garden plant and a weed. The user, by selection of an appropriate chemical and choice, of the right method and time for application, ensures that weeds, not garden plants, are killed.

Application of Herbicides. Both dry and liquid chemicals are available. Some of the forms are applied direct to weeds, others are dissolved in water and applied as sprays or from sprinklers. Dry herbicides may be spread by hand but care is needed to ensure even distribution and correct dosage. Alternatively the various mechanical fertilizer distributors may be used to spread herbicides. Some manufacturers prepare lawn fertilizers blended with chemicals to kill lawn weeds, thus doing two jobs in one.

Liquid herbicides, after dilution with water, may be sprayed with any garden spraying equipment but there is often danger of spray drifting where it is not wanted. Herbicides are more safely applied from a watering-can or special applicator fitted with a fine rose or sprinkler bar, from which the liquid can be delivered almost in contact with the weeds or soil. Sprinkle bars of different widths are available to suit particular requirements, e.g. a very narrow (3-in.) bar for applying herbicides between garden plants or in awkward places and a wide (18-in.) bar for covering lawns or large areas of vacant ground.

All equipment should be well washed after use. It is best to keep equipment solely for application of herbicides, thus reducing the risk of the chemicals getting on to garden plants.

Calomel, (Mercurous chloride). Used either alone or in combination with sulphate of iron as a moss killer, particularly on lawns. For this purpose it may be supplied as a liquid for spraying or sprinkling or as a powder for application direct according to manufacturer’s instructions. It is rather slow in action but persistent, and is most effective in early autumn. It is harmful to fish.

Chlorpropham. A selective residual herbicide usually offered for garden use in mixture with other chemicals such as fenuron and propham. It is available as a liquid or wettable powder to be applied to the soil as a spray and by itself is used mainly to prevent growth of weed seedlings in various bulb crops and around blackcurrants, gooseberries and strawberries. It is liable to cause damage to plants on light soils lacking in humus.

2,4-D. A translocated selective weed-killer which will kill a good many weeds in lawns without injury to the grass. It is often mixed with other herbicides, such as mecoprop to widen its band of efficacy. It is most effective if applied when grass and weeds are growing actively. Lawns should not be cut for a few days after application to give the chemical time to act. It is harmful to most garden plants other than grass and also to fish, so care should be taken to prevent drift. It is not persistent in the soil. It is available in various forms to be used according to manufacturer’s instructions.

Dalapon, A translocated selective herbicide which will kill grass, including couch grass, also reeds, sedges and other monocotyledons but is much less toxic to dicotyledons. It is particularly useful for killing grass in orchards and around bush fruits. It should not be used near Apple Cox’s Orange Pippin in winter. Sensitive plants should not be planted on treated ground under six weeks. Dalapon is sold as a powder to be dissolved in water according to manufacturer’s instructions and applied as a spray or sprinkle to the weeds.

Dicamba. A translocated selective herbicide used either alone or in combination with other herbicides such as MCPA for the control of weeds in lawns. It is supplied as a liquid to be diluted with water and applied as a spray or sprinkle to the lawn or weeds. It is harmful to most plants, other than grass, and also to fish.

Dichlobenil. A total residual herbicide which can be used to keep paths, drives etc. clean for many months or in carefully limited doses can also be used to prevent growth of seedlings and small weeds around established shrubs, trees, blackcurrants, gooseberries etc. It is purchased in granular

form for application direct according to manufacturer’s instructions.

Diquat A total weed-killer which is inactivated by contact with the soil. It is not itself poisonous to plants but is changed into a poisonous substance in the plant leaf by photosynthesis. Light is therefore essential for its action which is most rapid in warm, bright weather. It is available as a liquid for dilution and can be used to clear ground for sowing or planting or to kill weeds around growing plants, provided care is taken to apply it to the leaves of the weeds and to keep it off the leaves or green stems of garden plants. It does not matter if it falls on the soil.

Fenuron. A residual herbicide which in large doses will kill most plants but in carefully regulated quantities can be selective. For garden use it is often combined with other herbicides such as chlorpropham or propham. It is available as a liquid or wettable powder to be mixed with water and applied as a spray or sprinkle according to manufacturer’s instructions, its principal garden use being the repression of seedling weeds among perennial plants, shrubs, etc.

Ioxynil. A translocated selective weed-killer primarily used to kill weeds in young seedling lawns. It must be used 7 to 10 days after the germination of the grass seed when each seedling grass plant has approximately two leaves. It is purchased as a powder to be dissolved in water according to manufacturer’s instructions and applied as a spray or sprinkle.

MCPA, A translocated selective herbicide used mainly in gardens for the control of weeds on lawns. For this purpose it may be used alone or in combination with other selective herbicides such as 2,4-D or dicamba. Both it and these mixtures are used in the same way as 2,4-D.

Mecoprop. A translocated selective herbicide chiefly used in gardens for the control of weeds in lawns. It is more effective than either 2,4-D or MCPA in killing clover and is often offered in mixture with 2,4-D to provide a weed-killer with a wide band of effectiveness. Method of use, either alone or in mixture, is the same as for 2,4-D.

Paraquat. A total herbicide allied to diquat and with similar properties. It is a better grass killer than diquat and mixtures of the two are offered to provide a herbicide with the widest possible band of effectiveness. Method of use either alone or in this mixture is the same as for diquat.

Propham. A selective residual herbicide which is used either alone or in combination with other herbicides such as chlorpropham and fenuron to prevent the emergence of weed seedlings among certain plants and vegetable crops. It is available as a liquid or wettable powder for mixing with water and application as a spray or sprinkle according to manufacturer’s instructions. Propham is not so effective on soils containing a high percentage of peat. It is dangerous to fish.

Simazine, A residual herbicide which in heavy doses will inhibit the growth of all plants and can be used to keep paths, drives, etc. clear of weeds for long periods. In smaller, carefully controlled doses it can be used selectively to prevent growth of weeds in rose beds, round ornamental trees and shrubs, in orchards, around bush and cane fruits, etc. It is available as a powder for dissolving in water and application as a spray or sprinkle direct to the soil according to manufacturer’s instructions. It moves about very little in the soil.

Sodium Chlorate, A total herbicide which is both translocated in the plant and also effective in the soil. It is useful for clearing waste land and for keeping paths, drives, etc. clear of weeds. Drawbacks are its readiness to move about in the soil where it may easily be carried to places where it was not intended to be; the difficulty of knowing just how long it will remain effective, since it is easily washed out by rain yet may be retained for a long time in heavy soils or in dry weather; and its inflammability. To counter this last danger sodium chlorate is often mixed with a fire depressant. It is applied both as a powder (or granules) for use dry or for solution in water and application as a spray or sprinkle either to the weeds or to the soil. Clothing wetted with sodium chlorate may become inflammable.

Sodium Monochloroacetate. A contact herbicide that is used to kill seedling weeds among brassicas (cabbage, kale, etc.), also leeks and onions. It is harmful to bees and livestock including poultry. It is available as a powder to be dissolved in water according to manufacturer’s instructions and applied as a spray.

Sulphate of Iron. This is sometimes used as a moss killer on lawns and elsewhere, either by itself at 2-4 oz. per gallon or more usually in combination with other chemicals such as sulphate of ammonia, to make lawn sand, or with chlorpropham. It kills moss more rapidly than chlorpropham but is not so persistent.

2,4,5-T, A translocated selective herbicide used primarily as a nettle and brushwood killer. It is often mixed with 2,4-D to make it effective where both herbaceous and woody plants are to be killed without injury to grass. It is available as a liquid to be diluted with water according to manufacturer’s instructions and applied as a spray or sprinkle to the leaves and stems of plants to be destroyed. It has no effect in the soil. It is harmful to fish.

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