Garden Weeds and their Control
The suppression ofin a garden is of the utmost importance. If they are not kept under control a garden will soon become a wilderness.
Most garden weeds are native species which prefer to grow in disturbed ground or in an open situation where there is little competition from other plants, and their capacity for producing and distributing seed is such that, no matter how thoroughly and regularly the ground is hoed, there will always be new crops of weeds to replace those that are removed.
Fortunately, chemical weedkillers have been greatly developed in recent years so that weeds associated with a wide range of vegetable, fruit and flower crops can now be controlled by chemical means.
These chemicals have been produced primarily for the commercial grower, and they are most useful when large areas of a single crop are grown. In the normal garden many different crops are grown in a small area, and the weedkiller suitable for one crop may have disastrous effects on another growing nearby if it is accidentally contaminated. Nevertheless, there are times when chemicals can be used safely in the garden for weed control and this saves a great deal of hand labour. The recommended dosage should not be exceeded, or the plants may be damaged.
The discovery of the selective action of the growth-regulator weedkillers 2,4-D and MCPA in 1942 first demonstrated the great economic potentialities of chemicals for weed control. It was found that these compounds would kill many broad-leaved weeds growing among crops of the grass family, and that they were as effective on cereals as on. A single application will kill such weeds as plantains, creeping buttercup and sorrel; others, such as dandelions and ragwort, need to be treated again four to six weeks later; and a few, such as white clover, are more susceptible to the related compound mecoprop. If pearlwort is the principal weed in the lawn, treatment with lawn sand will also be necessary.
METHODS OF APPLICATION
MCPA, 2,4-D and mecoprop are obtainable under many different trade names, some of which are included in the chart which follows. Apply these carefully at the rate suggested by the manufacturers, either with a sprayer or a watering can fitted with a rose. The advantage of using a watering can is that there is less danger of the spray drifting on to sensitive plants. It does, however, distribute the chemical less evenly than a sprayer, and the rates recommended for application by means of a watering can are usually higher than those for spraying. For spot treatment of, some growth regulators are now available in aerosol containers.
BEST CONDITIONS FOR APPLICATION
Growth regulators are most effective if applied when conditions favour rapid growth. The application of a dressing of nitrogenous fertilizer about a fortnight before treatment will help to give good results. It encourages the weeds to grow faster and the grass to grow over any gaps left when the weeds die.
2,4-D and related compounds can be used safely on well-established lawns but can injure young grassand newly laid turves. Do not apply for six months after sowing.
Most flowers, fruit and vegetables are extremely sensitive to growth-regulator weedkillers. Take great care that they do not come in contact with these plants, either through drift of spray droplets or through failure to clean a watering can or sprayer thoroughly after it has been used for weed control purposes. If possible, use the equipment with which weed-killers are applied for this purpose only. If this is not possible, wash the equipment thoroughly after use, preferably with hot water. Wash several times and leave the container full of water overnight; then empty it and wash once more.
Do not use clippings from the first mowing after treatment as a mulch until the grass has decayed, when the chemicals will have broken down.
One possible way of avoiding problems of drift and contamination is to use one of the weeder bars on the market. A weeder bar has a waxy base impregnated with weedkiller, so that when it is dragged over a lawn the chemical rubs off on the foliage. Coverage of weed foliage is less complete than with liquid application, however, so that results are more variable and, even though no drift is possible, contamination can still occur through waxy material adhering to the soles of shoes, tools and hands.
Mosses need different treatment and are unaffected by the chemicals used for broad-leaved weeds, fawn sand is, how-ever, an effective method of control. It can be made by mixing 3 parts sulphate of ammonia, l part calcined sulphate of iron and 10 parts silver sand and should be applied at 1 to 2 oz. per sq. yd. Lawn sands containing calomel give more lasting control of moss, but badly infested lawns will often benefit as much from improved drainage, aeration and encouragement of the grass with fertilizer as from chemical treatment.
PATHS AND WASTE GROUND RESIDUAL CHEMICALS
Prevent the establishment of weeds on paths by applying a residual,-acting herbicide such as or monuron. These chemicals are insoluble and tend to stay near the surface of the soil where they kill the weeds as they germinate. At medium rates of application, their action persists for 6 to 12 months. But they must penetrate some distance into the soil to exert their full effect, and they act best when the soil is moist or when rain follows shortly after application. At the higher rates of application recommended for the destruction of they may persist for two years, but care must be taken to ensure that no roots of susceptible trees or shrubs are below the treated area. Plums and other stone fruits are particularly likely to be damaged in this way.
Weeds in paths and on waste ground can be killed with sodium chlorate, used alone or mixed with one of the residualto increase the persistence. Apply as a spray or as a fine powder on the leaves of the weeds.
Sodium chlorate will kill many annual and perennial species. If applied in spring it acts rapidly and can persist for about six months. Its principal disadvantages are that the chemical and any organic material soaked in it are highly inflammable when dry, sometimes igniting spontaneously. The fire risk can be reduced by using one of the formulations containing a fire-retarding agent. When there is lateral movement in the soil — for example, on sloping ground in wet weather — plants growing alongside the treated path can suffer damage from the chemical draining down the slope.
Where couch or other grasses are the principal weeds of uncropped ground, they are best controlled with dalapon. This chemical is applied as a solution in water and enters the plant mainly through the leaves. It is most effective if applied in spring, when growth is rapid, or in autumn.
With couch a certain amount of regrowth often follows the application if the ground is left undisturbed; there is less regrowth when the ground is dug over or cultivated deeply two to six weeks after spraying. Except under very dry conditions susceptible crops may be safely sown six weeks after dalapon treatment, as long as the ground has been deeply cultivated.
Many garden plants are susceptible to injury by dalapon, but it has relatively little effect on broad-leaved weeds; such weeds as buttercups, docks and dandelions often take over when couch has been killed.
Amitrole is another possible grass-killer. It has more effect on these other weeds than dalapon and, although as yet it has been less widely used, it promises to be a very useful chemical.
For waste ground and fence lines where docks, thistles, bindweed and other perennials have gained a hold, and where there is no danger of damaging near-by susceptible plants, use MCPA or 2,4-D. Either will kill the weeds down to ground level, but a single application will rarely give a complete kill of the roots. If nettles or brambles predominate, the brush-killer 2,4,5-T is more effective.
Painting a solution of the chemical on the leaves of weeds is laborious, but it may make possible the treatment of small areas, too closely surrounded by sensitive plants to allow other methods of application to be used.
in apple orchards can be controlled with growth regulators, but take care to keep the chemical off the tree foliage. Do not carry out this treatment during the blossoming period. Dalapon can also be used to kill couch under well-established apple and pear trees, but is not safe under plums and cherries.
Control annual weeds among established plantings of bush and cane fruits through the summer by applying low rates of simazine (l to 2 lb. per acre of active material) to clean ground in the spring. Simazine is fully effective only if the soil is moist, and it should, of course, be kept off the fruit plants as much as possible. Take care also not to exceed the dosage recommended. Higher rates applied for several years may lead to a build up of chemical in the soil, and so cause trouble with subsequent crops.
If couch has infested fruit plantings, dalapon may be used as a control, but only during the dormant season: even then there is a risk of damage with some crops.
Established black currant bushes can be safely treated with doses of up to 8 lb. per acre; but gooseberries and raspberries are rather more susceptible, the latter being liable to injury if dalapon is applied at rates of more than 4 lb. per acre.
Few safe herbicidal treatments have yet been developed for use among flowers. But with bulbs, an autumn or winter application of chlorpropham (CIPC) or a mixture of chlorpropham and diuron made before the shoots appear will prevent the emergence of weeds until well into the spring. Chlorpropham is especially effective against chickweed and annual grass.
Roses have shown a high degree of resistance to simazine and this material may be used between rose bushes as-described for bush fruits under Fruit Plantings above.
A number of chemical methods of weed control have been developed for use with vegetable crops. In the average garden, however, any one vegetable is rarely grown in a large enough area for advantage to be taken of these methods, and the herbicides in question are not usually marketed in small enough quantities.
The following crops can be sprayed successfully, and the appropriate chemicals are given in the chart: asparagus,, runner beans, , car-rots, and . Further information may be obtained from the Weed Control Handbook (see Bibliography). The technique used to apply a herbicide is relatively unimportant as long as the correct amount of chemical is distributed evenly over the area being treated. Many types of small sprayer are suitable; those with plastic containers are usually cheaper than those made of metal, and some are designed especially for small-scale garden use. In many cases, a watering can fitted with a fine rose is adequate.
It is most important to follow as closely as possible the instructions supplied with the herbicide. With some chemicals the margin between the dose which will kill the weeds and that which will injure the crop is small, so measure out the required amounts carefully.
The amount of water used to apply the weedkiller is not so important, provided distribution is even. But with those chemicals which are taken up through the foliage, the amount of water used must not be so high that there will be excessive run off.
The chemicals mentioned are all of low toxicity to man and animals, but it is a sensible precaution to keep the concentrated materials well out of the reach of children and away from foodstuffs. They should also, of course, be kept well away from seeds and fertilizers.
Dispose of used containers in such a way that any remaining chemical will not find its way on to growing plants. Sodium chlorate is highly inflammable and needs to be treated with special care.
As many garden plants are highly susceptible to chemical weedkillers, take every precaution to ensure that the weed-killers make contact only with the weedy area being treated. Avoid spraying in windy weather at all costs.