Herbicides

Herbicides that are applied to the seedbed or growing crop must have a selective action, i.e. kill the weed but leave the crop undamaged. This selective action may succeed for one of several reasons. Chemicals often affect different plant families in different ways. The broad-leaved turf weed, daisy (Bellis perennis, a member of the Asteraceae) is controlled by 2,4-D, leaving the turf grasses (Graminae) unaffected.

Sometimes plant species are affected by different concentrations of the chemical to a degree that can be exploited. The correct concentration of selective chemicals may be vital if the crop is to remain unharmed.

It can thus be seen that a concentration of 25 ppm of propyzamide applied to lettuce would leave the crop unaffected, but control all the weeds except groundsel.

The following relative values (parts per million, ppm) for the amount of propyzamide herbicide required to kill different plant species illustrate this point:

Crops			Weeds
carrot	0.8		knotgrass	0.08
cabbage	1.0		black nightshade	0.2
lettuce	78.0		fat hen	0.6
			pennycress	78.0
			groundsel

A third form of selectivity operates by correct timing of herbicide application. A seedbed with crop seeds deep enough below and weed seeds germinating at the surface may receive a contact chemical, such as paraquat, which permits germination of the crop without weed competition. A similar effect is achieved when a residual herbicide, such as propachlor, is sprayed onto the soil surface to await weed seed germination. The situations for weed control are summarized in Figure 16.9.

Herbicides may conveniently be divided into two main groups: the foliage-acting, and the soilacting (residual) chemicals.

Foliage-acting herbicides
These enter the leaf through fine pores in the cuticle or the stomata. The herbicide may move through the vascular system (translocated chemicals) to all parts of the plant before killing plant cells, or it may kill on contact with the leaf. Four active ingredients are described, each belonging to a different chemical group, and each having a different effect on weeds:

• Glufosinate-ammonium is a non-selective, non-residual herbicide available to both amateur and professional growers. It is commonly used to kill top growth of a wide spectrum of weeds in stale seedbeds, after harvest, in bush and tree fruit, or in waste land. It is translocated from leaves to the roots. It is quickly absorbed in damp soils, thus allowing planting soon after its application. It should not be used on foliage of vegetables, annual bedding plants or on turf. Similarly, after spraying this chemical, it is crucial that soles of boots contaminated with the chemical do not inadvertently walk across grass.
• 2,4-D is available to both amateur and professional growers. It is an auxin and causes uncontrolled abnormal growth on leaves, stems and roots of broad-leaved weeds, which eventually die. It is a useful selective herbicide on turf because the protected meristems of grasses can survive unaffected. It must be kept well away from nearby border plants and from some crops, such as tomatoes, which are extremely sensitive to minute quantities of the chemical. In formulations for the private garden, 2,4-D is mixed with other ingredients, such as dichlorprop, to give a broader spectrum of weed control.
• Amitrole is available to professional growers. It is used in similar situations to glufosinate-ammonium, but is more residual, surviving in the soil for several weeks. It stops photosynthesis, scorching both grass and broad-leaved weeds (non-selective). It is especially useful on uncropped land and, when applied in autumn, it is translocated to underground rhizomes of couch that are then killed. It should not be sprayed onto the foliage of growing plants.
• Glyphosate is available to professional and amateur growers. It enters the foliage of actively growing annual and perennial weeds (unselective) and is translocated to underground organs, subsequently killing them. It is commonly used several weeks before drilling or planting of crops, around perennial plants such as apples or in established nursery stock trees. Glyphosate is inactivated in soils (particularly peats), thus preventing damage to newly sown crops. It may cause damage if spray-drift to adjoining plants or fields occurs.

Soil-acting herbicides
These are either sprayed onto the soil surface or soil incorporated (see Figure 16.9). They must be persistent (residual) for several weeks or months to kill the seedling before or after it emerges. Root hairs are the main point of entry. Increased rates may be necessary for peat soils (since they inactivate some herbicides). The chemical may be applied as a spray or granule before the crop is sown (pre-sowing stage), before the crop emerges (pre-emergence) or, with more selective chemicals, after the crop emerges (post-emergence). Three active ingredients are described, each belonging to a different chemical group, and each having a different effect on weeds:

• Sodium chlorate is a residual inorganic ingredient available to amateur and professional growers for total weed control in non-crop situations. It enters leaves and roots and has a rapid action. No plantings can be made until six months after the chemical has been applied.
• Chlorpropham, available only to professional growers, is a relatively insoluble compound. It is applied as a pre-emergent spray to control many germinating weeds species, such as chickweed, in crops such as bulbs, onions, carrots and lettuce. It usually persists for less than three months in the soil. In light, porous soils with low organic matter, its rapid penetration to underlying seeds make it an unsuitable chemical. Earthworm numbers may be reduced by its presence.
• Propachlor, available only to professional growers, is a relatively insoluble compound; it is applied as a pre-sowing or pre-emergent spray to control a wide variety of annual weeds in brassicas, strawberries, onions and leeks. For weeds in established herbaceous borders (such as rose), the granular formulation gives a residual protection against most germinating broad-leaved and grass weeds.

Mixtures
The horticulturist must deal with a wide range of annual and perennial weeds. The somewhat specialized action of some of the herbicide active ingredients previously described may be inadequate for the control of a broad weed spectrum. For example, in the case of chlorpropham, the addition of diuron enables an improved control of charlock and groundsel, while a different formulation containing chlorpropham plus linuron is designed to have greater contact action and thus control both established and germinating weeds in bulb crops. Careful selection of the most suitable mixture of active ingredients is therefore necessary for a particular crop/weed situation.