A weed is a plant that is growing in the wrong place.

This definition can apply to all plants and all situations. Even a precious orchid may be regarded as a weed if it grows in the wrong place. Herbicide treatments are applied as a means of solving weed problems efficiently, either as total herbicides or as selective weedkillers. A simple knowledge of the biology of weeds and the behaviour of chemicals in plants will give us a greater understanding of the use of herbicides in solving these.weed problems.

Herbicides may be used in turf to give selective control of broad-leaved weeds and moss, and today we are seeing a new trend towards the management and control of specific grass weeds such as Poa annua.

Perennial broad-leaved plants such as shrubs, trees and hedges have special weed problems with both grass and broad-leaved weeds and therefore require appropriate remedies.

Weed problems

(a) Turf weeds

i) Establishing turf

Infestation of ‘open’ soil from seeds, both in air and buried in the soil. Little or no competition from young grasses therefore, can be a serious threat to new swards.


Sow seed at correct time of year. Thorough preparation of seed-bed. Use clean seed.

Herbicides – seed-bed preparation. Presowing contact applications also post emergence treatments.

ii) Mature turf

Periodic re-infestation from imported weed seeds and from creeping perennials. Broad-leaved weeds will seize the opportunity presented by any open soil (caused by damage, disease, worm casts etc.) and have the ability to establish themselves rapidly.

Please do not dig up or destroy precious wild flowers.


Cultural –

hand-weeding. mowing.

Chemical – the use of herbicides with properties that can be exploited to give selective control.

(b) Weeds in shrubs and other broad-leaved perennial plants

the presence of ‘open’ soil allows

invasion by weeds.

Control – mechanical removal costly and often

very difficult.

chemical using the selective

treatment of herbicides, e.g.

herbicides placed selectively, as

directed applications. The use of

soil acting residual herbicides is

possible for many deep-rooted

established shrubs.

WEED SPECTRUM – Types of weeds found depend on:-

i) Previous land use – arable, grassland, non-crop.

ii) Soil type

physical structure


nutrient status

moisture level (drainage/rainfall/irrigation).

iii) Modifying factors – climate. Management

frequency of soil disturbance (aeration, hoeing)

frequency of mowing

frequency and type of top dressing


worm control.


1. Interception of the spray droplets by foliage

For foliage applied herbicide, the toxicity is likely to

be related to amount of spray retained. In some cases the

greater quantity of spray intercepted by weeds contributes

toward selectivity in the crop, rarely is it wholly

responsible for it. To be effective, a foliar herbicide

must be retained on the leaf surface long enough to enable

penetration to occur.

Differential uptake can give rise to some mechanisms of selectivity:

Directed spray -herbicide intercepted by weeds only.

Large flat leaves intercept a lethal dose of herbicide. Narrow grass leaves intercept a sub-lethal dose. This factor usually enhances biochemical selectivity.

Wax on leaf surface may cause spray to run off one type of leaf and remain on others. Hairiness on leaves can also cause differential spray retention.

The age of the foliage can affect the hairiness/waxiness of the leaves and give rise to selectivity against weeds which emerge after the crop has become established, e.g. the tri-foliage leaves of the clover are less retentive than the cotyledons. If, as is often the case in agricultural grassland, it is desirable to preserve clover, then treatment of established swards is recommended.

Tear off the lower epidermis of a pea leaf or the dicot and mount slide in a drop of water, under a cover slip. Examine at XlOO under a microscope to see this jigsaw puzzle effect.

The leaf surface is covered by cuticle which varies in thickness and composition throughout the life of the plant and from one spp. to another. It is water repellent and forms a protective barrier. Aqueous solutions may enter through the stomata, but these together are lined by a thin layer of cuticle, and entry via this route relies on surface tension being lowered by wetting agents. Note also that in some plants, stomata are confined to the lower surfaces of leaves and are not readily accessible by spray droplets.

There are probably two main routes for leaf penetration, one for flat soluble materials, the lipoidal route (this works under all conditions) and one for water soluble compounds, the aqueous route (which works when humidity is high).

The rate of penetration of cuticle is affected by:-

a) Solubility in fats (cuticle structure fat/protein/


b) Concentration gradient across cuticle.

c) Light.

d) Humidity.

e) pH – charged ions cannot follow lipoidal route.

3. Uptake by plant roots

The precise mechanisms of all chemical uptake by roots are difficult to study and therefore our knowledge is sketchy.

Chemicals may come into contact with the roots by:

a) Interception – the root makes direct contact.

b) Mass flow – the chemical is carried along with

water being imbibed.

c) Diffusion – the chemical travels through water via

a concentration gradient. As chemical is absorbed by the roots, the water in the immediate vicinity becomes depleted of chemical – thus more molecules move in from more concentrated areas to replace it.

Entry of herbicide into the root can in some cases be quite independent of the entry of water. Herbicides may also enter below ground through subterranean stems.

4. Movement of herbicides in the plant

Herbicides travel through both living (symplast) and non-living (apoplast) pathways.

5. Short distance movement

a) Uptake, involves short distance travel.

b) ’Contact’ herbicides must move a short distance in plants.

c) Translocation over longer distances requires a short-term movement to take herbicide to the long distance systems which are internal.

Herbicide movement through living tissue is precarious since the herbicide may injure the mechanism by which it is transported within the plant.

Volatile herbicides may move through intercellular spaces.

6. Long distance movement (translocation)

Particularly important for the control of perennial weeds.

a) Phloem systems – foliage applied translocated herbicides are normally carried from leaf only in phloem. Since the phloem is a living system, too high concentration in the leaf can lead to poor activity due to inactivation of the transport system.

Sugars are normally transported in the phloem system from.the leaves to the .roots and storage organs. Thus, this system carries the herbicide to the rhizome and hence zeros in on the target.

Note: Very young leaves do not export sugars, neither do senescent leaves, therefore the transport system needs the right growth stage and environmental conditions conducive to growth in order to be able to transport a translocated herbicide.

b) The xylem system – concerned in upward movement of water and minerals from roots to aerial parts of the plant. This is a non-living transport route. The rate of uptake and transport through this system depends on:-

i) Water loss through leaves – temperature, wind, 1ight, humidity.

ii) Water stress in root zone – overrides the transpiration loss factor.

Uptake .through the roots and transport through xylem system provides ideal distribution for herbicides whose mode of action affects photosynthesis.

Transfer of material from one transport system to another (xylem <— phloem) is not uncommon.


Many factors operate to give different modes of action on a whole plant’ level, e.g. variations in mechanisms of uptake. Once the chemical has reached the final target cell, it may affect different cell functions. The main ones are covered below:-

1. RESPIRATION – breakdown of sugars for energy.

Much of this process occurs in the mitochondrion and involves a chain of enzyme reactions which are in ‘fine’ balance under normal conditions. Such enzyme pathways are vulnerable to inhibition at many different sites:-

Many products are known to inhibit respiration.

The inhibition of respiration is an efficient way of killing since it shuts down the cellular powerhouse.

Note that herbicides may have an effect on more than one cell function.

2. PHOTOSYNTHESIS – trapping energy from the sun.

This process again involves chains of enzyme reactions which are vulnerable to attack. Production of chlorophyll pigments is also an area of vulnerability.


Very important to normal cell function, particularly in growth of cells and in maintenance, e.g. continued replacement of ‘worn’ parts, e.g. enzymes.

4. Other major cell functions which can be affected include: interference with plant hormone balance. Nucleic acid synthesis/transcription. Synthesis of lipids (fats).

Selectivity of certain herbicides

At a cellular level there are three ways of achieving selectivity.

i) De-toxication – a mechanism which breaks down the herbicide to non-toxic products found in resistant species.

ii) Toxication – a mechanism which breaks down a non-toxic substance to a herbicidal one in susceptible plants. An example of this occurs with MCPB and 2,4-DB. These phenoxybutric acids are converted to MCPB and 2,4-DB in susceptible plants.

iii) Biochemical specialisation – where different pathways exist between one spp. and another, attacking enzymes in these ‘unique’ processes gives selectivity.

Note: The resistance to herbicides changes with the growth stage of the plant, therefore timing of application can give a mechanism of selectivity.

Timing of herbicide application

The mode of action of a herbicide may dictate when it should be applied for maximum effect.

Pre-emergence herbicides (soil applied) have a greater degree of flexibility in timing but their persistence in the soil should be considered.

For post-emergence herbicides, a ‘window’ exists between seedling emergence and flowering, which is the optimum timing for application.


Formulation: Chemicals for weed control may be available various forms, the most common being as follows:-

1. Liquids

concentrated material to be diluted with water, prior to use. Note that CDA sprayers – controlled droplet application – tend to use an ‘oil’ carrier in place of water. It is a thin oil not unlike a paraffin.

Wettable powders – powders which although not always

soluble in water, have been prepared for suspension/dilution with water prior to use.

Powder and dusts – materials to be applied in the dry

state, without dilution.

4. Granules

solids of large particle size normally fertilisers, but sometimes with herbicides added. Application is without dilution to soil or turf. Also as insecticides, e.g. TEMIK.10G.

Dilution rates

The above may require dilution and also may have to be applied at a set rate per hectare. Basically, two cases occur:-

i) the material is used neat but applied at a fixed rate per hectare, e.g. solid fertiliser/herbicide applied at a set rate per square metre.

ii) the material is diluted prior to application but applied at a fixed rate of concentrate per hectare. Example: a selective weedkiller such as 50% 2,4-D should be applied at a given rate of concentrate per Ha. The concentrate is diluted prior to spraying. The addition of water is purely a means of applying a small quantity of concentrate over a large area. In fact, the amount of water added may be varied to suit the type of equipment used for application, and the type of equipment will be chosen to be suitable for the scale of the operation. Large water volumes and large droplets tend to reduce drift.


The type of equipment required depends:

a) on the form of the chemical, and

b) on the area to be treated.

Enlarging on this:-

Liquids and wettable powders are applied in spray form; small areas being treated with a watering can or knapsack sprayer; larger areas with trolley or tractor mounted spraying equipment.

Powders/granules are evenly spread, either by hand or mechanical spreaders.


Best kill before flowering, because:

i) most products interfere with plant growth.

ii) many need growth for uptake/translocation.

iii) prevents seed formation – slows spread of weeds.

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