The weeds are mainly classified into 2 ways i.e
- According to the lifecycle.
- According to the morphology
According To The Lifecycle
In this classification, weeds are classified according to the length of the lifecyle i.e the time they take to grow, mature and die.
Under this classification, the following groups of weeds existence.
These weeds complete their lifecycle within a year or less e.g. black jack, goat weed, pig weed e.t.c
Bi – Annual weeds
These complete their lifecycle in more than one year but not more than 2 years. In the 1st year may exhibit vegetative growth and flowers and produce a seed in the second year.
They complete their lifecycle in more than 2 years e.g coach grass, tick berry oxalis. Perennial weeds are often very difficult to control because they possess perenating organs e.g. Rhizomes, bulbs, corns e.t.c.
According To Morphology
Broad leaved weeds.
These weeds are usually dicots and they have net venation leaves e.g thorn apple, wandering jew, maxican mangled e.t.c
Narrow leaved weeds
They have parallel veins and they are mainly monocots. They are subdivided into the following
- Grasses e.g spear, star .coach grass.
- Sedges.This group has a triangular stem e.g Cyperus species.
- Woody weeds
They have strong wood stems and they are mainly controlled mechanically by uprooting e.g Sodom apple, lantana camara e.t.c
These are the weeds with thick water filled / succulent stems e.g water hyacinth.
Methods of controlling weeds.
There are a number of methods that are used to control weeds by farmers and those include:-
- Cultural method
- Mechanical / physical method
- Biological method
- Chemical method
- Legislative method
Integrated weed control.
The use of any , of the above methods keep on varying from one place to another depending on the type of weed, the type of plant grown, the economic status of the farmer and environmental conditions.
Mechanical / Physical
This method involves physical destruction of the growing weeds.
The main aim of this method is to encourage the germination of as many as possible so that they are destroyed at tender /young stage.
It is very effective for controlling annual weeds but for the case of perennial weeds , they must be weeded periodically because they continue to grow new shots from their perenating organs.
The methods used in mechanical weed control include the following.
- Hand pulling/ uprooting of individual weeds to expose their roots so that they eventually die.
- Hand hoeing using garden tools like hand hoe, forked hoe.
- Mechanical cultivation /tillage. This involves use of implements e.g ox- drawn implements, tractor, drawn e.g weeders, this buries the weeds before they produce the seeds.
Problems associated with frequent tillage to control the weeds.
- Too much cultivation may lead to destruction of soil structure.
- Encourages water loss from the soil through evaporation.
- It may improve the conditions that encourage the germination of weed seeds previously buried in the soil.
In some cases, the implements and tools used cause damage to the root of crops or destroy the whole crop plant.
Flaming: This is the use of heat or fire to control the weeds among the grown crops. For it to be effective.
- The crop must be more resistant to the flame than the weeds.
- The crop must have grown higher than the weeds.
This is done using cutlasses/ slashes/mowers to destroy the weeds. It is more effective in controlling tall and soft weeds than creeping weeds.
Advantages of Slashing
- It is effective to control tall annual weeds
- It is faster /quicker to use.
- Useful in tilling weeds in harsh or hard condition to cultivate.
- It can be fairly used in all top topography
- Can’t control the weeds that are below the ground
- It is not easily applicable where crops are not well spaced
- Does not encourage improved soil drainage and aeration.
Advantages of Mechanical Method
- It is not toxic to crops and the animals.
- Its use requires little skills.
- It is cheap in the longrun i.e tools are used for long time.
- It is quicker, especially when machines are used.
- If the weeds are cultivated out, the cultivation promotes aeration, drainage and water percolation in the soil.
- It enables burying of the surface vegetation and crop residues for easy decomposition.
Disadvantages of Mechanical Method
- Destroys soil structure.
- The crops may also be damaged in the process of removing the weeds.
- It is also slow especially when hand tools are used.
- It may lead to the oxidation of nutrients in the soil.
- It encourages soil erosion.
- Its efficiency and effectiveness may be affected by wet weather.
This involves use of natural enemies to control weeds. These natural enemies are commonly animals that feed on weeds. Other living organisms can also be used.
Use of rabbits to control the Mc Donald’s eye in Banana plantation.
Control of water hyacinth using beetles
Control of lantana camara by lantana burgs and goats
- Does not pollute the environment.
- Doesnot affect the soil structure.
- It is cheap once the biological agent has been identified.
- The biological agent of weed control may later become a pest especially when the weed is eaten up.
- It requires a lot of research in establishing the biological agent hence making it tiresome.
- It does not destroy the underground parts of the weed.
- It takes a long time for the weed to be eaten up from the garden thus becoming expensive.
- Some weeds donot have appropriate biological agents.
- The agents may destroy predators and pollinator at the same time.
- It cannot eradicate weeds with seeds that remained dormant in the soil.
Chemical Weed Control
This involves the use of chemicals known as herbicides (weed killers)
Classification of Herbicides
Classification according to time of application
Pre- emergency herbicides.
These are applied to control weeds from the surface before crops emerge.
- Crops emerge in a weed free environment.
- The crop grows vigourously and covers the ground before weeds recover.
- If rains water leaches , the herbicides to where the crop seed is, there may be delayed or incomplete germination.
- Post – emergence herbicides
These are applied after the crop seedlings have emerged out of the soil.
Classification according to mode of action
These will kill only the part of the weed with which they come into contact e.g paraquant (Gramaxone) . They are mainly used to kill the shoot system of the weeds.
These are absorbed and translocated into the plant and will kill the weeds by disrupting the physiology of the weed. They are also called translocated herbicides.
Classification according to selectivity of the herbicides
Selectivity of the herbicide refers to the capacity of a herbicide to be able to kill a given species of weeds and not another plant .
These kill certain weeds in preference to another.
Non – selective herbicides
These kill only weeds to which they have been applied.
Examples of Common Herbicides
- 2, 4 – D, It is a selective translocated herbicides that controls broad – leaf weeds. It is applied as a post – emergency herbicides
- MCPA is selective herbicides that control broad – leaf weeds. it is applied as a past – emergence herbicides.
- Paraquant (Gramaxone)
It is a non- selective and non- systemic herbicide that controls broad leaf weeds and young grasses.
- Simazine; controls broad – leaf weeds and grasses.
- 2, 4, 5 – T. It is systemic and kills woody weeds.
- Atrazine, used for broad – leaf weeds and grasses. It can be applied both past – emergence and pre – emergency.
- The method is quick and less tiresome.
- It’s effective as it kills the weeds completely
- It’s effective where mechanical tillage is imposed e.g. In area with less labour.
- It preserves the soil structure.
- There is minimum disturbance of the soil
- It is based in crops where the monohology doesn’t favour mechanical cultivation.
- It is the best method where the topography doesn’t favour use of machines to control the weeds.
more effective in controlling weeds in the rows of crops where tillage would injur the roots of the crops.
Controls weeds in good time before competition sets in e.g pre- emergency herbicides.
Effective in controlling problematic perennial weeds e.g Oxalis Latoforia, digitaria scalarum.
It requires less labour.
Disadvantages of Chemical Weed Control
- Chemicals pollute environment.
- Expensive since it involves purchasing of chemicals.
- Herbicides are poisonous to man either when absorbed through plants or by direct contact.
- Their application needs skills which are lacking in most local farmers.
- Chemicals affect PH of the soil.
- They are often hard to obtain in most areas.
- Some chemicals have long residual effects e.g Atrazine.
- If applied carelessly, they can spoil crops especially non- selective
Precautions to Take When Using Herbicides
Read the labels and instructions carefully before mixing the chemicals.
Wear protective clothes e.g overalls, gloves, masks, rubber boots.
Trouser with turn outs where granules / dust particles can collect avoid them.
While in fields, avoid drinking , eating and smoking
Do not keep chemicals in unlabeled container i.e avoid transferring chemicals to beer bottles, containers e.t.c
Keep all chemicals in locked places out of children’ reach.
Dispose of empty container, safely by burning or burying the m to avoid them from being leaked by animals.
Donot blow blocked with your mouth.
Wash the protective clothes thoroughly with soap.
Wash your hands thoroughly with soap after handling chemicals.
In case a farmer in hales unknowingly drinks or comes in contact with chemicals , seek medical advice.
How to Ensure Effectiveness of Chemicals
Do not spray in windy conditions to avoid chemicals being blown to unintended fields.
Dilute the chemical with the right amount of water.
Use the chemicals for the intended crop.
Allow the correct time to elapse between spraying and harvesting of the crop.
This involves laws and regulations to control introduction of weeds into the country or from one country to another.
Outline the conditions for effective legislative weed control.
Integrated Weed Management
This involves the use of more than one method in a way that is friendly to the environment . It resorts to herbicide use as the last method.