Classification of weeds

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.

Annual weeds

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.

Perennial  weeds

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

Herbaceous  weeds

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.

Slashing /mowing

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.

Biological Control

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

Contact herbicides

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.

Systemic  herbicides

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 .

Selective herbicides

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.

Legislative Control

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.