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Weed is any plant growing where it is not wanted. They are plants that undesirably grows profusely where it is not wanted such as gardens, cultivated land, lawns and parks. This is usually of great significance to farmers because they compete for limited nutrient available in the soil. Hence, reducing the productivity of the cultivated crops.


Weeds have certain effects in agriculture, which are mostly in the form of different harmful results but  there are also some beneficial effects.

Direct Losses by weeds

1. Weeds cause reduction in crop yield through competition for light, nutrient, water and space. They can also reduce the yield of crop through the release of toxic substances or exudates which inhibit crop growth.

2. Weeds can reduce the quality of harvested agricultural products.

3. Weeds interfere with harvest operations and increase the cost of harvesting in both small holder and large scale farms.

4. Weeds may poison animals. Some weeds can adversely affect livestock because of the high nitrate content of the shoots.

5. The cost of controlling weeds is high.

6. The presence of weeds can impede water flow in irrigation canals.

7. The presence of weeds in lakes and reservoir can increase loss of water by transpiration.

Indirect Losses caused by weeds

1. Weeds serve as alternate hosts to many plant diseases and animal pestse.g insects, rodents, birds etc that attack crops.

2. The presence of weeds imposes a limit on farm size.

3. The presence of weeds can also reduce the economic value of lakes by preventing or limiting fishing activities.

4. Weeds such as Imperatacylindrica become fire hazards in the dry season throughout the savanna vegetation zone.

Beneficial Effects of Weeds

1. Weeds provide a vegetative cover that protects the soil surface against erosive action of rain and wind.

2. Weeds play an important part in nutrient recycling. Roots of weeds tap nutrients from the lower soil depths and return these to the soil surface as litter when the weeds shed their leaves or when the entire plant plants dies and decays.

3. Weeds add organic matter to the soil both from the roots and from the above ground parts.

4. Many plants that are designated weeds are used as potherbs(edible weeds) e.gTalinumtriangulare.

5. Weeds provide food and cover for animal. Wildlife generally depends on weeds for survival as food and shelter.

6. Weeds serve as an important source of genetic materials for crop improvement such as breeding for resistance to pests and diseases which are made possible by genetic materials provided by wild species of the crop plants.

7. Weeds serve as hosts beneficial insects, and at the same time provide nectar for bees.

8. Many weeds help to beautify the landscape. e.g a good ground cover of Cynodondactylon beautifies the home.


(a) Wind
Many seeds are well adapted to wind travel. Cottony coverings and parachute-like structures allow seeds to float with the wind. Examples of wind-dispersed seeds include common milkweed (Asclepiassyriaca), common dandelion, Canada thistle, and perennial sowthistle (Sonchusarvensis). 

Factors that influence wind dispersal:

  • seed weight
  • seed shape
  • structures (wings or pappus)
  • height of release
  • wind speed and turbulence

(b) Water
Aquatic weeds disperse largely through water. They may drift either as whole plants, plant fragments or as seeds with the water currents. Terrestrial weed seeds also disperse through irrigation and drainage water. Weed seed often moves with surface water runoff into irrigation water and ponds, where it is carried to other fields. Weeds growing in ditch banks along irrigation canals and ponds are the major source of weed seed contamination of irrigation water.

Weed seed often remains viable in water for several years, creating a “floating seed bank” and allowing weeds to disperse over large areas in moving water.

(c) Animals
Several weed species produce seeds with barbs, hooks, spines, and rasps that cling to the fur of animals or to clothing and then can be dispersed to long distances. Farm animals carry weed seeds and fruits on their skin, hair and hooves. This is aided by special appendages such as Hooks (Xanthiumstrumarium), Stiff hairs (Cenchrus spp), Sharp spines (Tribulusterrestris) and Scarious bracts (Achyranthusaspera). Even ants carry a huge number of weed seeds. Donkeys eat Prosophisjuliferapods.

Weed seeds are often ingested and passed through the digestive tracts of animals. Animal droppings provide an ideal nutrient and moisture environment for weed germination. Only a small percentage of the seed remains viable after exposure to an animal’s digestive enzymes. Some ingested weed seeds are passed in viable form with animal excreta, which is dropped wherever the animal moves.

(d) Dispersal by Man
Man disperses numerous weed seeds and fruits with raw agricultural produce. Weeds mature at the same time and height along with crop. Due to their similar size and shape as that of crop seed man unknowingly harvest the weeds also, and aids in dispersal of weed seeds. Such weeds are called “Satellite weeds” e.g. Avenafatua, Phalarisminor.

(e) Dispersal by machinery
Weed seeds often are dispersed by tillage and harvesting equipments. Seeds move from field to field on the soil that sticks to tractor tires, and vegetative structures often travel on tillage and cultivation equipment and latter dropping them in other fields to start new infestation. Disc-type cultivation equipment is less likely to drag vegetative plant parts than are shovels or sweeps.


  1. Speargrass (Imperatacylindrica)
Imperata cylindrica
  • Tridax (Tridaxprocumbens)
Tridax procumbens
  • Bahama grass (Cynodondactylon)
Cynodon dactylon
  • Wild sunflower (Aspiliaafricana)
Aspilia africana
  • Elephant grass (Pennisetumpurpureum)
Pennisetum purpureum
  • Carpet grass Axonopuscompressus) Axonopus compressus


 Common NameBotanical NameDescriptionAgent of DispersalReason for the Dispersal by that AgentMethod of Control
1TridaxTridaxprocumbensIt is an annual weed. It has serated leaves with long flower stalk. The entire body is hairyWindSeeds are very light. Seeds have hairy structures(Papus) which aid their propagationPhysical control by cutlass, mechanically by mower or chemically by spraying herbicides.
2Blue featherCommelinanudifloraIt is a creeping plant with adventitious roots. The leaves are simple and at the top of the stem are flowers which are bluish or purple in colour.AnimalsThe seeds are sticky with hooks which adheres to coats of animalsPhysical control by cutlass, mechanically by mower or chemically by spraying herbicides.
3Goat WeedAgeratumconizoidesIt is a simple plant with slender stem with simple leaves. It has numerous small flowers and fruits on the stem.WindThe fruits are light and powdery which are easily blown away by windPhysical control by cutlass, mechanically by mower or chemically by spraying herbicides.
4Water leaf plantTaniliumtriangulareA simple herbaceous plant. It has succulent stem and leaves. Flowers and seeds are borne on top of the plant.By explosive mechanismThe fruits/seed split open by itself when ripe.Physical control by cutlass, mechanically by mower or chemically by spraying herbicides.
5Pig weedBoehaviadiffusaIt is a simple plant with many branches. It has slender stem with broad leaves, Its seeds/fruits are sticky.Man or AnmialSticky seed/fruit and they easily detach from the plant to adhere to the coat/skin or cloth of animal and man.Physical control by cutlass, mechanically by mower or chemically by spraying herbicides.
6Spear GrassImperatacylindricalIt is a rhizome with underground stem which runs horizontally. Food is stored in the stem and roots are adventitious.It is propagated by stem cutting by man’s activity. Chemically by spraying herbicides. It is resistant to drought and burning because of its watery rhizome and difficult to uproot
7Stubborn WeedSidaacutaIt is a perennial shrub. The leaves are narrow, pointed and serrated. Along with the stem are buds, flowers and fruitsExplosive Mechanismnd man and AnimalsThe fruit is a capsule which splits on its own, thus scattering the seeds in the farm. The sharp edges of the seeds can adhere to human and animal skin.Physical control by cutlass, mechanically by mower or chemically by spraying herbicides.
8SpigeliaSpigeliaanthelmiaIt is an herbaceous annual weed that grows near streams. The main stem is hollow and bears only one or two branches at the only node.Fruits are dispersed mechanically.When fruits are matures, it splits open mechanicallyPhysical control by cutlass, mechanically by mower or chemically by spraying herbicides.
9Guinea grassPanicum maximumIt is a dominant pasture grass of the rainforest. It is a bunchy erect or tufted grass. It can grow up to 2m   
10Elephant grassPennisetumpurpureumIt is also common in the rainforest zone. It is a perennial grass with cane like stem and dull purplish leaf blade   



Cultural weed control refers to any technique that involves maintaining field conditions such that weeds are less likely to become established and/or increase in number. Examples of cultural weed control would be crop rotation, avoiding overgrazing of pastures or rangeland, using well-adapted competitive forage species, and maintaining good soil fertility.


Mechanical weed control refers to any technique that involves the use of farm equipment to control weeds. The two mechanical control techniques most often used are hoeing, tillage (Using hand to pull off weed) and mowing.


Biological weed control refers to any technique that involves the use of natural enemies of weed plants to control the germination of weed seeds or the spread of established plants. This is a rapidly expanding area of weed control with many examples. Examples of biological weed control include introduction of insects and herbivorous animals.


Chemical weed control refers to any technique that involves the application of a chemical (herbicide) to weeds or soil to control the germination or growth of the weed species. Common examples of chemicals used to control weed include paraquat,2,4-dichloroacetic acid(2,4-D),2,4,5 trichloroacetic acid (2,4,5-T)


  1. a. Describe briefly four(4) Method of weed dispersal.

b. Describe three (3) cultural and three(3) mechanical methods of weed control

WASSCE; June 2002)                               e

  • State four (4) reasons why weed are difficult to control.   (WASSE; 2008).
  • State four ways in which weeds are beneficial to farmer. (WASSE; June 2012).
  • a)Define the term Weed. b)Describe four (4) methods of weed control c) Give Six (6) reasons for controlling weeds on the farm. (WASSE; June 2008).
  • Name ten (10) weeds on the farm including their common names, botanical names and method of seed dispersal.