Weed is any plant that grows in a place where it is not expected to grow or a plant which grows out of place. Weed can also be defined as a plant that grows where it is not desired in such a way that it constitutes nuisance either to man, livestock or crops.

All farmers have to work hard to prevent their crops from being invaded by weeds. Losses in crop yield due to weeds may be greater than those due to plant pests and diseases. If a farmer fails to control weed growth, the chances of successful crop production are seriously reduced.



The harm done by weeds is due mainly to their competition with crops. The effects or economic importance of weeds are:

  1. Weeds Compete with Crops for Space: When a young crop is developing, weeds rapidly crowd and smother the crop, and the space needed for healthy growth is reduced.
  2. Weeds Compete with Crops for Sunlight: Weeds are capable of producing many large leaves which shade crop seedlings and therefore reduce the amount of sunlight which is available for photosynthesis.
  3. Weeds Compete with Crops for Nutrients: The roots of weeds rapidly develop and remove a considerable proportion of the available plant nutrients, thereby reducing the healthy growth of the crops.
  4. Weeds Habour Crop Pests and Diseases: Some weeds may habour crop pests and diseases by acting as hosts, i.e. providing the insects with food when crops are not available.
  5. Weeds Compete with Crops for Soil Moisture: Their roots are capable of developing fast and removing a large portion of soil moisture required for the normal growth of the crops.
  6. Losses in Crop Yield: The combined competition of weeds with crops eventually will result in low yield of crops.
  7. Weeds Compete with Crops for Soil Oxygen: Weeds are capable of developing long roots which remove a considerable amount of oxygen required for crop root respiration.
  8. Losses in Quality of Crops: The quality of harvested crops will reduce and this will affect the market value of such crops.
  9. Losses in Income of Farmers: The low yield coupled with the reduction in the quality of produce, and increase in the cost of controlling the weeds consequently lead to losses in the income of farmers.
  10. Reduction in the taste of some grasses: Weeds also invade pastures, reducing the taste of useful grasses.
  11. Some Weeds Are Toxic to Farm Animals: The leaves and stems of some weeds contain toxins which may be dangerous to farm animals.


Summary of Reasons for Controlling Weeds

  1. Weeds cause losses in crop yields.
  2. Weeds lead to increased cost of farm operations, thereby leading to inefficiency of farm operations.
  3. Increased cost of harvesting.
  4. Reduce the quality of crop products.
  5. Reduce the quality of animal products, such as milk by imparting bad odour and taste to it.
  6. Certain weeds are poisonous and hence kill animals that feed on them.
  7. Form alternative hosts for pests.
  8. Form alternative hosts for pathogenic organisms.
  9. Compete with crop plants for nutrients, space and light.
  10. Reduce land value.
  11. Poisonous weeds are dangerous to man.



However, some weeds are still useful to farmers in the following ways:

  1. Weeds help in checking soil erosion.
  2. Some weeds are useful as food for humans.
  3. Some weeds serve as feed for livestock.
  4. Certain weeds have medicinal properties.
  5. Weeds can be ploughed into the soil to replenish soil nutrients.
  6. Some weeds are used for roofing, making mats and baskets.
  7. Weeds can be cut and used as mulch.
  8. For compost making.
  9. Leguminous weeds harbour bacteria that help to fix atmospheric nitrogen into the soil.



Weeds often possess characteristics which increase their growth habit and mode of dispersal. Factors which aid the growth habits of weeds include:

  1. Production of Large Quantity of Seeds: Most annual weeds usually produce a large quantity of seeds which germinate and grow rapidly.
  2. Some Weeds Reproduce by Seed: Most weeds are deep-rooted and reproduce themselves by seeds. This mode of sexual reproduction enhances their spread and growth.
  3. Some Weeds Reproduce by Vegetative Propagation: Most perennial weeds, e.g ., grasses are usually propagated by means of rhizomes or stolons which spread rapidly.
  4. Growth of Large Leaves within a Short Time: Rapidly growing weeds may produce many relatively large leaves within a short time. These may shade the seedlings of crop plants, if they are not controlled early.
  5. Weed Growth Is Favoured during Rainy Season: Most weeds germinate and grow rapidly during the rainy season as water is available in sufficient quantity to enhance their growth and spread.
  6. Most Weeds Are Wind-pollinated: Most weeds are pollinated by wind and this enhances the spread without hindrance, e.g. tridax.
  7. Production of Large Quantities of Pollen Grains: Most weeds have small, inconspicuous and odourless flowers which produce large quantity of pollen grains.



Most seeds and fruits of weeds are generally dispersed by wind and animals. They produce large number of fruits and seeds, many of which fail to establish themselves due to the fact that they do not find favourable soil conditions.


Mode of dispersal of Seeds weeds include:

  1. Dispersal of seed by Wind: The fruits and seeds of many weed species are specially adapted to dispersal by wind. Weed species which spread their seeds through the agency of wind often have feathery, winged or powdery seeds, e.g ., Tridax procumbens, goat weed, guinea grass, elephant grass and many other species of the grass family.
  2. Dispersal of Seeds by Animals: Animals like rabbits, rats, squirrels, antelopes and man are involved in the dispersal of weed seeds. Animal-dispersed weeds usually have sticky, hooked, edible or colourful fruits or seeds. The sticky and hooked types of fruits and seeds adhere to the coats of grazing animals and are carried from one place to another. Some animals, particularly birds, swallow the seeds of edible fruits. These pass unharmed through their digestive tracts. Examples of animal dispersed weeds are pig weed (Boerhavia diffusa) and Commelina spp.
  3. Dispersal of Seeds by Water: Water-dispersed weeds are common in riverine areas. Fruits or seeds of such weeds have waterproof cuticles, air spaces within the seed/fruit mesocarp and light weight.
  4. Dispersal of Seeds by Explosive Mechanism: Few plants and all leguminous weeds are dispersed by explosive mechanism, e.g. talinum triangulare, centrosoma, pueraria, calopogonium and mucuna.

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