Main methods of controlling weeds

What is wrong with weeds?

Weeds compete with crops for space, light, nutrients and moisture.  They can bind around crops, climb up them and strangle them or more likely, crowd them out with root expansion. Some weeds are parasitic on crop plants, others are poisonous to cattle. The most troublesome weeds are often related to the crops affected. This makes them difficult to spray and may also harbor the diseases and pest of the crop.

Tropical climates are favourable for rapid and heavy growth of weeds during the rainy season and many weeds have adapted to survive a dry season and an annual burn.

Weed competition seriously damages crop yields in the tropics leading to losses of as much as 50% in fields with heavy infestation. Weed control in the first 4-6 weeks of the life of the crop is vital.  With some humid tropic crops over half the labour devoted to producing the crop is used in weeding.

Methods of controlling weeds.

Land Cleaning for annual crops and weeds

Burn the land after harvest once all the useful material has been removed. This will kill a large proportion of the weed seeds and destroy any pests and disease from the previous crop.

  • After the first rains, any remaining annual seeds will germinate so the first pass of a plough will destroy them. The buried material will act as a manure too.
  • Inter-row weeding 2-3 times in the early stages of crop growth is required.  After this time, the crops will be sufficiently established to compete with the weeds and the plant shade and crowd out the weeds. Where cultivation is done in ridges, earthing-up will bury weeds.

Perennial weeds are not usually found on regularly farmed land.  They are more characteristic of land that has been left fallow or newly farmed land.. Perennial weeds usually have vegetative means of reproduction and are harder to clear than annuals.

Examples of perennial weeds are Sourgum Halepense (Johnson Grass) and Imperata Cylindrica (Swordfish).

Land cleaning for perennial weeds

As with annual crops, cut down and burn grass and weeds.

  • Underground rhizomatous material should be dragged out of the ground using a tine ripper and gathered in heaps for burning.
  • The first crop that is grown should be a cleaning crop. Potatoes and maize are good cleaning crops.

Parasitic weeds

Parasitic weeds are plants that grow on crops getting their nourishment from the host plant.  This can impair the growth of the crop and its yield.  Striga is a common weed which is parasitic on the roots of maize.

Controlling Striga

  • Rotate crops so the weed does not have a chance to build up and so dies out.
  • Plant trap crops with the maize. These stimulate Striga to germinate, but the  parasite is unable to penetrate the root and the seed dies.  Cowpeas are an example of a trap crop.
  • Repeated pulling of Striga will lessen it, but pulling does stimulate the weed.
  • Plant Striga resistant seed.

Spraying with herbicide (chemical weed-killer)

Herbicides are available in many countries to control weeds.  Sprays must be selective; they must be able to differentiate between the crop and weed and destroy only the crop.  The weeds must be correctly identified if herbicides are to be correctly deployed.

Herbicides must be treated with care as some of them are poisonous so strict safety regulations should be observed such as wearing protective clothing and disposing of old chemical containers responsibly.

The disadvantages of non-chemical control of weeds are the use of hand tools,forks and cutlasses can damage the roots and shoots of crops. Continuous hoeing of the soil damages the soil structure leading to soil erosion and aids the decomposition of the soil organic matter. This would make chemicals a no-brainer in terms of addressing weed control This is true for large estates (with larger budgets) and at high altitudes, but chemical spraying for weed control for small farmers has had little impact for the following reasons:

High cost of herbicides makes them unsuitable for small farmers.  Hand weeding does not cost cash.

  • Where intercropping is practiced, it is difficult to find a herbicide that will take out weeds in a mixture of different species.
  • There is a general low level of education and a lack of advice for farmers pertaining to chemical use.
  • Herbicides historically were not readily available in small quantities.  This is gradually changing though.
  • Chemical herbicides harm fish and aquatic fauna.
  • Counterfeit pesticide and illegal imports can be highly damaging to crops, land and people.
  • Introduction of herbicides can kill off one weed and enable another weed to take its place.

Other ways of controlling weeds

  • Plant cover crops which smother weeds.  These, often creeping plants, must not compete too much with the main crop.  Examples are Muncana planted with maize in Benin has been successful in controlling spear grass.
  • Rotating crops reduced weed build up associated with one particular crop.
  • New varieties allow closer planting of seed which crowds out weed competition.
  • Fresh tadpole shrimp in paddy fields helps control the weeds by eating the young buds and roots of the weed plant.


Good luck with the elimination!

Written by Fiona Johnson

October 2011

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