Thursday, July 24, 2014

How to Kill Weeds: Guide to Controlling Weeds


Weeds are any plants that grow where they are not wanted. They compete with the cultivated crops for nutrients, moisture, sunlight, and space. They shelter pests and diseases that attack the crop. They reduce crop yields and farmers’ incomes. Controlling weeds can be a lot of work.

How to Kill Weeds

In conventional farming, tillage is a major way to control weeds. Farmers plough repeatedly in order to suppress weeds and have a clean field when they plan their crop. Ploughing buries many weed seeds, but it also brings other seeds back to the surface, where they can germinate. Burning crop residues may also stimulate the growth of some types of weeds.

Conservation agriculture reduces weed numbers in several ways:
  • It disturbs the soil less, so brings fewer buried weed seeds to the surface where they can germinate.
  • The cover on the soil (intercrops, cover crops or mulch) smothers weeds and prevents them from growing.
  • Rotating crops prevents certain types of weeds from multiplying.

How to manage weeds

Controlling weeds is vital in conservation agriculture. If you do not control weeds properly, they may take over your field, and you will be left with little or no yield! It is important to control weeds at the right time, before they become a problem. Do not allow them to compete with the crops, and do not let them grow long enough to produce seeds. You may have to slash weeds even after harvesting the crop in order to prevent them from producing seeds.

Weeds can be a big problem when you first start using conservation agriculture. You may have to work hard in the first couple of years to control weeds. Be patient! If you do it properly, weeds will become less of a problem later on.

You can manage weeds in many different ways:
  • Using crops and other forms of soil cover.
  • By hand weeding or using equipment to cut or crush the weeds.
  • Using herbicides.

You will probably need to use a combination of these methods to control weeds. It is best to prevent weeds from growing by using various forms of soil cover. These methods are cheap and avoid disturbing the soil. You can then kill any weeds that do grow by using a hand hoe or machete, or with herbicides.

Managing weeds with soil cover and crops

There are various ways to control weeds using crops and other forms of soil cover. They include planting the main crop and intercrops at the right spacings, planting cover crops, using mulch, rotating crops, and intercropping.

Crop spacing

You can plant crops closer together to shade weeds that try to grow in between. The best crop spacing suppresses weeds but avoids competition between individual crop plants. The ideal spacing depends on:

  • Soil moisture and temperature - Weeds grow quickly in hot, wet areas, so closer spacing is needed to smother them.
  • Soil fertility - Weeds grow fast in fertile soil, so closer crop spacing is recommended.

Cover crops

Good cover crops spread over the soil quickly and suppress weeds before they can grow.

Select cover crops that have several uses (food, fodder, fuelwood, etc.), and that produce a lot of green matter that covers the surface rapidly. Cover crops such as lablab can cover the soil completely 2 months after planting.

You may have to weed once to give the cover crop a chance to get established. You can also use a pre-emergence herbicide after planting maize and lablab to stop weed seedlings from emerging.

If the rainy season is long enough, consider planting the cover crop as a relay crop. It will spread over the soil and smother weeds after you harvest the main crop.

Some cover crops (such as black oats) control weeds by producing chemicals that prevent weeds from growing.

Crop rotation

Planting a different crop on each field breaks the life cycle of weeds. There are fewer weeds, and they are easier to control. A good crop rotation prevents the buildup of weed populations. If you cannot rotate your main crop, try to plant a different cover crop or intercrop each season.


Intercropping helps cover the soil and smother weeds that grow between the rows of the main crop. Choose a crop that spreads quickly and produces a lot of vegetation. Legumes, pumpkins and sweet potatoes are a good choice.

Weeding by hand or with equipment

Hand weeding

You can pull out weeds by hand, or slash them with a machete, sickle, slasher or billhook. You can also use a hoe for weeding, but this disturbs the soil surface. Hand weeding is often the job of women and children.

Here are some advantages of hand weeding:

  1. Uprooting weeds by hand disturbs the soil less than using most types of equipment. Try not to disturb the soil too much if you use a hoe or other implements.
  2. Hand tools are cheap and can be bought in most markets.

Hand weeding has several disadvantages:

  1. It is hard work and takes a long time.
  2. The weeds may regrow easily.
  3. The stalks may not be crushed well, making it difficult to plant crops through the residue.

Animal- and tractor-drawn weeders

To use an animal- or tractor-drawn weeder, plant the crops in rows with the same spacing as the cultivator blades.

Weeding by draught animal or tractor power is quicker and easier than by hand. Using an animal-drawn weeder can take less than one-fifth of the time needed for hand hoeing.

However, weeders have several disadvantages:

  • They can damage crop roots. This may be a problem especially in arid and semi-arid areas.
  • They disturb the soil. They bring up weed seeds to the surface and let them germinate more easily. That means more weeding later on.
  • They may carry weed seeds from place to place.
  • They do not work properly if the field has crop residues or mulch from cover crops.


A knife-roller kills the cover crop and weeds by bending them over and crushing them. It is used before planting the main crop. It can be pulled by draught animals or by a tractor. Knife-rollers are fairly simple, and can be designed and made locally.


In some places, there are not enough people to do the weeding. If this is the case, consider using herbicides.

Herbicides are quick and easy to apply, and do not disturb the soil. Some herbicides kill only certain types of weeds.

Not many smallholder farmers use herbicides because they are expensive and hard to find. They also need special equipment, such as sprayers or wipers. It is important to use the right amounts of chemicals, mix them with clean water, and handle them safely. If you are considering using herbicides, get training on how to use them the right way.

Herbicides can be applied in different ways:

  • Weed wiper (such as a Zamwipe);
  • Knapsack sprayer;
  • Hand-pulled sprayer;
  • Animal-drawn sprayer;
  • Tractor-mounted boom sprayer.

Before Deciding on Herbicide Use

Herbicides are an important tool for weed control, but should not be relied upon solely for weed elimination. Follow these steps before deciding whether or not an herbicide is even necessary and which one is the best to use.

  1. Assess the amount of weeds in your lawn. If you have a low population of certain broadleaf weeds, such as dandelions or purslane, hand pulling may be the only means of control necessary.
  2. If your lawn is otherwise healthy, learn to tolerate a certain amount of weeds. Even in the best-managed lawns a number of weeds will appear.
  3. Consider using an herbicide only for invasive weeds such as ground ivy, clover, wild violet, and grassy weeds.
  4. It is important to identify the weeds. This will help in the selection of the appropriate herbicide and also will determine the timing of the herbicide application.
  5. Try to determine how the weed was introduced into your yard and the conditions that favor it.
  6. Look at cultural practices, along with poor growing conditions, as the possible reason for weed encroachment.

Weed wipers

A weed wiper looks like a broom with a sponge on its head. A small tank on the handle holds the herbicide, which flows down into the sponge. The best known type of weed wiper is the "Zamwipe", used in Zambia.

Wipers are ideal for small farms. Use them to apply herbicide on weeds between crop rows, or to kill cover crops before planting the main crop.

Their advantages include:

  1. They are light and easy to use, and ideal for women farmers.
  2. They are fairly cheap (about US$ 17) and easy to maintain.
  3. They have low application rates (only 20–25 litres/ha), so use little water.
  4. There is no risk of wasting herbicide or damaging the crop because of spray drifting in the wind.

Wipers have some disadvantages:

  1. They are useful only for small plots of land.
  2. Farmers need to know how to use the wiper properly.
  3. Wiping weeds on uneven ground is difficult.

The Zamwipe works best just before planting the main crop. Use it to kill weeds that have started to regrow after they are slashed, when they are 10-12 cm tall. Use it also to deal with individual weeds. It can also be used to control weeds in between rows of maize or sorghum, when the crop is kneehigh - i.e., at the same time as when farmers normally weed their fields.

Never use dirty water, and clean the wiper immediately after use.

Knapsack sprayers for kill weeds

Many small-scale farmers already have a knapsack sprayer. These sprayers may be manual or powered by a petrol engine. They are reasonably cheap and easily available.

Mounting a shield prevents the spray from drifting onto crops, so the sprayer can be used after the crop has emerged. Knapsack sprayers are not suitable for large farms.

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