Herbicide spraying is not allowed in organic farming. This method uses various plant poisons to kill the weeds, all of which are harmful to natural systems and some of which can stay in the soil for a very long time and become part of the food we eat. There have been many examples in East Africa especially from flower companies renting land from small farmers, returning that land poisoned to an extent that attempts at selling export food crops from this land has been rejected from the export markets due to too high levels of herbicide contamination (reports from farmers in Limuru area who have rented land to flower farms).
Discussed below are the most common types of weeds and how to control them without having to engage in use of herbicide use.
These are all the weeds germinating from seed along with every crop and going through a full lifecycle from germination to flowering to setting and dropping seeds in one season. All healthy top soils have myriads of different types of weed seed, and every time the soil is disturbed a new lot germinates, in order for the ground to keep itself covered. If we leave these weeds to grow unchecked, the crop we are trying to cultivate will not do well as there is too much competition.
Control of annual weeds
Many methods have been devised to combat annual weeds at an early stage to get optimum yield of food crops:
- Digging or pulling the weeds and removing them from the field (in small gardens). The weeds can be composted; this will ensure their seeds do not grow again if left on the farm
- Shallow cultivation at an early stage and leaving the weeds to dry on top of the soil either by hand or by animal or tractor mounted equipment. Tools include row cultivators, small grain seed spring harrows and hard brushes for row treatment. These two first methods as well as burning are developed in the cool temperate climates and generally aim to keep the soil in between crop plants bare. Under the African sun, bare soil is also unprotected soil. Unprotected soil is prone to erosion, crusting after heavy rains and general soil degradation. There will also be a flush of germination of weeds after every rainfall as nature strives to repair the damage, and repeated
energy consuming weeding operations will need to be carried out.
- Slashing weeds at ground level when 10 to 15 cm tall and definitely before flowering, then leaving them on the ground as mulch, reduces the weeding work substantially. It is much lighter work to slash than to dig with a hoe (jembe) and intervals between slashing can be longer than between traditional weeding. This method can also be using either hand slashing or animal/tractor drawn equipment such as mowers or the knife.
- Planting of ground cover plants especially legumes to crowd out further “weed” germination. This not only provides ground cover but also enriches the soil, and will eliminate further weeding operations. The legumes will often continue growing after the main crop is harvested, providing soil protection until the next crop is planted. They can also become useful for feeding livestock or for incorporating into the soil as green manure.
- Burning. Mechanised farming can choose between a variety of equipment for weed control by burning, both back pack types and tractor mounted equipment. This method will not improve soil fertility, but instead burn off badly needed humus in the top layer of the soil.
These are weeds with a root system that survives the dry seasons and stay alive for two or more seasons. If not controlled, perennials can completely crowd out crops in some cases by sending a dense network of underground roots in all directions. They are very difficult to control as the roots go deep and a very small piece of root or stem can regrow after weeding and create new networks.
Perennials such as couch grass and sedges have a function though: they help the soil restore aeration and natural life in the patch of ground where they grow. They also protect the soil from soil erosion, being carried away by water or wind and the grasses provide fodder for livestock. . If these perennial weeds cover unproductive corners of the farm or steep hillsides they are not harmful, so far they do not invade the crop area. Control of perennial weeds
Management of sedges:
- Ground covering legume plants and mulches can play a very important role in both improving the soil fertility and combat perennial weeds. Clover and other leafy and strongly growing legumes planted in sedge infested land will both overpower the sedges and enrich the soil. In the case of watergrass or nutsedges, harrowing only makes the problem worse, as the root nuts will be separated from the stems and given the opportunity to send out many new shoots.
- Solarisation. Covering a sedge infested piece of land with black polythene after wetting it, and leaving for some days with hot sunshine, will completely eliminate any of the sedge species. However plastic is expensive, so if it cannot be afforded try the first option: ground cover with legumes. For more information on solarisation click here
- Mulching. C. rotundus has been successfully controlled with heavy mulching. Initially the weeds grow prolifically, but after a wet period they are easy to remove by careful hand pulling making sure the “nut” does not stay in the soil. This does not work on hard unmulched soils.For more information on mulching click here
- Hand digging. Very careful hand digging with a knife ensuring all the little underground bulbs are removed can give a small reduction in oxalis populations (annual or perennial), but is very time consuming and bound to leave a few bulbs here and there which will waste no time in germinating.
Weed control in row crops
Best weed control there is, is to keep the soil covered with useful plants during the growing seasons and with mulch or tree cover as far as possible during dry seasons. Once the green manure crops cover the soil in between the maize or cassava, there is no more need for weeding. Remember weeds/plants are nature’s tool to create biodiversity and a healthy soil. Help nature create the biodiversity and the heavy weeding work (fighting nature) becomes much lighter.
Weeds or green manure left in the field after a crop should be left to grow during the dry season if they can. If not they will at least protect the soil which can then be prepared for planting just before the rains are expected.
On slopy land which has not been terraced, cultivate in strips across the slope only. Any soil run off will then be caught by the next strip of weeds. Tractors pulled row cultivators are also available for those who can afford them. Alternatively hand weeding is needed.
Weed control in small grain crops
Small grains such as wheat, barley, etc are normally planted in dense populations as their rate of ground cover per plant is fairly poor. This poses problems for weeding. In Europe and other places there is equipment specially constructed for mechanical weed control in grain fields, but such equipment is not yet available in East Africa. Therefore, small fields of organic wheat should be sown in rows far enough apart to be able to weed at least once with a hoe. Measure the width of your holes and make sure they can fit in between the rows of grain when seeding.
If legumes are available for intercropping they should be sown directly after the first weeding. Blue vetch is a good legume to intercrop with small grains. Also peas in an oat field will both
improve the forage yield and nutritional value of the combined crop, while keeping other weeds under control.