Biology and Ecology of Weeds

Annual weeds

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 topsoils 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. 


For more information on purple witchweed (striga) click here 
 

Perennial weeds

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 and stolons 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. 

Sedges (Cyperaceae) have smooth leaves and triangular flower stems. The clustered seed heads differ according to species (there are more than 50 species in East Africa)(Terry 1976). They have underground bulbs, stolons or tubers which can remain dormant for long periods of time. They are often only minor problems in shambas with a mixed weed population. 
Sedges are not real grasses and most livestock only eat them if there is nothing else available. 
 

Sedge grass (cyperus longus) with underground stolons

(c) A. Bruntse, Biovision

The most common species of Cyperus includes Cyperus longus with underground stolons like couch grass and no visible "nuts". This is particularly troublesome in rice fields and other waterlogged locations.  C.rotundus , more common in hot areas, has underground nuts and thin connecting stolons.

Nutgrass

(c) A. Bruntse, Biovision

Another troublesome sedge especially for non-tillage farmers is "watergrass" - small plants with a tiny underground "nut" and a prolific seed producer. 
Sedges (including nutsedges and watergrass) release chemicals that reduce the growth of other plants near them, which is why most crops grow very poorly in the presence of sedges. 

Among the many weeds we see everyday some do not seem to have any function that we know of. But this does not mean they are useless. However, oxalis in spite of its tiny size has been found to reduce the yield of maize up to 24% (Terry 1984). 

If you look carefully, most of these perennial weeds are most serious where the soil is compacted, waterlogged or has generally become infertile, or on mechanized farms where annual weeds have been killed by herbicides.
 

Oxalis growing and some uprooted - note the small underground bulbs from which the stems break very easily

(c) A. Bruntse, Biovision

Couch grass (Cynodon dactylon) is a perennial grass, with underground rhizomes and on the ground runners.

(c) Charles T. Bryson, USDA ARS, www.insectimages.org

For more information on couch grass click here 
 

Weeds and soil fertility

In studying the relationship between weeds and soil fertility a clear connection appears. Of all the seeds stored in the topsoil, those suited to the soil status will be the ones germinating. Certain weeds will germinate on very poor and damaged soil and very different ones will grow on a soil in good fertility.

Weeds, as any other plants, take up nutrients from soil and air and return them to the soil when they die. On a poor soil the weeds growing will be those that are able to extract or fix nutrients the more demanding plants are not able to take up under the same conditions. The poor fertility plants/weeds will therefore enrich the soil and slowly improve it, if left to do their job. High fertility weeds left to do the same on fertile soils will improve the soil even faster as they take up/fix higher amount of nutrients. 

It is known that too many weeds reduce yields, but not much research has been carried out on retaining some weeds for soil protection but keeping them down to a manageable level, so as not to interfere with the main crop. 
On slopes there are many recommendations for strip cropping in order not to loose valuable top soil. The grass strips than become soil conservation measures. However, very good results have been achieved by The Conservation Agriculture research team on replacing weeds as ground cover with legumes, both as far as yields and reduced workload for the farmers is concerned. (IIRR 2005)

Weed Control

Control of annual weeds

Many methods have been devised to combat annual weeds at an early stage to get optimum yield of food crops:

1. Digging or pulling the weeds and removing them from the field (in small gardens). The weeds can be composted. 

2. 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 no 5 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. 

3. 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/digging. This method can also be using either hand slashing or animal/tractor drawn equipment such as mowers or the knife rollers developed by Conservation agriculture studies in East Africa (IIRR 2005). 

4. 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.

5. 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.

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). 

 

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.

Tractor 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 hoes 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.