Biology and Ecology of 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
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.
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.
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.
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)