Why do weeds grow ?

Environmental effects on weediness

In the tropical zone, the presence of species is principally related to the physico-chemical nature of the soil and to its humidity. After a number of years, the composition of the weed flora evolves after an area has been cultivated.

• · At the beginning, the flora consists of non-competitive species present in the natural environment. These call for very little weeding and are not really adapted to the repeated disturbances brought to the agricultural environment.
• · Rapidly, this flora is replaced by species biologically better adapted to the agricultural context and which proliferate with time. These new species extend to other plots through contaminated seeds, tools, animals, irrigation water, wind, …
• · Husbandry practices influence the rate of flora’s evolution and the selection of species best adapted to the context. In a few years, weeds almost monospecific appear, against which farmers have no control with the classical techniques.

Variations during a crop cycle

During a crop cycle, the floristic composition of the plot and the abundance of species evolve. The species present at the beginning of the cycle are responsible for most of the competition against the crop, while those at the end of the cycle have an adverse effect on the harvest and its quality and increase the weed seed stock of the soil. The study of the weediness must therefore take into consideration the whole of the crop cycle.

Definition of a level of harmfulness

A rational management of a plot’s weediness must take into account different levels of harmfulness :

• Direct biological harmfulness : competition with the crop – drop in productivity
• Indirect harmfulness : decrease in crop quality harvested, adverse effect on the work at harvest
• Local ecological harmfulness: management of the flow of the potential seeds stock of the area
• Damage to the regional agro ecology: possible effect on the weed seed stock in the soil

Finally, the study of a particularly invading species must deal with its biological strategy. The objective will be to understand all the characteristics that provide to the species the capacity to adapt itself and be invasive taking advantage of the agro-ecological context.

Return to the page "Weed science : everything about weeds"