Weeds are the plants that are unwanted in an given situation and may be harmful, dangerous or economically detrimental. They are serious threat to primary production and biodiversity that reduce farm and forest productivity, displace native species and contribute significantly to land and water degradation with second ranking in habitat loss and causing biodiversity decline. In short “Any crop that are unwanted on a given situation can be considered a weed.”

            
                                                                   Every year, thousand of weeds infests the crop beyond their threshold level, reducing their economic yield. Under world conditions, about 30,000 species are grouped as weeds. But it is of concern, the high reproductive potential of weeds that has made them so  adaptable in any environmental conditions around the world. Weeds generally propagate by two methods: Sexual and Asexual propagation.

  
                                         Weeds crop too show allelopathic effects ( chemicals or exudate produced by one plant species) on the inhabiting crop determining the germination, growth of another plant species sharing the same habitat. Allelochemicals are produced by plants as end product, by-product and metabolites belonging to phenolic acids, flavonoids and other aromatic compounds.

Some  techniques, to get rid out of this:

Crop rotation to minimize the undue effect of herbicides

Using rotation heribicides with different mode of action

Integrating cultural, mechanical and chemical weed control methods.