WEEDS are the plants, which grow where they are not wanted. They grow in the fields where they compete with crops for water, soil nutrients, light and space and thus reduce crop yields. They also harbour insects pests and micro-organism. Certain weeds release into the soil the inhibitors or poisonous substances which are harmful to the plants, human beings and live-stocks. They increase the expenditure on labour and equipment, render harvesting difficult, and reduce the quality and marketability of agricultural produce. They block the drainage and impede the flow of water in canals and water-transport channels and their growth in the rivers renders navigation very difficult. The dense growth of weeds in water pollutes the water because they deoxygenate the water and kill the fish.