Weeds are unwanted plants grown in lawns and gardens. They are not sown intentionally by farmers. Weeds are harmful to other plants. They reduce crop yield by competing for water, light, soil nutrients and space. Some of them are poisonous to cattle and horses. They act as carriers of disease-causing organisms for plants. Wild mustards, prickly lettuce, cockleburs, and saltbush etc. are the examples of common weeds.