What is a Weed? A weed is a plant that is growing where it is not wanted, it can have strong and healthy growth, and is able to overgrow valued plants by overcrowding, thus depleting soil nutrients and moisture that would otherwise be available to preferred plants.
Examples of weeds Dandelion
English Plantain
Nettle
Poppy as a weed in a field of crops
Poppy as a wanted plant in a garden
Prickly Pear Cactus
Area in Queensland, Australia, covered with prickly pear cactus (Opuntia stricta), which had expanded rapidly after being introduced in 1926.
The same area three years later (1929) after the cactus moth (Cactoblastis cactorum) was introduced as a biological control agent for the cactus.
Use of pesticide in crop spraying
Spraying DDT to get rid of mosquitoes