Weed control is a particularly acute problem in developing countries. Weed plants grow more vigorously and regenerate more quickly in tropical than in temperate zones because of the heat and higher light intensity. Much more is known about weeds in the developed than in the developing countries, and the techniques of control - mechanical and chemical - are often unsuitable to the physical, social and economic conditions of developing countries. Biological controls are a relatively cheap self perpetuating control and are available and proven in some countries for major tropical weeds including water hyacinth. The problem of water weeds has been aggravated by development projects such as reservoirs, irrigation canals and dams. The greater quantities of human effluent and fertilizers in these waters make them richer them natural waters and thus weeds flourish more easily. The effects of an abundance of weeds include slower water flow (by as much as 80%) and blockage of shipping and fishing.
Destructive weeds include purple nutsedge, Bermuda grass, barnyard grass, jungle rice, goosegrass, Johnson grass, Guinea grass, water hyacinth, cogon grass, and lantana. They occur in every major agricultural area in the warmer regions of the world.