From Envirowiki

Monoculture generally refers to the agricultural or forestry practice of planting only a single species of a plant over a large area. For example, many industrial farms will have many acres of only one type of corn or wheat or whichever crop; logging companies may plant trees to replace those they cut down, often choosing to use only a single type of tree that grows quickly and produces the type of wood they want to harvest.

Monoculture may be efficient in the short term, as it can be easier to maintain. However, it is typically viewed as bad from an environmental perspective because:

• It is a threat to biodiversity and often damages natural habitat for native plants and animals
• Areas of monoculture are more susceptible to diseases and pests, often increasing the need for pesticides
• All of the plants are drawing the same nutrients and producing the same excrement, the soil can become depleted and unblanaced, often increasing the need for fertilizers.