A weed may be broadly defined as any unwanted plant. A corn plant, for example, is a weed if it is in a strawberry planting. Most weeds are plants that are generally considered undesirable by gardeners. They are neither eaten nor considered attractive. They are frequently invasive and difficult to control.
Weeds compete with crop plants for moisture, nutrients and light.
They may also harbor insects and diseases, promote other diseases by increasing humidity, decrease vegetable quality and make harvests difficult. Vegetable plants grown under weedy conditions will have reduced yields or they may not survive to produce at all. A weedy garden or flower bed is also unattractive. Weeds are highly adapted to where they grow. They have large, efficient root systems, grow rapidly and frequently produce tremendous amounts of seeds.