A weed is any plant that grows where people don’t want it to grow. For example, when we plant a lawn, we don’t want other plants growing in the lawn. Other plants spoil the appearance of the lawn, and take minerals and water from the soil that the lawn needs. So we call any other undesired plant a weed.
Plants whose seeds scatter easily are usually called weeds because they come up in many unwanted places. However, weeds are not always useless. Some serve as food for wildlife. Also, weeds hold the soil in place, and help to prevent erosion.
Many plants that people widely regard as weeds also are intentionally grown in gardens and other cultivated settings. The term also is applied to any plant that grows or reproduces aggressively, or is invasive outside its native habitat.
While the term “weed” generally has a negative connotation, many plants known as weeds can have beneficial properties. A number of weeds, such as the dandelion and lamb’s quarter, are edible, and their leaves or roots may be used for food or herbal medicine. Burdock is common over much of the world, and is sometimes used to make soup and medicine in East Asia.
Some weeds attract beneficial insects, which in turn can protect crops from harmful pests. Weeds can also prevent pest insects from finding a crop, because their presence disrupts the incidence of positive cues which pests used to locate their food.
Direct Losses by weeds
*Weeds cause reduction in crop yield through competition for light, nutrient, water and space. They can also reduce the yield of crop through the release of toxic substances which inhibit crop growth. This is called allelopathy. Uncontrolled weed infestation can lead to 95% yield loss in cassava, 40% in maize and 53% in cow pea, soybean and pigeon pea.
*Reduce the quality of harvested agricultural products.
*Interfere with harvest operations and increase the cost of harvesting in both small holder and large scale farms.
*Weeds may poison animals e.g. Amaranthus spp can adversely affect livestock because of the high nitrate content of the shoots.
*The cost of controlling weeds is high.
*The presence of weeds can impede water flow in irrigation canals and in lakes and reservoir can increase loss of water by transpiration.
Indirect Losses caused by weeds
*Serve as alternate hosts to many plant diseases and animal pests e.g insects, rodents, birds etc that attack crops.
*Weeds impose a limit on farm size.
*Reduce the economic value of lakes by preventing or limiting fishing activities.
*Weeds such as Imperata cylindrica become fire hazards in the dry season throughout the savanna vegetation zone.
Non Agricultural Losses
*Affect health of humans, stinging nettle can cause skin rashes and the flowers of some other weeds can be associated with allergies in humans
*Impair visibility along roads and railway lines.
*Uncontrolled weed growth reduces the value of real estates.
*In situations where farmers depend on human labor for weeding, children have to miss school at peak of weeding periods. This reduces the quality of education that these children can get during their early years.
Content for this question contributed by Tom Justice, resident of Cincinnati, Ohio, USA