Any plant that grows where it is not wanted can be described as a weed. Weeds are a nuisance that farmers fight all the time.
They have high resistance to disease and it is not unusual, for instance, to see Irish potatoes or tomatoes growing to maturity in gardens where they are not wanted without any use of pesticides, which would be extremely rare in gardens meant for the crops.
Weeds also tend to grow vigorously even in periods of drought and they are heavy feeders, benefiting from soil nutrients supplied by both organic and synthetic fertilisers which are meant for crops.
They use water to grow, they occupy space, and they compete with farmed plants for light. Weeds also make crop inspection difficult for the farmer since many of them are poisonous or irritating to the skin.
A lot of weeds have leaves that stick to the farmer’s clothes. Their seeds have a high survival rate and quickly germinate when the rains begin.
They form safe hiding places for pests, snakes, and rodents. They reduce farmers’ profits because the farmer must hire labour, use fuel consuming machines or purchase herbicides to keep the weeds under control. They degrade the soil since they use up humus much as some of them are nitrogen fixing. They can cause stunted growth for the crops and reduce their quality.
They are, however, important for preventing soil erosion since they protect the ground against raindrops while their roots hold the soil firmly in place. Some farmers use weeds to make compost manure while many others bury them in the soil during ground preparation to make their gardens fertile.
They can also be used to make “manure tea” by immersing them in water for some days.
Some perennial weeds last for years and spread their roots under the ground, making it very difficult for the farmer to remove them physically.
For smallholder farmers weeds can be controlled by cultivation with simple tools like a hand hoe.
Other farmers avoid weeds by planting as soon as the rains begin for crop seeds to germinate before the weed seeds do.