Weed problems in smallholder farming areas in Zimbabwe

A weed is generally described as a plant which grows where it is not wanted. If weeds are left uncontrolled, they will compete with the crops for nutrients, light and water. Consequently crop growth is reduced leading to low yields. The affected crops have reduced heights, thin stems and show nutrients defficiency symptoms (for example maize leaves may appear yellowish showing nitrogen defficiency). In most cases weeds are left to flower and produce seeds. This means that during each cropping season, weed seeds are always added to the soil. As a result large seed banks are formed. Tillage, for example, ploughing action has a tendency to bury seeds in the soil and also bring other seeds to the surface of the soil. Weed seeds which are burried to about 25 to 30 cm may become dormant. The seeds will only germinate when they are brought close to the soil surface by tillage activities. Most weeds in Zimbabwe germinate after the early rains in October and November, which is the start of the cropping season. Some weeds, for example Leucas martinisensis germinate at the end of the season i.e. in May. Weeds such as Richardia scabra will continue to germinate throughout the season. On the other hand weeds such as Eleusine indica and Acanthorspemum hispidum may germinate during the early part of the season. The main weeds which are commonly found in smallholder farming areas are as follows: Richardia scabra, Acanthospermum hispidum, Hibiscus minuta, Cleome monophylla, Bidens pilosa, Eleusine indica, Setaria verticillata, Rottboellia conchichinensis, Cynodon dactylon, Commelina benghalensis, Rhynchylitrum ripens, Cyperus species, Striga asiatica, Striga gesneriodes and Alectra vogellii. The top picture shows maize infested with Richardia scabra, which has tiny white flowers. Acanthospermum hispidum is shown in the second picture.