Worldwide, weeds reduce food crop yield by up to 20%. In fact, it has been calculated that for every kilogram of weed growth, farmers lose a kilogram of crops produced.
With statistics like these, it’s crucial for farmers to take urgent action the moment they observe new weeds emerging. Many farmers wait too long before applying herbicides. In some cases, this means that the weed grows large enough to recover and send out new roots. Typical examples are Portulaca, which is a succulent, and Commelina benghalensis.
Post-emergence herbicide may simply be ineffective if the weed has reached a certain stage of development. For example, quickweed (Galinsoga parviflora), which affects carrots, is not controlled by the post-emergence herbicide Linuron once it starts to flower.
Whatever the case, you usually get better weed control using less herbicide at the correct stage. Many of our weeds are invasive aliens introduced through carelessness. Fodder imported by the British Army during the Boer War accounted for some of these.
Imported seed lots of cultivated crops are also sometimes contaminated with weed seeds. It takes only one weed seed in a ton of imported seed to introduce this weed into the wild. It can be spread by water, implements, muddy shoes, wind, animals, birds and in other ways.
Act rapidly before it is too late
When I was working in the Hoedspruit area, many farmers struggled with Flaveria bidentis. This weed, a member of the marigold family, originates from South America, has yellow blooms and is able to resist most herbicides.
Many years later, after moving to Gauteng, I noticed a few of these plants growing alongside a railway line and brought this to the attention of the neighbouring farmer. No action was taken, and now it has spread for kilometres along the railway line, ready to invade the adjoining countryside.
We tend to become aware of a dangerous new weed only after it has become a permanent resident.
The invasive pompom weed
Another species invading the grasslands of South Africa is pompom weed (Eupatorium macrocephalum). This is a beautiful pink flowering plant originally brought in from South America as an ornamental member of the daisy family. It is now a Category 1 weed, meaning that it is illegal for a property owner not to take action to destroy it.
Yet when it grows alongside road verges, the provincial authorities do nothing about it. This inaction is disgraceful; the weed is spreading rapidly and our veld is being rendered less productive as a result.
E. macrocephalum produces thick tuber-like roots and can withstand adverse conditions, making it a permanent problem unless destroyed. If every farmer worked together with the state, it could be eradicated.
Farmers need to become more aware of the weeds that occur in their areas and keep an eye out for new weeds that can become a problem in their cropping system. Most importantly, they need to destroy them before they become permanently established.
In forthcoming issues, I shall highlight more of the major weed species to look out for and discuss how to ensure that they do not become established.