Invasive alien plants affect nearly one tenth of the surface area of South Africa, including waterways, riparian zones, and grassland or scrubland. More than 150 introduced plant species have become invasive. These plants use more water than indigenous plants, because indigenous plants are adapted to the dry conditions in South Africa and become dormant during the dry season. Alien plants have longer roots and continue to draw water from the rivers and groundwater reserves during dry seasons, thus reducing the amount of water available to support other terrestrial and aquatic communities.

Alien plants also destabilise riverbanks, because chemicals from their leaves and roots penetrate the soil and prevent other plant species from growing underneath them. This creates large areas of bare soil along the banks, which are thus more susceptible to erosion. Eroded sioil causes increased turbidity and sedimentation in the rivers, smothering of interstitial habitats and fish gills, and increasing the likelihood and severity of flooding.

South African ecosystems are adapted to regular burning, but when alien trees dominate the system the fires are often more intense because of the addition fuel load. This can result in damage to the soil, vegetation and surrounding infrastructure, and a reduction in the viability of indigenous seeds.

Alien vegetation also poses a threat to biodiversity. South Africa is an exceptionally biologically diverse country, with nine times more plant species, eight times more breeding bird species, six times more mammal species, and twice as many amphibian species, compared with the world average. Alien invasive plants dominate natural habitats, by competing with natural species for space, water, sunlight, and other resources. They also reduce the structural diversity of the vegetation, and disrupt ecosystem dynamics, which can impact on the number and type of animal species that can be supported by the vegetation in that habitat.

The Working for Water Programme was launched by the national Department of Water and Sanitation to try to control the problem of alien invasive plants. The Programme uses a combination of methods to tackle the problem: