Some plant species that have been brought to Southern Africa from other countries pose a serious threat to the environment, including water resources, indigenous vegetation, agriculture, and the economy.

Some of these plants were imported intentionally to address specific needs, like sand stabilisation or sylviculture, or as ornamental garden trees or shrubs, or unintentionally, like cosmos and khaki weed (kakiebos), which were brought in along with horse feed for the British army during the Anglo Boer War. The pretty pink, white, and sometimes purple or crimson cosmos flowers that adorn country roads in spring is an invasive alien plant, while khaki weed is regarded as a nuisance that sticks to everything, especially your socks!

The wattle is one of the Australian acacia species that is regarded as an invasive alien in South Africa. Photo: Pixabay

The alien threat

Species that are alien to South Africa may have natural enemies that curb their spread in their countries of origin. Where there are no natural enemies, like insects or fungi, they rapidly reproduce and spread.

This poses a threat to their adoptive country

  • They deprive indigenous plants of space by their rapid encroachment, threatening biodiversity
  • As they usually consume more water than indigenous plants, they deprive the local vegetation of water
  • They deplete underground water resources and in heavily infested areas can reduce water run-off by up to 30%
  • They provide fuel for intense veldfires that destroy vegetation and soil structure, infrastructure and threaten human and animal lives.
  • As a result, they threaten biodiversity and disrupt the ecosystem services that we need for our livelihood, such as food, water, firewood, medicine, and clean air.

Eucalyptus flowers produce copious amounts of nectar. Photo: Pixabay

Recognise them

According to CapeNature the top 10 invasive alien plants affecting the Western Cape include rooikrans, black wattle, Port Jackson, silky hakea, long-leafed wattle, stinkbean, Australian myrtle, spider gum, cluster pine and blackwood. Ken Coetzee of Conservation Management Services, believes that prosopis, or honey mesquite (Suidwesdoring) should head the list in the more arid areas such as Namaqualand and the northern Cape’ as this invasive species has already invaded vast areas of the arid rangelands.

The South African government, CapeNature, the Gouritz Cluster Biosphere Reserve, and many other organisations provide for the management of these invasive plants by clearing land, making it available to agriculture, and to ensure a sustainable water supply. Clearing is done by biological, manual, mechanical and chemical methods.

Cut them down to size

Biological management involves the use of the alien plants’ natural enemies, such as beetles, flies, fungi, and pathogens that are sourced in the countries of origin. Examples of this include the use of the cactus moth, Cactoblastis cactorum, and the cochineal insect, Dactylopius austrinus, on prickly pear, and the gall midge, Dasineura dielsi on rooikrans, to name a few. Manual management involves removing small invaders or young plants from the soil by hand, while mechanical management involves chopping down plants with a hatchet or panga or using a bow saw or chainsaw to cut the plants down.

Ringbarking is also effective and involves removing a 30 to 40 cm strip of bark around the trunk. This prevents the plant from taking nutrients from the soil to the branches and leaves, and the plant dies. Chemical management include the use of herbicides to kill the plant. The stumps of trees that had been cut down should in most instances be painted with a registered chemical agent to prevent it from sprouting again.

Be on the lookout

The following tips will help you do your bit to stop invasive aliens in their tracks while they are still manageable. These actions are suitable for controlling invasive alien plants on a small scale in your garden, as well as on a bigger scale on your smallholding or farm.

  • find out which plants are aliens and how to control and remove them;
  • share your knowledge with others;
  • join or form a team to control alien plants in your area;
  • remove small plants as they sprout in your area;
  • get rid of the cuttings and make sure the roots don’t sprout again by painting it with a registered chemical;
  • replace them with indigenous plants suitable for your area;
  • plant only indigenous, waterwise plants in your garden or public area; and
  • by removing aliens before they become a problem, you will promote not only indigenous plants, but also conserve water resources.

The flowers of the eucalyptus tree are a vital source of nectar for honeybees. Photo: Pixabay

Aliens save the bees

However, not all aliens are bad. Some eucalyptus species, native to Australia, are also a well-known feature of the South African landscape. The eucalyptus is lauded for its benefits, including being a fast-growing source of wood, oil that is used in cleaning materials and an organic insecticide, and their ability to dry up a swamp. The latter is unfortunately also a negative feature, especially in a water scarce country such as South Africa. Their most important feature is their ability to produce copious amounts of nectar, which is needed by honeybees.

According to the Alien and Invasive Species Regulations, which were promulgated under the National Environmental Management: Biodiversity Act, 2004 (NEMBA) published in the Government Gazette on August 1, 2014, these species include the river red gum (Eucalyptus camaldulensis), spider gum (Eucalyptus conferruminata), sugar gum (Eucalyptus cladocalyx), karri (Eucalyptus diversicolor), saligna gum (Eucalyptus grandis), and forest red gum (Eucalyptus tereticornis). Even though these species are listed as invaders in South Africa, they only need to be removed if they are in a place where they have a negative effect on the ecosystem, including biodiversity, water resources, erosion, and an increased risk of fire.

However, if these gum tree species on your property forms part of a bee forage area, a plantation or woodlot, a windrow or lining an avenue, you can apply for a Category 2 permit and keep them. In the next issue, the thorny subject of prosopis invasion of vast areas of land will be under the microscope.

For more information contact Ken Coetzee at 076-227-5056 or


Alien Vegetation Management (n.d.). CapeNature alien-vegetation-management

Gums and bees: A roadmap for landowners in South Africa. (2018). South African National Biodiversity Institute (SANBI) uploads/2018/05/gumsbees-webversion-hyperlinks.pdf