Prickly intruders

They grow everywhere – in gardens, along the sidewalk, on fallow land and on slopes and they’re taking over our living space. In Namibia there are 287 known invader cacti species, and these two women from Windhoek are taking on the fight.

18 August 2019 | Environment

Windhoek • Theresa Lang

Gunhild Voigts carries gardening tools with her wherever she goes. Peek in her car and you will find rakes, saws and axes. While driving around with her she tells me, “here we cleared the whole slope” or, “this whole area was overgrown with cacti,” while waiving her hand out of the car window.

She’s in the backseat, giving directions. Her husband is behind the wheel, while the co-driver seat is reserved for interested guests.

Gunhild has already taken government officials and sceptical land owners on this same special tour. At a walking pace, the car rolls through the streets of Windhoek. However, on this trip the well-known Tintinpalast and the Christuskirche are not included. No, this time visitors see Windhoek from the perspective of a passionate conservationist.

Gunhild – slim, tall, with snow-white hair – loves sharing her successes: Clean, cactus-free front yards and large piles of cleared cacti on the roadside. “Actually, we were hoping to compost the cacti, but it’s too dry this year,” she says.

Just as the National Botanical Research Institute (NBRI) calls for cleaning operations in the botanical gardens, Voigts does the same with her Cactus Clean-Up Initiative, only she and her team remove these spindly plants and other intruders from Windhoek’s sidewalks. In the city, she does so at the request of the municipality. For private property owners, she provides reliable and experienced workers who free the garden of cacti at a fee.

Despite her success, there is still a lot that needs to be done. Again and again, our tour takes us past “cactus-contaminated” land, as Voigts calls these areas. “We do not want to leave no vegetation for the next generation.”

She can name any tree on the roadside – sometimes with a bitter aftertaste. “Just look, everything is covered by eucalyptus,” she says, shaking her head. “Eucalyptus is actually used to dry out swamps because the plants extract so much water from of the ground. However, in Namibia with our water shortages, we really can’t allow that.”

Getting on board

In her cacti purges, Gunhild often encounters reservations and sceptical glances from residents who decorate their gardens with pretty ornamental plants or use cacti to protect their property from intruders.

With much patience, she explains why cacti and other neophytes (a plant species recently introduced to an area) are so problematic for Namibia’s flora. Often she has to compromise. “This couple is afraid of burglary, so I agreed with them that the cacti can stay until I gave them an alternative,” she says while pointing to a high fence, reinforced with a roll of barbed wire on top.

However, longer spines lurk under the fence. A cactus covers almost the entire area under the fence.

She says a good alternative is the so-called “gemsbok horns” or Sanservieria pearsonii. “Those are pointed up and harmless from the side. No burglar will get in,” she promises with a laugh.

Since the problem is not only a Namibian one, Gunhild also wants to share her methods with others: “I submitted the Cactus Clean-Up Initiative to the Dubai Expo Global Best Practice Program for the 2021 World Expo,” she says.

“In Athens, what’s left of the Acropolis is also threatened by cacti. Our initiative can serve as an example of how a community can solve its own environmental problem,” she says. “The project meets all the criteria: It is inexpensive, easy to implement and it is effective.”


In future, however, Barbara Curtis of the NBRI and Gunhild want to tackle the invaders not only with their hands and garden tools, but with a “bio-control agent” that is supposed to kill the cacti.

Earlier this year a committee of the Namibian Chamber of Environment approved the import of this agent from South Africa. “It’s really small insects infesting and killing the cacti,” Gunhild says. There is no danger for other plants, Barbara assures. “The bio-control agents are so specifically bred that they infect only one cactus species at a time. We now have three different kinds of these agents. As soon as the temperatures don’t drop so much at night, they will be released.”

Many of the invasive plants are beautiful, boasting colourful flowers and fruits. They are robust and well adapted to the Namibian climate. It’s no wonder then that many residents plant Cryptostegia grandiflora, a creeper with the pretty pink flowers, in their gardens. But you shouldn’t let its appearance deceive you. The “rubber vine” is highly invasive. It grows quickly and branches into trees from where it removes all light and water. An infested tree dies slowly.

Such is the case with many neophytes, says biologist Barbara. “They take over the habitat and suck water away from the native plants.”

Thus the women plead: “Don’t plant invasive cacti to begin with. Many people believe that they have control of the ornamental plants in their garden, but that is not the case,” Barbara warns. “Birds eat the seeds. Where they leave their faeces, a new plant grows. You cannot control that.”

Barbara is working on a new edition of the Atlas of Namibia, a book that lists all the country’s fauna and flora, as well as roads, geological features and much more. In contrast to the last version, invasive plants will also be included this time. That is why she is looking for places where these invader species grow.

She’s hoping that Namibians will help. “People can contact me if they spot invasive plants. It is best to send me a photo and the GPS coordinates, along with a rough description of the place and plant.”

It is important to her that the maps are supported by enough material so that they do not give the wrong impression. “If too few affected locations are placed on the map, then it may create the impression that the problem is not that big,” Curtis fears. She wants to counteract this by not looking for the invaders alone, but collecting as many clues from Namibians.

Anyone who discovers an invasive plant, can email the information to Barbara at [email protected] Ideally, please include GPS data and a photo, too.

For more information on the Cactus Clean-Up Initiative, visit or contact Gunhild at [email protected]

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