"We've got the reserve under control now and I know the big (non-indigenous) trees by name -- and their days are numbered."

Carstens, a retired history and English teacher, is the advance guard of a growing movement to control the spread of non-indigenous plants in South Africa.

In October, National Weed Buster week implored South Africans to "stop the invasion, plant indigenous".

Environmentalists say alien plants and trees displace native species, intensify flooding and fires and can cause watercourses and dams to silt up or dry out entirely.

"Alien plants waste about 7 percent of our water resources," said Rand Water, which supplies water to many South Africans and operates schemes that pay unemployed people to remove alien plants from water catchment areas.

"Tree invaders of river courses and catchments drastically reduce the volume of water that reaches rivers and dams, and can even cause streams to stop flowing or wetlands to dry up."

In a country with large areas of arid semi-desert which relies on seasonal rains to top up its numerous dams for agricultural, industrial and domestic water supplies, such a threat is of serious concern.

WEED BUSTERS' BIBLE

"Invasive alien plants are a massive threat to the ecological functioning of natural systems and to the productive use of land," Agriculture and Land Affairs Minister Thoko Didiza wrote in a foreword to "Alien Weeds and Invasive Plants", the bible for weed busters like Carstens.

An alien A-Z, it explains, for example, that the downy thorn apple or Datura Innoxia from North and Central America with its large tubular flowers is poisonous and irritant, classed "Category 1: prohibited on any land or water surface in South Africa, must be controlled, or eradicated where possible".

Thanks to its geographical location and settler history, South Africa has plants from all over Europe, the Americas and Asia and Australia.

Some were brought deliberately for settlers' gardens, others by accident.

One example is the cosmos, a flower that covers corn fields and rough ground in rippling seas of pink and white each autumn.

Carstens says it was introduced with imported fodder for British cavalry horses during the Anglo-Boer War of 1899-1902.

For some people unaware of its foreign origins, the cosmos is as South African as biltong or boerewors -- the dried meat and sausages eaten every weekend around the braai, or barbecue.

CONFLICT WITH LOCALS

Weed-killing cosmos sometimes threatens to bring Carstens and her team of environmental workers into conflict with locals who cannot see the problem with leaving pretty flowers alone.

Some residents also resent branding jacarandas "declared invaders" -- meaning the trees, whose magnificent October blooms turn avenues in Johannesburg and Pretoria into floral avenues of lilac snow drifts, may eventually die out as no further planting is allowed without special permission.

But Carstens says she avoids confrontation with owners of prohibited plants, turning a blind eye to the towering five-metre (16 ft) queen of the night cactus flourishing in a garden behind the reserve's fence, its mass of large yellow flowers promising a teeming new generation of alien offspring.

The plant -- of American origin like all cacti -- has large spines which act as an irritant, making it a popular fencing plant among residents of the crime-ridden city.

Carstens says she is reluctant to take residents to task over plants in their private gardens while public council-maintained land is rife with invasive alien species.

"There's no control," she said.