The program currently runs 300 projects in South Africa's nine provinces. Teams canvass infested areas to hack out and destroy invasive species. Participants receive skills-training and education.
Preston, who is also a member of the Global Invasive Species Program, says the South Africa government has voted a substantial sum of money to tackle the triffid menace.
But the problem with triffid weed and other invasive species in South Africa has generally grown so big that authorities are looking at new ways of getting the public more fully behind the anti-invasive program.
In some regions, nonnative plants from around the world have taken so well to their new habitats that they are posing a serious threat to the rich plant and animal life that makes South Africa the world's third most biodiverse country.
Water hyacinth and other plants are clogging up lakes and rivers and invading forests, mountains, grasslands, and vast semi-deserts. In addition, the alien species often pose a fire hazard, making vegetated areas more fire-prone and sapping the country's precious water resources.
Preston believes the anti-invasive campaign must become more nuanced to get the public support it deserves. "The emphasis needs to shift from what is necessary to what is feasible as well."
Demand for a more flexible approach may be seen Pretoria, the nation's capital, which is famous for its jacaranda trees.
The tree, an import from Argentina, was first brought to the city in 1880 and large-scale planting started soon after. A hundred years later, more than 50,000 jacarandas adorned the city's streets, parks, and gardens.
South Africa's parliament passed an act three years ago that declared 198 exotic species as "weed and invader" plants. The law provides for stiff fines and jail sentences for trading in or keeping the plans on public or private land, including suburban gardens.
Officials recently listed the jacaranda under a special category of invasive species, which allows the city to keep existing trees but not to replant them when they die. The idea was that attrition would gradually eliminate the water-hungry alien trees from the city.
But local residents have put up such a fight that city authorities may relent and allow them to replace dead trees.
Preston, of Working for Water, says the alien-invasive problem is so immense that it may well make sense to get the distractive fighting over the jacarandas out of the way.
"We need all the support we can get. We don't want to alienate people," he said. "Jacarandas are a problem, but not to the extent that they should be allowed to get in the way of the bigger objective. We have to box clever and rather ensure support for the fight against far worse species."
But in general authorities are not taking a more lenient approach. Cities employ municipal officers dubbed "plant police," who knock on suburban doors to inspect gardens. Errant owners get warned to remove listed alien-invasive plants or be charged.
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