How an idealistic tree-planting project turned into Kenya's toxic, thorny nightmare

The Guardian

22-05-2025

For his entire life, John Lmakato has lived in Lerata, a village nestled at the foot of Mount Ololokwe in northern Kenya's Samburu county. 'This used to be a treeless land. Grass covered every inch of the rangelands, and livestock roamed freely,' he says.
Lmakato's livestock used to roam freely in search of pasture, but three years ago he lost 193 cattle after they wandered into a conservation area in Laikipia – known for the fight over land access between Indigenous pastoralists and commercial ranchers.
'Some of my cows were shot dead,' he says. 'People were killed.' Of the 200 cattle Lmakato once owned, only seven remain.
One of the main reasons the livestock of Lmakato, 48, crossed over into the conservation area was the mathenge, as the mesquite shrub (Neltuma juliflora, formerly classified as Prosopis juliflora) is known in Kenya. The grassland landscape is so dominated by inedible mathenge trees that cattle have to wander further to graze. Introduced in 1948, mathenge – a plant native to South America – became widespread throughout east Africa in the 1970s.
It was seen as a remedy for creeping desertification, providing tree cover and preventing soil erosion in drylands, as well as a source of fuel and animal fodder. In Kenya, the UN Food and Agricultural Organization and the government actively encouraged its planting.
As the shrub spread, it quickly became a nightmare. A closely related species is now considered one of the world's worst invasive floral species.
Since its introduction, mathenge has spread across the country, and nearly three-quarters of Kenya is at risk of being invaded. It has colonised large swaths of the country's arid and semi-arid regions, choking vast rangelands and drawing moisture from the soil with its deep roots.
The Kenya Forestry Research Institute (Kefri) estimates it has encroached on 2 million hectares (7,700 sq miles). According to Kefri scientists, it is spreading at a rate of up to 15% a year.
In 2006, its harmful effects led to a court case against the Kenyan government, when people in Baringo – one of the country's hardest-hit regions – filed a petition seeking compensation for the impact of the introduced mathenge trees. The court ruled against the government.
'The spread is so fast that it has caused entire communities to be displaced, schools to close, and even disrupted river flows, as the plant blocks watercourses – contributing to flooding and displacement,' says Davis Ikiror, Kenya-Somalia country director for Vétérinaires Sans Frontières (VSF) Suisse, an organisation that has worked in Kenya for more than two decades.
In Samburu county, where more than 60% of the population are pastoralists and 30% mix herding with small-scale farming, livestock is a lifeline. Some animals die from mathenge toxicity after ingesting it in large amounts. While grazing, the plant's tough thorns injure the animals by lodging in their feet and its sweet pods – high in sugar – cause dental decay and the loss of teeth among the animals.
Livestock have become unwitting vectors in mathenge's dissemination: as animals graze, they eat the pods and deposit the seeds with their dung.
In 2008, Kenya declared mathenge to be a noxious weed and passed laws requiring people to clear invasions of the plant or report them if the infestation was unmanageable.
'There's no way we're going to eliminate the plant. That's why we need to control it,' says Ramadhan Golicha, an environmental officer with the Isiolo county government.
But with resources yet to be allocated for a national action plan on clearing the plant, some projects are exploring the possibility of turning it into raw material. By making mathenge a commodity to be exploited, they hope to keep its spread under control.
Lmakato was among 25 community members taking part in one such project, run by VSF Suisse along with the University of Nairobi and the regional governments of Samburu and Isiolo. 'We've learned how to use it to our advantage,' he says.
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One of the first techniques they adopted was turning mathenge pods into livestock feed by grinding and mixing the seeds with straw, bean husks and acacia pods. 'That allows us to produce fodder, which we then store for the entire dry season,' says Martina Lenanyangerra, another community leader involved in the project.
A study of this combination of feed is being carried out nearly 180 miles (300km) away, at the University of Nairobi. In June last year, Edward Musya, a vet and master's student studying animal nutrition, travelled to Merti – one of the worst-affected areas in the country – to collect more than 20 types of local plants to combine with mathenge.
'The goal is to formulate a feed that is high in nutrients and easily digestible for livestock,' says Musya. 'When mixed, the sugar content in the pods decreases, making it a safe feed while also helping to contain the plant's spread.'
The training in Samburu also introduced briquette production as a fuel source, using barrel-shaped kilns to turn mathenge wood into charcoal. In Samburu – where more than 95% of people rely on firewood or charcoal for cooking – this has helped ease pressure on native trees while also offering a new source of income.
But to rein in the plant's spread, larger interventions will probably be required. Mathenge's deep root system, reported to go as down as far as 35 metres below ground, siphons off precious water that other species require.
Its branches grow so thickly and its roots are so deep that an incursion can even alter the course of rivers. In some parts of Isiolo, such as Ires a'Boru, displacement of water beyond the banks caused flooding that forced communities to relocate.
The slowed water flow and dark canopies create ideal breeding grounds for mosquitoes – which worsen the spread of malaria and Rift Valley fever, and kala-azar, or leishmaniasis, which is carried by sandflies.
The plant's chokehold has wrought havoc on biodiversity. Once seeded, it forms a dense canopy that stops other plants growing by blocking out light, says Douglas Machuchu, project manager at VSF Suisse.
To protect entire ecosystems, the shrub must be kept in check. 'Where mathenge grows properly, nothing grows underneath,' he says.
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