Words and pictures by Ida K. Rigby, for Let’s Talk Plants! June 2024.
Fever Tree
Featuring the fever tree, Vachellia xanthophloea, takes us to South Africa’s infamous Crooks Corner on the storied Limpopo River. In 1902 Rudyard Kipling’s Just So Stories fixed fever trees and the Limpopo in the European imagination.
(Editor’s note: Does this article make you want to read Kipling’s 1912 edition of Just So Stories? You can read the Project Gutenberg eBook of Just So Stories for free: “This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org.”)
Crooks Corner lies at the confluence of the Limpopo and Luvuvhu Rivers where they separate Zimbabwe, Mozambique and South Africa. In the early 20th century, the area was a haven for smugglers, gunrunners, fugitives, illegal labor recruiters and ivory poachers because when rangers came after them all they needed to do was cross a river into another country. Donkey trains carried ivory to the port of Sofala, Mozambique. Fig, jackal berry and fever trees, festooned with vines line the two rivers and the banks of their tributaries, where fish eagles perch in the fever trees.
We walked on sand bars, but stayed away from the pools out of which crocodiles could have lunged at any moment. The day before we arrived a tourist on a walking safari foolishly approached an elephant, who swiped her with her trunk, and the elephant was killed. The elephants all over the area were on edge because the news of her death had spread among the herds. Our driver stayed much farther away from the agitated elephants than usual.
Until 2005 the fever tree was labeled Acacia xanthophloea. Vachellia refers to George Harvey Vachell (1789-1839), a British chaplain who collected plants in China. The species name xanthophloea comes from the Greek for yellow bark.
Fever trees are semi-deciduous, grow up to 80 feet tall, have a spreading crown and are short-lived. They usually occur in groups, creating their own forests. (Trees South Africa, treesa.org)
The tree’s smooth, photosynthetic bark is lime green and covered with a sulphur-colored powdery bloom. The bark is slightly flakey.
Since the fever tree grows in areas with shallow water tables, marshes, pans that periodically flood and riparian zones along rivers, malarial mosquitoes are often prevalent in fever tree forests. Early settlers associated the trees with malaria, hence the common name “fever” tree. By contrast, indigenous people used fever tree bark as a malaria preventative.
Pairs of strong, white spines grow along the branches, which helps protect birds’ (such as weavers’) nests from snakes. Nodules on the roots contain nitrogen fixing bacteria. The trees are open and sculptural as the structure of the greenish branches is clearly visible.
Flower spikes at nodes produce a ball of sweetly scented, bisexual flowers from August through November; towards the end of branches up to 10 flower spikes form clusters.
The small leaves open in the morning and close at night and in dry weather. They also close slowly when touched.
Fever tree wood is used for furniture and carvings but must be seasoned so it won’t crack. The wood is also used for posts, construction, firewood and charcoal. In some areas the largest branches are used to keep hippos out of the fields. Leaves and pods are fodder for livestock. The bark is used to treat fevers and eye conditions and to make a good luck charm.
Elephants eat the young branches, leaves and roots; giraffes and vervet monkeys eat the leaves and pods. Monkeys and grey go-away birds (so called because their cry sounds like “go-away”) eat the flowers. Insects pollinate the flowers. (pza.sanbi.org)
Baboons eat the green seeds and gum; a water-soluble sugary substance exuded to seal wounds and prevent infection by bacteria and fungi. Bush babies also eat the gum. (Tree South Africa, treesa.org.)
Baboons were enjoying the flowers when I visited Crooks Corner in August 2016.
The fever tree has many virtues as a landscape tree, including its form, coloration and fast growth. On the other hand, it’s a good example of the dangers of introducing alien species into gardens. In Australia, and even in its home in the African savannah, the fever tree has the potential, as a dominant pioneer in areas newly cleared for grazing, for becoming an invasive pest.
Therefore, government policy in Queensland and Western Australia is one of prohibition, detection and eradication before the fever tree becomes a major pest in riparian and semi-arid and arid habitats. (Government of Queensland, daf.qld.gov.au)
Specimens detected in a zoo in Perth and in gardens in Queensland have been destroyed. It’s a good reminder that seemingly climate appropriate, enticing plants can be potentially destructive, invasive species.
