Willem says: 'The Phyto-Philes are for plant lovers of every size and shape, colour and flavour. As with my Colours of Wildlife column, I'll be featuring one species per article, illustrated with sketches, paintings, and/or photos. Over time I hope to be showcasing the amazing diversity of weird and wonderful plants that occur in South Africa, while also from time to time looking at the flora of other countries. While featuring many spectacular species, I'll not be neglecting the smaller, more humble kinds that are nevertheless fascinating in their own right.'
I have for you yet another truly bizarre plant – the Transvaal Sesame-Bush, Sesamothamnus lugardii. Yes, this is a relative of the plants that yield those sesame-seeds! Sesame plants, Sesamum indicum, are slender annual herbs, while this is a very stout, long-lived, semi-succulent shrub or tree. The link with the sesame plants can best be seen from the flowers: sesame bushes have showy trumpet-like flowers similar to those of sesame plants.
Let's look at the name of this species. 'Transvaal' is the name of a province of the 'old' South Africa, north of the Vaal River. Today what used to be the Transvaal has been divided up, into the Gauteng, Northwest, Mpumalanga and Limpopo Provinces. This sesame bush occurs only in the Limpopo Province and marginally in the furthest north of Mpumalanga. It also occurs in Botswana and Zimbabwe, but in those countries it also stays close to the Limpopo River Valley. Another name for it is the Eastern Sesame Bush, since there are three other sesame bushes that occur in Namibia, further to the west. But then there are related species in Kenya, Ethiopia and Somalia, even further to the east! In the end I would vote for 'Limpopo Sesame Bush' as the most appropriate name for it.
In South Africa, this species is rare except in the very far north. It can easily be seen in the Messina Baobab Reserve and in the Mapungubwe National Park. It inhabits very hot and quite dry regions, usually in dry woodland, with baobabs, corkwoods, Star Chestnuts and other weird plants. The sesame bush is a very striking plant. It doesn't grow very tall, 6m/20ft at the most, but develops a proportionately very fat trunk. It looks like a mini-baobab. Actual baby baobabs look quite different … they're often not recognized for what they are! But the sesame bush can easily be distinguished from baobabs by its coppery, flaking bark. Baobab trees have a thick, reddish bark sometimes reminiscent of congealed wax. Also, sesame bushes bear (in spring and summer) very small, undivided, greyish leaves, while baobabs have (also only during the rainy season) large, bright green leaves, single when small, and palmately (i.e. like a hand) divided when large.
Sesame bushes can also look very similar to star chestnuts. They can be separated from them by their growth form, with their branches and twigs typically being more 'upright' while the star chestnut's branches tend to spread more. The sesame bush has a bronzy peeling outer bark, with a greenish underbark, while the star chestnut's bark peels in several layers each with a different colour, from wine-red to light cream. In leaf, the sesame bush's simple, narrow, greyish leaves are very different from the star chestnut's delicate roundish to three-pointed leaves.
Lastly, if you inspect a sesame bush closely, you'll see that it bears short, straight (or slightly curved) spines on the branches. Both baobabs and star chestnuts are spineless. So this is the final giveaway. All taken into consideration, you can identify a sesame bush with 100% confidence if you're in the region where it grows and can have a good look at a specimen!
And quite a bizarre specimen it is. Sesame bushes look like they should be growing on Mars or something thereabouts! Indeed the region where they grow has a rather otherworldly ambience quite apart from the sesame bushes. The northern Limpopo Valley is dry and sparsely vegetated, with rocky hills and other formations that look like piles of rocks stacked by giants, or walls they had built aeons ago. These geologic features also look ancient, as indeed they are. The rocks can have various colours, from black, to vivid red, to grey or yellowish. Soil and rock is frequently exposed, the plant cover being so sparse. But the plant cover that does exist features such huge things as baobabs! Many other species of tree in the region could be considered giant succulents. Of these the sesame bushes are around in the middle size-wise. But they are very gnarly and striking in appearance.
It is strange then that such craggy trees bear such delicate flowers! Sesame bush flowers have a tube up to 10 cm/4" long, and are up to 6 cm/2.2" in diameter. The petals are white and rather frilly. These tubular, sweetly scented flowers are probed by the long bills of sunbirds, for their nectar. They're followed by the fruit, which are capsules up to 5 cm/2" long. These dry out and split open to release the seeds. The seeds are unlike sesame seeds, being flat and papery, enabling them to be blown far and wide by the wind.
We still don't know much of the ecology of this tree. Not many animals are known to browse it, but elephants sometimes do, and may even uproot entire plants. Humans don't seem to use them either … at least, for any other than decorative purposes! They are not much grown, but are sought after by succulent collectors. It is said that they are hard to grow from seeds … I did not find them so, I got about twenty-five plants growing from seeds I bought from Silverhill seeds. I also successfully grew them from cuttings. In both cases they should be planted in sandy, well-draining soil and kept warm and somewhat on the dry side, definitely not wet, but with a bit of watering every week or two. In my garden I now have a number of nicely sized-and-shaped individuals. They take on their characteristic form from the start, and fatten up nicely. They flourish in hot and dry regions; they can take only the lightest frost. They could probably become interesting container specimens kept in a sunny place indoors in colder climates. They're definitely worth trying to make bonsais from!
This and other sesame bush species belong to the Pedaliaceae, a small plant family found in Africa, Madagascar and Asia. The species range from small herbs to these tree-like plants, and all of them are quite interesting! There are a few other thick-trunked genera such as Pterodiscus of Africa and Uncarina of Madagascar. Most of them have very interesting flowers. They display interesting features in their distribution also, the sesame bushes as well as Pterodiscus being found in dry regions in Southwestern Africa as well as Northeastern Africa – with a huge region in between where they are not found at all! This indicates that long ago a dry corridor linked these two dry regions, but then moist climates intruded and separated these regions. The period of time they've been separated since has allowed for the evolution of different yet still closely related species.