The bushveld candelabra tree (Euphorbia cooperi) can reach up to seven metres high. Here it grows in shallow soil on this large granite dome at Gutschwa Kop, north of Nelspruit.

Through history, the spotlight has often shone on the euphorbia, or spurge, family of plants, which has been known to humans for millennia. Euphorbia was reputedly named after Euphorbos, said to be a physician to Juba, the ruler of the ancient kingdom of Mauretania in west Africa more than 2 000 years ago.

The euphorbia family, Euphorbiaceae, is also found far and wide across the temperate and tropical world and consists of more than 8 000 plant species.

ENDEMIC EXCITEMENT

What is exciting about euphorbias in southern Africa is the high proportion of species that are endemic. About 172 species are found in our subcontinent – defined in this study as Botswana, Lesotho, Namibia, South Africa and Swaziland. Almost three-quarters of these, or 74%, are endemic and so grow naturally only in this region.

What is more, almost all are succulent. This is not at all the case, for example, in the Americas, in Eurasia and the northern hemisphere generally.

TAKING A LOOK

Settlers at the Cape such as soldier and traveller Colonel Robert Gordon (1743-1795) were early cultivators of euphorbia in their gardens. In the late 18th century, plant collectors at the Cape such as the Swede Carl Thunberg (1743-1828) and the Scot Francis Masson (1741-1805) sent herbarium and live specimens of South African euphorbia back to Europe. More were added the following century by James Bowie (c.1789-1869), Christian Ecklon (1795-1868), Carl Zeyher (1799-1858) and the Drège brothers, Carl (1791-1867) and Johann (1794-1881).

CLASSIFYING FINDS

At Kew, the plants attracted the attention of Nicholas Brown (1849-1934), assistant keeper of the herbarium, who carried out the last revision of the southern African euphorbia species for volume 5, part 2, of the Flora of Tropical Africa, published in 1911/1912, and for volume 5, part 2, of the Flora Capensis, published in 1915. South African botanist R. Allen Dyer (1900-1987) worked on the succulent euphorbias of the Eastern Cape and co-authored the major two-volume work on southern Africa’s succulent euphorbias in 1941 with the Americans Alain C. White & Boyd L. Sloane.

Regional accounts for Namibia and Botswana have appeared in the eight decades since then. Much more extensive exploration has also brought new species to light, mostly in formerly inaccessible areas such as northern Namibia’s Kaokoveld. This new book brings together the results of the exploration and research of the last 80 years, ensuring that the distributions of species and their variations have been clarified.

Another evolution in southern African euphorbias is this modified growth form, where short shoots are packed into a dense mound of plants, such as with this Euphorbia clavarioides at Ubala, north of Kokstad. 

RAIN SMART

Among its many species, euphorbia has adapted to a wide range of rainfall. In southern Africa, for instance, E. giessii, E. lignosa, E. phylloclada and E. virosa grow in the hyper-arid coastal Namib desert with less than 50 millimetres of rain a year.

In the Drakensberg’s moderately moist (mesic) grasslands with at least 1 000 millimetres annually, E. clavarioides, E. gueinzii, E. natalensis and E. striata occur.

Wetter still are the granite domes of northern Mozambique where annual rainfall reaches 2 000 millimetres. E. marrupana, E. mlanjeana, E. namuliensis, E. unicornis are among the locally plentiful species of euphorbia there.

In southern Africa, many of the species that are found most widely can tolerate a wide range of rainfall. E. avasmontana, for instance, grows equally well receiving between 50 to 600 millimetres a year and E. clavarioides from 250 to more than 1 000 millimetres a year.

North of Molepolole, Botswana, bekruipbos (Euphorbia spartaria) grows among scattered trees in pale pinkish Kalahari sand.

ROAD CHALLENGE

When you are travelling over the holiday, you are likely to spot euphorbia in most habitats in southern Africa, except in dense montane forests. Species inhabit sands, flat areas from stony to heavily loamy, stony slopes and shallow soils on solid granite ‘whalebacks’.

Some smaller species benefit from the presence of protective shrubs but do not wholly depend on protection from other plants for their survival. They may even start off life as a small seedling in the open, protected by some stones or larger rocks.

Unlike plants belonging to the stapelia genus, which can be difficult to find in nature and are often very scattered, plants of euphorbia are mostly easily found and they are usually plentiful where they occur. Some of the small species with spreading, usually short, branches from a thick stem (medusoid) species occur sporadically so may be more difficult to find but most species are big enough for you to spot from a passing car.

This noorsdoring (Euphorbia ferox) growing near Kommandodrift, south-east of Cradock, has its branch tips covered in cyathia, an inflorescence that looks like but is not actually a true flower.

SANDVELD

Sandy habitats are plentiful in southern Africa. The low-lying, flat, coastal sandy areas of the western Cape sandveld are fairly rich in euphorbia species.

Many euphorbia are widespread across these sands and may sometimes be dominant. Only E. stapeloides is endemic here, unlike the many species of succulent ice plants (Aizoaceae).

What are widely called Kalahari sands cover a large portion of central southern Africa, stretching from eastern Namibia and parts of the Northern Cape through Botswana. This ‘Kalahari desert’ is not intensely dry and receives about 200mm of rainfall a year in the southwest and as much as 500mm in the north.

But there are not many euphorbia species in the region – watch carefully for the very scattered occurrences of E. duseimata, E. maleolens and E. spartaria. Look on calcrete outcrops around shallow pans along sand-choked drainage systems for E. crassipes and E. juttae.

Euphorbia species become less and less diverse among the sands as you move east across southern Africa. On the region’s eastern flank, only the geophytic E. gueinzii and the herbaceous wild spurge or inkalamasane, E. natalensis, inhabit the grassy, settled dunes of the KwaZulu-Natal and southern Mozambican coast.

Where the patches are sufficiently isolated and not too regularly burnt, a wealth of succulents may develop, especially aloe, bobbejaantou (Cynanchum), euphorbia, kalanchoe, plectranthus and several members of the periplocoideae, a subfamily of the dogbane (Apocynaceae).

Many species belonging to the section within the euphorbia genus that bear pairs of spines at the base of leaves, along the stem and branches, are found only in these habitats, especially in Angola and Mozambique.

In those species which become large, such as the naboom (E. ingens) once the branches are out of reach of browsers, the spines disappear.

On the left, this noorsdoring (Euphorbia ferox) from Steytlerville in the Eastern Cape is densely armed with spines, one appearing in the axil between each leaf and shoot. On the right, the spines are more densely clustered on this bobbejaannoors (Euphorbia polygona) from Toorwaterpoort in the Eastern Cape.

In South Africa, such domes are present on the eastern and western sides. Those on the eastern side may be inhabited by species such as E. clavigera, E. cooperi, E. schinzii and E. vandermerwei but only the last is restricted to them.

In western South Africa in the winter rainfall parts of the country, the much drier domes of gneissic rock do not have any distinctive euphorbias on them, although they may also be rich in succulents.

This edited extract is adapted from the introductory chapters of the two-volume work Euphorbia in Southern Africa by Peter V. Bruyns (Springer Nature Switzerland).
Peter Bruyns (peter.bruyns@uct.ac.za) retired in 2022 as a professor in the Department of Mathematics at the University of Cape Town. He has published many peer-reviewed papers on various groups of succulents, especially the stapeliads and ceropegia of the Apocynaceae, euphorbia, Crassulaceae, Didieriaceae and, as co-author with Cornelia Klak, on the vygies (Aizoaceae). His two-volume, sole-authored work Stapeliads of Southern Africa and Madagascar was published in 2005.

Gommelkbos (Euphorbia gummifera) grows in arid conditions such as near Halenberg, east of Lüderitz, in the Namib desert.