Euphorbia grandicornis (Guest column by Theo Heijnsdijk)

Name
Euphorbia grandicornis is a representative of the large group within the genus of shrub-like succulents with spine pairs, also referred to as the diacanthium section. Plants belonging to this group usually have 2 spines that are facing outwards from the edge of a shield. Another example of a member of this group is E. canariensis. Sometimes there are 3 or 4 spines and sometimes only one (for example, in E. unispina).
Grandicornis literally means ‘with large horns’.
In English-speaking countries, the plant is also called ‘Cow’s horn Euphorbia’. The thorns remind me more of the tentacles of a snail. See Fig. 1.

In the next image, we see (between the spines) the scars of the tiny leaves that are formed in the new growth and that will soon turn yellow, shrivel, and fall off.

Other names that are widely used are ‘Big horned Euphorbia’, ‘Rhino thorn’ and even ‘Zig-Zag Cactus’. There is also a cristate form and this looks even much more ferocious than the normal plant. In Afrikaans, the plant is called renosterdoring (a renoster is a rhinoceros and doring stands for thorn).

History
The plant has probably been in cultivation for quite a long time, but for many years it went undescribed.
We first come across the name Euphorbia grandicornis in 1889 in the first part of the 2-volume book ‘Pflanzenbiologische Schilderungen’ by the German professor of botany dr. Karl Immanuel Eberhard Ritter von Goebel. In a general story about Euphorbias, he discusses a number of aspects of E. grandicornis as if it were a well-known plant. It is nothing like what we mean by a description nowadays. However, we do find a beautiful pen drawing of a top cutting (Fig. 3).

In addition, there is a drawing of the leaves in the new growth and of a cross-section of the trunk in which the peculiar, twisted shape of the winged ribs is clearly illustrated (Fig. 4).

Von Goebel reports that he has not been able to find any data on the habitat. However, from the physical appearance of the plant with the wide thin ribs and therefore a large surface area – similar to, for example, leaf cacti- he concludes that it is unlikely to be exposed to long dry periods in its natural environment.
After von Goebel it was quiet until 1893. In that year, an article by a Mr. J.E. Weiss, Reader in botany, appeared in the periodical ‘Dr. Neubert’s Deutsches Garten-Magazin’ under the title “Empfehlenswerte Cacteen” (“recommendable cacti”). He took the concept of cactus rather broadly because the last ‘cactus’ discussed in his article is Euphorbia grandicornis. There is even a photo, the very first one published, but the print quality was rather poor in those days (Fig. 5).

Original description
The plant was still not officially described at the time. That was done in 1897 by Nicholas Edward Brown in part 26 of Hooker’s Icones Plantarum. That is why the author’s citation which is always added to the species name in scientific literature, in this case is ‘Goebel ex N.E. Br.’. The description by Brown, who was employed by Kew Gardens near London, was based on a plant that had been cultivated there since 1876. The description with Latin diagnosis was accompanied by two drawings of quite a big specimen. I don’t particularly like these drawings. Brown also mentioned a locality: South Africa, Umfolosi River, Zululand.

Habitat
Nevertheless, there was apparently uncertainty as to where the plant occurred in nature, because in 1904 in Engler’s ‘Botanische Jahrbücher’ ( ‘Botanical Yearbooks’) in a monography on Euphorbia’s in the section Diacanthium the author, F. Pax, writes: “Vaterland unbekannt, vielleicht Africa”  (Native country unknown, maybe Africa). And in 1907 Alwin Berger in his ‘Sukkulente Euphorbien’ says: “Homeland ?”  and further down:
The origin of the plant is unknown. However, it is not plausible that it was only recently introduced, because the large specimen in Kew mentioned above, indicates that it is an old inhabitant of our greenhouses.). Brown and others later compiled the part about the Euphorbiaceae in Sir William T Thiselthon-Dyer’s ‘Flora Capensis’. This part appeared in 1915 and here E. grandicornis is also treated.  The last line I find quite funny: “Distribution: Easter Region: Zululand, stone! Marriott! And cultivated specimens!”.
I suspect Marriott is a name, but I haven’t been able to find any information. But especially all those exclamation marks intrigue me. I have no idea what the writer wanted to emphasize with this.
In modern literature, it is made clear that the range is quite large: South Africa (Kwazulu-Natal), Swaziland, Mozambique, Kenya. There the plants grow at low altitudes (up to 400 m) in small groups between grass or shrubs. They have 3 or 4 ribs and can be up to 2 m tall. It seems that large plants often succumb to their own weight. The variety sejuncta (described in 1970 by Leach), which remains smaller and sometimes grows lying down, has 2 or 3 ribs and is known only from a site in Mozambique. There it grows in the company of Aloe chabaudii, Euphorbia corniculata and E. tirucalli on granite slopes at altitudes between 380 and 700 meters.

Flowering
In a sunny location, a plant that is preparing to flower stands out because of the vivid red colour of the developing cyathia (Fig. 6).

These appear on the uppermost and therefore youngest, segments of the stems. There are always 3 cyathia together, but often only the middle one will fully develop. It only produces male flowers (stamens). The two outer cyathia are bisexual. First, the male flowers develop (Fig. 7)

and when they dry out, the female flower (pistil, fig. 8) follows.

All cyathia are bright yellow in colour. This type of inflorescence with a central male inflorescence flanked by 2 bisexual cyathia is often found in Euphorbias, for example in E. canariensis.
Ripe fruits are about 8 mm in diameter and purplish-red in colour (Fig. 9).

Cultivation
A well-drained mixture with little organic material and a lot of additions such as lava, pumice, clay chunks, etc. is recommended. It’s best not to use peat. Give ample water from March to September. In winter, the temperature should be at least 12 °C, but preferably a little higher. Propagation is by cuttings, which root quite easily, or by sowing. Seed is fairly well available and it is fascinating to see how a wildly thorned plant develops from the delicate seedling with its 2 cotyledons (Fig. 10).

In frost-free areas, E. grandicornis is recommended for hedges. Pruning is well tolerated. Goats eat the corners off but leave the thorny sides alone. The juice doesn’t seem to hurt them.

Literature:
Berger, A. (1907). Sukkulente Euphorbien: 52-53.
Brown, N.E. (1897). Hooker’s Icones Plantarum 26, plates 2531, 2532.
Brown, N.E.; Hutchinson, J.; Prain, D. (1915). Euphorbiaceae in Thiselton-Dyer, Flora Capensis 5, sect 2, part 2: 367-368.
Goebel, K. von (1889). ‘Pflanzenbiologische Schilderungen’ 1: 62-63.
Pax, F. (1905).. Monographische Übersicht über die afrikanische Arten aus der Sektion Diacanthium der Gattung Euphorbia in Engler’s Botanische Jahrbücher 34: 74.
Weiss, J.E. (1893). Dr. Neubert’s Deutsches Garten-Magazin 46: 291.

First published in Succulenta 92 (4) 2013. Translation from the Dutch by F.N.