Introduction

Somalia is very appealing to many lovers of succulent plants for various good reasons. Somalia is a very difficult and dangerous travel destination and yet it is the natural habitat of a great number of rare and difficult to cultivate, oddly shaped and ultra succulent plants of various plant families. Whitesloanea crassa, once believed extinct and then rediscovered,  is just one example. More interesting for us is that the family of the Euphorbiaceae also contains some very special jewels originating from Somalia. To name just a few; Euphorbia turbiniformis, atrox and ponderosa. Also Euphorbia horwoodii belongs to this small group of species.

Frequently found in succulent literature

In various articles one can read about how this species was found by the late Frank Horwood (Carter 1978). The stunning difference between the juvenile spherical and the mature shrubby appearance of Euphorbia horwoodii is legendary and rarely seen in the botanical world. Almost all plants found in nature had the shape of a ball with a flattened apex.. Only rarely was a plant found with a shrubby appearance. Oddly enough the situation in cultivation is just the other way round. Nearly all euphorbia-lovers have never seen a spherical Euphorbia horwoodii in the flesh.

When one has a good look at a juvenile Euphorbia horwoodii, one can clearly see the side-branches on the sphere, or better said, see the place where the side-branches should be. It looks as if the side-branches are pressed into the sphere. The spine shields easily come off and often are totally absent. The resemblance with Euphorbia turbiniformis is striking. As soon as such a plant comes into cultivation, or I assume in nature under more favourable conditions, the side-branches start growing and it evolves into a sparingly branched  spiny shrublet. The branches are relatively thick and sturdy and have a marvellous colour pattern, which also adds considerably to its popularity amongst growers of euphorbias.

Natural habitat

Euphorbia horwoodii is closely associated with the famous grower of succulent plants Frank Horwood. However it is most likely that Frank Horwood is not the discoverer of the plant that bears his name. It is John Lavranos who earlier found just one spherical plant 55 kilometres south of Garoe, and which most likely represents (since this specimen did not survive, perhaps it would be better to say “which could possibly represent”) Euphorbia horwoodii (Horwood 1976). This single plant however died during the journey. So we will never be sure and so this find never went into the books as Euphorbia horwoodii.

Later, during the same trip, Frank Horwood again found a spherical Euphorbia 50 kilometres east of Sinugif. Initially it was believed to be a form of Euphorbia turbiniformis and surprisingly it received not much attention. Only a few plants were collected and given the collection number Lavranos & Horwood 10153. Just one plant was found, which had left the spherical phase behind and had grown into a shrublet. At that time it was thought to be a hybrid, which seemed very reasonable at the time. (Perhaps this was the later described E. umbonata? – I do not know of any positive proof that E. horwoodii occurs away from the Isskushuban area)  Euphorbia horwoodii appeared to grow in a vast area as it was found again at a 300 kilometre distance in another locality. This locality is 90 kilometres southwest of Isskushuban and here the type material was collected, Lavranos & Horwood 10188. 

Observations in cultivation

What is written above is just a short extract of all that has been written over a few decades about Euphorbia horwoodii in succulent literature. As this has already received its deserved share of attention, this was not the purpose of this article. There are a few points about the existing plants in cultivation labelled as Euphorbia horwoodii which should be brought to the attention of the interested grower of euphorbiasree different forms.

The oldest and most widespread form in collections of Euphorbia horwoodii is characterized by its thick and sturdy branches with a very striking colour pattern. The rounded branches have vertical rows of spine shields joined by a dark olive green band. In cultivation in a sunny position or under some degree of stress the branches tend to colour purple. The flowers are formed on fairly long peduncles. Because I assume this plant was the first one to be in cultivation of the three forms I want to discuss here that it most likely originates from one of the two localities visited by John Lavranos and Frank Horwood in 1973.

In 1985 and 1986 a bigger group visited Somalia and they visited the above mentioned localities again. During these journeys the collected material was initially distributed in collections of succulent plants in the U.S.A. with the designation B&L, which is short for the names of the succulent collectors Jerry Barad and Seymour Linden. The trip of 1986 is extensively described in a highly recommended article in The Euphorbia Journal (Linden 1994).

Two different forms with a B&L-number are growing in my glasshouse for several years now. The first to present to you is Euphorbia horwoodii B&L589. This particular form has thinner and less sturdy branches than the previous form and  the colour pattern on the branches is of a more dull lighter green, yellow-green and grey-green and is not made of vertical bands but of irregular spots. Most easy to recognize are the downward or sideways curved spines. This form grows the fastest, but is also the most reluctant to flower of these three forms in cultivation. Also a branch without a growing point of Euphorbia horwoodii B&L589 is the easiest one to form offsets out of a flowering eye. These offsets initially have the spherical appearance of a seedling and also have the juvenile characteristic feature of having four equally sized spines per spine shield.

The second plant with  a B&L-number to be discussed here is Euphorbia species affinis horwoodii B&L 887. Although I received it as Euphorbia horwoodii it is my opinion that it is not this species. Just like the first form dealt with here B&L 887 has quite sturdy, however somewhat thinner branches, which have the habit of growing quite vertically when grafted in the growing conditions in my glasshouse. Quite striking is the grey colour of the branches and when grown in sunny conditions they only have a haze of green. The pattern of different shades of grey is much less pronounced and is less appealing as well. They also have the habit of turning purple under stressed conditions. The spination is much weaker with small straight spines. Of the three  plants discussed here, B&L 887 is the easiest one to flower in cultivation and its flowers  attract the most attention by the bigger cyathia and brighter yellow glands. These differences  make it hard to imagine that they represent one and the same species. If it is really a different species it must originate from a different locality in nature and the mentioned differences should be constant in this separated population.

Most species closely related to Euphorbia horwoodii are found only in a small area in nature or even known only from one single locality. Euphorbia horwoodii on the other hand is known from a very wide area, which also might be an explanation for the differences found. I think the answers to this can only be found in the field. It is a pity I could not find the habitat information of these B&L-numbers. Have you tried to contact Jerry Barad? or John Trager at the Huntington Bot. Garden where the living plants were taken? 

Two faces

Euphorbia horwoodii sometimes is called the euphorbia with the two faces, a subglobose plant and a shrublet (Jenkins 1998). The seed of Euphorbia horwoodii is for a euphorbia rather small, as is the seedling after germination. The seed shell remains on top of the sticky seedling. Grains of sand and dust cling onto the tiny seedling, which obviously protects it against the harsh climate in which it has to grow and also makes it almost impossible for predators to find it. This survival strategy is found also in other related species in this group of plants.

How well adapted they might be in their natural environment ,but how difficult these seedlings are raised in our hothouses in Western Europe! Their great need of light and warmth, slow growth rate and their extreme vulnerability to any excess moisture are the cause of this. Even if we are realistic, it is a true challenge to raise these plants from seed, and we accept the self-imposed burden and pretend it can be done. Some success can be achieved however and very small spherical seedlings, only a few millimetres in diameter, can be successfully grafted on to a well growing stock. Several times we have observed that a seedling does not grow into a sphere but right away starts growing as a spiny branch (see picture 12).  Sadly I must admit that the few times we could harvest ripe seeds of Euphorbia horwoodii, we did not keep the different forms separate. For this reason we can only speculate on which forms have two faces or not. 

Offsetting of plants in cultivation

Above is already mentioned the two ways of making offsets of Euphorbia horwoodii B&L 589. There are some similarities to be seen between these seedling-like offsets and the shoots made to form a main stem of the euphorbias in the Section Triacanthium, which I described before in this journal (Van Veldhuisen 2006). The best way one can see that these offsets revert to the juvenile phase is that the spine shields with four equally sized spines form a cross. Also the spine shields easily come off. So far I was unsuccessful in rooting such a little ball, but grafting is an easy trick. The result is a much more handsome specimen than when a single branch is grafted. The branching of a seedling-like graft is at a lower level just above the stock and so the graft starts growing more  broadly. I have to add that both the other plants discussed here also have the habit of producing seedling-like offsets, but not as easily as B&L 589.

To conclude this article I would propose that for the interested grower of euphorbias it is worthwhile growing all these three different forms in your collection. The differences are easy to see and recognizable at a distance. They are examples of the most beautiful and special plants of the genus Euphorbia. The fact that even with such a  widely cultivated Euphorbia there is still so much to discover is for me very special..

References

• Carter, S. (1978): Succulent Euphorbia of Somalia – Part 3, Cactus & Succulent Journal (U.S.), 50(1): 25-29
• Carter, S. (1994): Euphorbia hunting in Somalia, The Euphorbia Journal, Vol. 9: 84-93
• Horwood, F. K. (1976): Succulent Safari to Somalia, Cactus & Succulent Journal (U.S.)
• Jenkins, T. (1998): Two Faces of Euphorbia horwoodii, British Cactus and Succulent Journal 16(4): 221
• Linden, S. (1994): Somalia 1986, The Euphorbia Journal, Vol. 9: 94-123
• Marx, G. (1992): Euphorbia horwoodii Carter & Lavranos, The Euphorbia Study Group Bulletin, 1:23-26
• Veldhuisen, R. van (2006): Euphorbia Subgenus Euphorbia Section Triacanthium Jacobsen, Euphorbia World, 2(2): 6-13