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Succulent Euphorbias--Part 5
The final group of succulent euphorbias consists of those commonly called the “twin-spined” euphorbias. A major, very distinct subset of these occurs on the island of Madagascar, and I’ll conclude my discussion of this giant genus with them, but hundreds of other, more closely related “twin-spined” species grow over much of the African continent and extend to Arabia, India, and a few as far as Thailand. The paired spines of these species are derived from appendages found at the base of leaves, generally, though not universally, assumed to be the stipules (minute, paired tendrils). The stems and branches of these species bear a set of spines at every node, up and down in longitudinal patterns. Twin spines actually begin as a juvenile pair of double spines, and some species retain this double pairing, resulting in four spines per node. In other species the spines are fused and seem single, or one pair becomes fused, resulting in three spines, and in a few species the mature stems lose their spines, but in all cases they start out with similar structures, providing botanical evidence of a phylogenetic relationship among them.
It’s easy to distinguish these euphorbias from those discussed earlier with spines derived from flower stalks, as the twinned spines occur in obviously symmetrical arrangements while peduncular spines look more scattered about a stem or branch. Aside from fundamental spine arrangement, however, this large group of species can take on almost any appearance imaginable, from a sixty foot high tree to a shrunken sphere less than an inch across. Let’s start with the bigger and more typical kinds, and then move on to some of the real curiosities of the group.
A couple of these tall growing euphorbias are almost ubiquitous in cultivation, and have been for years. Among them, Euphorbia trigona, which retains it good-sized leaves for longer than most of the spiny euphorbias, and E. lactea, often found in cultivation either in a crested or variegated form, have been widely cultivated for so many years that their exact place of origin is unknown. Both were described from plants found in India, and that country has a number of other large, tree or shrub-like species, including commonly cultivated plants such as E. antiquorum and E. nivulea. Less frequently encountered, Euphorbia royleana, from the Himalayan foothills of northwest India, produces long-lasting, rather succulent leaves at the ends of its cylindrical, unsegmented main stem and branches. It grows at altitudes high enough to receive regular frost in winter, but whether or not it could survive our winter rains and fog is another question. Traveling west takes us to Arabia and the Horn of Africa, where a number of tree euphorbias make their home, most widely distributed and best known probably being E. ammak (often, and mistakenly, called E. candelabrum, actually a rarely seen plant from Angola). The growth form of E. ammak typifies that of many tree euphorbias: a tall main trunk with a burst of upright growing branches that begins a about halfway up the trunk. Plants with this general shape grow up and down the continent of Africa, some with upright branches like ammak, some with sprays of horizontal branches. Different species have denser or more lax branching arrangements; in the case of the distinctive and very wide spread E. ingens, the horizontal spread of the branches may equal the height of the plant. The significant differences between species consist of flower details and the angles, joints and constrictions of the branches. As a result, even though they share the same general morphology, tree euphorbias display of wide variety of form. The semi-tropical E. bougheyi, from Mozambique, has delicate two angled branches when young that look almost like wavy green seaweed, and only develops a thickened, three angled trunk with considerable age. Many tree species, both wide ranging and quite rare, are South African. They include E. zoutspanbergensis, one of the most attractive, a graceful, slender growing tree with a very restricted range. E. excelsa is both stouter and much taller. E. grandidens grows a lofty trunk, to over sixty feet, with a sparse spray of branches near the very top. Most of these South African tree euphorbias live in dry but somewhat tropical regions, where temperatures never get too low, but several, among them E. triangularis, flourish in the botanical garden at Kirstenbosch, in Cape Town, where the climate matches that of the Bay Area almost exactly, and several of the widely distributed species have made their way into outdoor gardens around the San Francisco Bay.
Euphorbia cooperi occurs over a large portion of southern Africa, and resembles a somewhat smaller version of E. ingens, with a shorter trunk. Several western species from South Africa and Namibia have pretty much eliminated their trunks entirely and grow in the form of candelabras, with many upright, arching stems. Probably the best known of these is E. virosa, notorious for its highly toxic sap. It’s a beautiful plant, with a short, spiraling primary stem and straighter growing secondary branches marked with white, horny rims. Its segments are somewhat pyramidal in shape, wider at the bottom and tapering to the top, where the next segment grows. Although many euphorbias have a reputation for toxicity, E. virosa probably is among the most poisonous, with local use as an arrow poison or fish poison, and it’s advisable to handle the plants with caution. E. avasmontana is roughly similar in appearance, with a short, stocky main stem crowded with upright branches. Its segments are symmetrical rather than tapering, making it relatively easy to distinguish the two species. Several other candelabra shaped species, among them E. venenata, share these hot, arid Namibian plains with E. virosa, which has a huge range, from just south of the Namibian border all the way north to Angola. Unlike most of the tree euphorbias, species with this growth habit—E. avasmontana in particular—will develop a sort of miniaturized adult growth form when grown in a container.
E. memoralis, from a limited zone of chromium rich soils in Zimbabwe, and E. cactus (despite its name, less cactus-looking than various other euphorbias), from the southwestern part of the Arabian peninsula, form large shrubs with more of a tree-like shape than the candelabrum plants. E. kibwezensis, a smallish tree from East Africa, resembles E. cooperi rather closely, and like it, makes an imposing specimen, although not likely to survive a cold, wet Bay Area winter outdoors. Growing far from these species, Euphorbia canariensis, from the Canary Islands, has much the same general form, but with unsegmented, four angled stems of an odd olive-green color.
E. canariensis presumably evolved from plants native to North Africa, plants similar to Euphorbia echinus and E. resinifera, two Moroccan twin-spined species, the first with cylindrical, branching stems, the second lower growing and making large mounds of four angled stems. E. resinifera is one of the hardiest of this group, able to endure all the cold and wet that our area can give it. Euphorbia fruticosa, from Arabia, also grows into a spiny cushion of many twin-spined stems.
An enormous number of small and medium sized species inhabit semi-arid parts of East Africa, growing into spiny bushes of every imaginable variety. The best known of these come from Kenya, Tanzania and South Africa, but those are also botanically the most thoroughly explored areas, and many other species may still await discovery. Some of these plants become quite large, among them the widely distributed E. grandicornis, with well demarcated stem segments and long, very prominent spines. It will grow into a thicket sized shrub if given the chance. Less dramatically segmented and spined, E. coerulescens once dominated thousands of square miles of the South African interior, though now much of its habitat has been turned into farmland. It is one of the hardiest species for outdoor cultivation. E. ledienii, similar, but with less distinct segments and narrower stems, shares it habitat, but is slightly less hardy.
The stems of many other species are much thinner, generally less than an inch in diameter, and these euphorbias typically grow into bushes perhaps a foot or two tall and across. These types often have “shields” below the bases of their spine clusters, narrow pads often of a contrasting color. The most striking of these contrasting spine shields decorates the South African E. aeruginosa, a plant with bright blue-green stems and vividly copper colored spines and spine shields. E. cuprispina, from Kenya, is rather similar though not as brightly marked, while the Tanzanian E greenwayi also has pretty mottled, blue-green stems. Other species may have gray, brown, pinkish or even multicolored stems, vivid and colorful stripes (particularly when young) and many combinations and colors of spines. While E.heterochroma, a tall growing, thin stemmed species ranges over a good part of East Africa many of the species are endemics, restricted to a range of hills or a particular mountain, and the abundance of such habitats has led to this seemingly endless supplies of distinct species. E. nigrispina is a slender stemmed plant with black spines; E. cryptospinosa and E. erlangeri are thin-stemmed, clambering plant, with spines reduced almost to nonexistence. E. ndurumensis, from Kenya, has tall, very thin, spiny stems, E. malevola, from Zimbabwe, has distinctly four-angled stems that are proportionately a bit thicker, E. lydenbergensis, from the north-east of South Africa has blue stems and bright yellow flowers. The list of these smallish, spiny bushes could go on and on, and many of these plants are well worth growing. Their compact proportions make them well suited for container culture, they often have brightly colored flowers (mostly yellow but sometime red or other shades) and many don’t need quite as much light as their larger relatives.
The Kenyan Euphorbia uhligiana is a rather typical looking thin-stemmed plant, but has rather tuberous roots, and so points to another group of these plants. Small shrubs with larger branches and mostly underground main stems that intergrade into tuberous roots include South African species such as E. barnardii, E. perangusta, the very spiny E. grandialata, with pale yellow-green markings patterning its dark green stems, the smaller, even more vividly marked E. knobelii, and the very rare E. restricta, with a somewhat more clearly defined semi-tuberous base. E. restricta forms upright tufts of branches that don’t obviously emerge out of a caudex-like main stem, but several species have developed a definite caudiciform habit. E. vandermewei approaches the caudiciform body-type, but several species with proportionately smaller portions of their substance above ground and more below ground level exemplify the caudiciform style. E. stellata, usually with two angled stems held very low to the ground, E. squarrosa, typically with three angled stems, and E. micracantha, usually with four angles on its branches, put out relatively large numbers of relatively small branches, growing from a central point at the top of their underground, proportionately huge root-stock/caudex. With fewer branches, and main stems that blend imperceptibly into deep taproots, species such as E.clavigera and E. enormis, and most particularly E. tortirama, with as many as twenty segmented, twisting, foot long branches, and E. groenwaldii, with a half dozen unsegmented branches form a kind of continuum moving toward extreme caudiciform growth in response to increasing harshness of habitat. The caudices of species such as E. groenwaldii and E. tortirama may extend two feet underground, and dwarf that portion of the plant that exists in the open air. In the wild the branches of E. groenwaldii often are deciduous, dropping off in the dry season, and the Zimbabwean E. decidua takes this tendency to the maximum, living as an underground, branchless geophyte during the dry season, and annually putting out a crown of spiny branches with the new rainy season.
The tree euphorbias respond well to bright sun and a reasonable amount of water during their growing seasons, though many of them will be difficult to maintain in a reasonable sized container after a while. The thicket growers, such as E. grandicornis and the rather similar, but smaller spined, E. pseudocactus also will ultimately become rather difficult to maintain in a pot, as they want to sprawl, but they will behave themselves nicely for a number of years before requiring pruning and other maintenance. The candelabrum types, although ultimately becoming large, make good container subjects as most will display their mature form evening a dwarfed state. A bit more care with drainage and water isn’t a bad idea for the Namibian species. Typical thin-stemmed twin-spined plants do well with normal succulent soil, weekly water when in growth and bright light. In winter many do best with slightly more than average water, say every three weeks or so, and if kept too dry they will begin to shed their branches.
The tuberous rooted and caudiciform types need a bit more care when it comes to soil, light and water, but their demands are pretty much typical for succulent plants, water about once a week when growing, every four to six weeks in winter when dormant will work well for them. The species with really large caudices, such as E. groenwaldii, should be treated very carefully during transplanting, as injuries, even fairly minor ones, to the caudex may often prove fatal; the plants should be watched carefully before normal watering is resumed. The thick stemmed plants will root from cuttings, but the cuttings may have to be dried off for what seems like a really unreasonable amount of time, up to a year, before being replanted to avoid rot. Despite all these warnings, many of these plants are very easy to grow, will thrive for decades in containers and are excellent candidates for indoor growing in a fairly sunny spot.
The Garden has a varied collection of these plants, including some quite unusual species from East Africa, and both small and large growing types. We have a good collection of some of the interesting caudiciform species as well, including some very large, old specimens. We often have some members of this group of euphorbias to sell at sales.
In the next column I’m going to discuss some of the really bizarre spiny euphorbias of tropical Africa and Somalia, and then, finally move over to Madagascar and so complete this seemingly endless but still highly abbreviated survey of these plants.
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