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Succulent Euphorbias--Part 2
After November’s discussion of the some of the less highly specialized succulent euphorbias, this month we can move on to a second group that consists of species in which the succulent life-style has led to more dramatic physical modifications. Some of these species fall into obviously related groups within the genus, but in a number of instances the validity of some of the traditional organization of associated species, based as it generally was on large-scale morphological features, has come under question in the last several years. Pierre Bruyns, in South Africa, has reduced a number of species into synonymy, even as Gerhard Marx has been making (and describing) new discoveries, while Daryl Koutnik, writing in Vol. 10 of the Euphorbia Journal, has informally proposed a new organization of subgenera and sections of the genus, focusing on the succulent species. Regardless of how the taxonomic structuring ultimately plays out, the euphorbias covered in this and the next few columns include some of the most popular, most sought after, and also, some of the rarest and most difficult species in cultivation. With only a few exceptions, these euphorbias are confined to southern Africa, whether South Africa itself, or neighboring countries such as Namibia, Angola, Botswana and Zimbabwe. Their range encompasses a whole world of climate and environment; the only thing these diverse regions have in common is a marked dry season, and even that varies from place to place according to season¸ winter, summer¸ or almost any possible combination of months. In addition, the annual rainfall in these various euphorbia habitats can range from as little as two inches a year to as much as thirty.
Covering this vast expanse of land and variety of habitat and climate, it’s no surprise that the ever adaptable euphorbias have devised a wide array of succulent responses to environmental vicissitudes. As a first response, adopted by only a relatively few species, some euphorbias have evolved either tuberous, water-storing roots and caudices or what essentially are underground succulent stems. Probably the best known of the tuberous, caudiciform species is Euphorbia trichadenia, from north-eastern South Africa, and parts of neighboring Zimbabwe. In the wild this plant would look like nothing more than a small, fairly nondescript shrub, with rambling stems and linear, slightly glaucous green leaves, recognizable as a Euphorbia only by its flowers. But hidden underground, the bulk of the plant resides in a greatly enlarged, rather dome-shaped succulent tuber, which retains more than enough moisture to enable it to survive winter droughts. A smaller, related, very similar plant, E. pseudotuberosa, grows a bit farther to the south. At a considerable distance from these two, and almost certainly not closely related to them, a small group of associated species live along a wide expanse of the winter rainfall area, where they remain leafless and dormant during the summer months. Instead of taking the above ground appearance of small shrubs with leaves stretched out along their branches, with the coming of the winter rains these species present a small, stemless rosette of leaves flush with the surface of the soil, giving little indication of the succulent stem or stems below. The best known of these, E. tuberosa, has broad, sometime curly-margined leaves, while in a second species, E. silenifolia, the leaves are long and narrow, almost linear. In cultivation the tubers and underground stems of these geophytic species generally are raised above the soil, both to display the entire plant and to prevent then from rotting. Older specimens of E. tuberosa may have several underground stems, and when raised above soil level, they make some of the most attractive succulent bonsais.
The growth habit of an underground tuber or stem surmounted by a ground level rosette of leaves extends beyond South Africa. Perhaps the best known non-South African species with this form is E. primulaefolia, from Madagascar, but even a cursory look at its flower structures shows that this diminutive plant belongs with other non-geophytic Madagascan species instead of with the South African plants. A few very similar small euphorbias with the same habit occur in Madagascar while others appear sporadically on the African mainland as far north as Somalia. These mainland species in particular, combining the properties of almost complete invisibility in the field with genuine rarity, are practically unknown in cultivation.
A second group of plants, mostly from the eastern half of South Africa (with some extensions to the west) consists of plants which also bear large leaves at the tops of their succulent stems, but which, as opposed to E. tuberosa and its relatives, produce stems that grow above the soil. The best known of these is undoubtedly Euphorbia bupleurifolia, prized by growers for several centuries. Its most striking feature is its short, dark brown, woody, sharply tuberculate, rarely branched stem, which resembles a miniature pineapple (or, more accurately, a highly stylized drawing of a miniature pineapple). As is often the case with euphorbias, this species is monoecious, with individual plants either male or female. Its flowers appear at the tips of deciduous peduncles (flower stalks), often before the rosettes of narrow, bright green leaves. Several related species with taller, less distinctly tubercled, plain green, proportionately narrower stems include E. clandestina, with inflorescences hidden close to its stem, E. pubiglans, with persistent peduncles and a dense spray of green, round-bracted flowers, the rather similar E. clava and E. cylindrica, with respectively wider and narrower leaves, and the smooth tubercled, branching E. bubalina. A vaguely similar, but often much larger plant, E. monteiroi grows much farther north, in a band stretching across southern Angola and northern Namibia through Botswana and Zimbabwe into northeastern South Africa. Most of these plants need protection from cold, less than the brightest light, a reasonably rich (though fast draining) soil and adequate water.
A small group of somewhat similar plants, with prominent leaves but also with dense, persistent peduncles that harden off into spines after flowering include the tuberous- rooted E. oxystegia and E. loricata, single stemmed, densely branched, spiny, and up to three feet tall and almost as wide, the much dwarfed, symmetrically growing E.multifolia and the similar, somewhat larger, cushion forming E. eustacei (a cultivated, probably hybrid plant, sometimes called E. loricata, neither resembles nor has any particular relationship to the actual species). Though they share a general similarity of appearance with species such as E. clandestina, Koutnik has suggested that because of significant details of their flowering structures, along with the presence of peduncular spines, they actually may be more appropriately grouped with other peduncular spined species from farther east, such as E. horrida. These rarely encountered plants live in areas of summer drought, and should be grown accordingly in cultivation. If kept too wet in summer they have a bad habit of quickly rotting. In contrast, the leafy euphorbias mentioned before them do well with summer water and less (but not total dryness) in winter, though E. clandestina grows in the Little Karoo, generally a region of summer drought. E. bupleurifolia, despite responding well to more than average water when growing, should be left dry from the time its leaves drop until new growth starts. It also prefers a slightly shaded situation.
Some very rare species that certainly look like they belong with this group, but may instead represent an example of convergent evolution rather than common ancestry, come from many thousand miles away, in Somalia and the southwestern corner of the Arabian peninsula. Among these are Euphorbia hadramautica, the rare E. napoides, and the even rarer E. longituberculosa, small plants that resemble a miniaturized, unbranched E. bubalina, but which differ from that easily grown species by being extremely sensitive to both over and under watering, and which accordingly present serious difficulties for long term cultivation. Growing them with bottom heat may lead to somewhat better results in our fungus-happy local environment.
Also with upright, generally solitary or sparsely branched stems organized into tubercles, but much harder bodied and rather cactus-like in overall appearance, E. schoenlandii, E. fasciculata and E. restituta all grow fairly close to the South African west coast, with its winter rain and fog and summer drought (E. hallii, a related, taller and thinner growing species, lives a bit to the east). These plants thrive in the arid circumstances of their coastal desert home. In the wild they tend to lean slightly to the north, the direction of most sunlight in the southern hemisphere. Generally medium sized plants, they can approach three feet in height, and often grow in small colonies where they look like small sentinels on duty, surrounded by low growing, succulent scrub. They do well in cultivation particularly if given extra fast draining soil, a bit less than average water (and less even than that in the summer months) and intense sunlight. E. schoenlandii, at least, though literally growing within sight of the sea in the wild, and thus presumably sheltered from the intense cold of inland desert winter nights, will survive outdoors in the sunnier and warmer parts of the Bay Area.
The range of these generally unbranched “noordpool” plants overlaps that of a larger euphorbia sub-group, plants with a single thickened, sometimes underground main stem and a whorl of secondary stems that surround it like a swarm of snakes, the so-called caput-medusae group, named after the mythic Greek female with snakes for hair.
These medusoid euphorbias, though, will have to wait until next month.
The Garden has a small, but fairly representative selection of this group of Euphorbia species. Very occasionally we have some available for sale, but more can be found at various cactus and succulent specialty nurseries—a word of warning: many of the plants of this type offered in general nurseries will be misidentified, and may well be nothing more than unknown garden hybrids.
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