NOTE: Initially, I identified the plant in this post as Euphorbia grandicornis. Then I got a real Euphorbia grandicornis and wasn't sure what this plant was anymore. I've settled on a guess of E. pseudocactus, based largely on the pictures of it at davesgarden.com, but: 1) this could change again if I decide I'm wrong about E. pseudocactus too, and 2) the HTML address for the post still says E. grandicornis. Apologies for any confusion this may have caused. If it helps, virtually everything said herein about E. pseudocactus could also be said, just as accurately, about E. grandicornis.
I mentioned previously, in Euphorbia trigona, that in my quest to buy Euphorbia ammak, I bought two other plants by mistake first. This, obviously, is the other one.
This plant is not widely sold where I live (in fact, I've seen exactly two of them, and one of those was nearly dead), but it appears to be a lot easier to find elsewhere (in the desert southwest in particular, and California especially), judging by the sites that pop up in a Google search. I kind of get the impression that it's even harder to come by in its native habitat of southeast Africa (primarily South Africa and Swaziland), though this is tough to confirm.
A few websites I ran into, more than you'd think would be statistically likely, offer warnings along the lines of this plant isn't for beginners. I suppose there's an argument to be made, there, but I haven't seen any indication that this plant is any more difficult than other succulents from the same family. It needs a lot of sun, reasonable indoor temperatures (I wouldn't go below 50ºF / 10ºC), and water only when the soil is almost completely dry. A quick-draining, very lean soil is obviously helpful in preventing overwatering (and the heartbreak of rot that comes with overwatering). Too much sun, or too sudden of an increase in sun, will cause bleaching and spotting and all-around stress (e.g. if the plant's been inside a shop waiting to be sold for six months, and then you buy it and throw it out into the middle of your yard all summer).
Propagation is said to be pretty easy (I haven't tried it myself), and is essentially the same as what I described for E. trigona: cut off a piece, let it dry in a bright place out of the sun for a few days to a few months (2 weeks was the specific example given here, but I don't think the particular duration of time is as important as the fact that there is some callusing time. Planting immediately after cutting invites rot; waiting too long to plant could leave you with a curved plant (even if it's laying down on a bench somewhere, the cutting will still have a phototropic response: over time, this will result in a tendency for the plant to bow upward unless it's rotated). The site I linked to also used rooting hormone: I'm agnostic about the usefulness of rooting hormone with Euphorbias (or, in fact, pretty much anything else).
This species, and the many similar species of upright Euphorbias with many spines, are often used inaccurately in TV shows and movies to announce "cactus" or "desert" or (very occasionally, in your more symbolic shows) "mean person" to viewers. I'm fuzzy on the reasons why,1 but it happens often enough to be worth noting.
This is one of the more cactusy-looking Euphorbias, hence the botanical name (pseudo meaning "false," plus cactus meaning "cactus"), though a lot of the African succulent Euphorbias look enough like cacti to be confused with them. So, now might be a good place to go into detail about how the Cactaceae are not Euphorbias, and what's the difference, and why it's not just something botanists and pedants came up with to hassle people about. In the end, hopefully, you'll have laughed, you'll have cried, and you'll have learned a little bit about yourself, and those of you who were really paying attention might even learn a little something about the true meaning of Christmas, though I make no promises.
Let's start with why it's important. The Cactaceae / Euphorbia resemblance is an excellent example of convergent evolution, the term for when two species or groups which aren't terribly related to each other wind up in similar environments, and then solve the problems of those environments in ways that look and function more or less alike.
In the case of Euphorbia pseudocactus, we can see certain obvious adaptations to arid environments: a water-storing trunk, complete lack of leaves to minimize moisture loss via transpiration (some Euphorbia species, like E. trigona, continue to produce leaves, but these are small, pathetic, and apparently pretty disposable), thorns and irritating sap to discourage predation,2 a waxy coating to minimize water loss through the stem, a mostly-vertical columnar shape to cut down on sun exposure during the hottest part of the day, and so on. And with a typical cactus, say Cereus peruvianus? Well, we have a water-storing trunk, leaves reduced down to sharp spines to discourage predation, a waxy coating, a mostly-vertical shape, and so on.
What does this prove? Well, not much of anything on its own, but it's indirect supporting evidence for the theory of evolution:3 although there are plenty of indications that the ancestral cactus and the ancestral Euphorbia were very different from one another (e.g. their flowers are not particularly similar, Euphorbias have irritating white sap and cacti don't, cactus spines are modified leaves, and emerge from super-shortened branches called areoles, whereas Euphorbia spines are part of the stem, and some of them still produce leaves), there turn out to be only just so many good ways to cope with the conditions of desert-like environments.4
The most impressive photo I could find to show this is here, down at the bottom of the page: not only is there a cactus and a Euphorbia that look a lot alike (I think the Euphorbia might be E. lactea), but they also rounded up a third plant, from the Asclepiadaceae, or milkweed / Hoya family, that looks like the other two, which sort of strengthens the point further.
A very quick and rough guide to telling Euphorbia apart from cacti:
If the spines are paired and at an angle of about 100º to one another, as in E. grandicornis, it's a Euphorbia. Cacti almost always have more than two spines (usually more like eight) at an areole, pointed in every direction.
If the plant bleeds a white, latexy substance when poked: it's a Euphorbia. Cacti usually (I hesitate to say always, though I'm unable to think of any exceptions) have clear sap.
If the plant has conspicuous true flowers (as opposed to nondescript flowers surrounded by flashy bracts, as with poinsettias), it's a cactus. Euphorbia flowers tend to be small, yellowish green, and in clusters.
It's not generally all that relevant, practically speaking, whether you're looking at a thorny cactus or a thorny Euphorbia: they take more or less the same care. But sometimes one wants to know anyway.
The more-cactusy-than-cactus phenomenon that leads Euphorbias to sometimes find themselves in botanically and historically inaccurate TV settings (there was an actual example of this not that long ago, but of course I can't remember the show in question because I didn't write it down immediately: bloggers take note) is a bit like the more-cowboy-than-cowboys phenomenon: Japanese tourists who go to rodeos in cowboy boots and ten-gallon hat, city slickers who wear cowboy shirts and big belt buckles to clubs but couldn't ride a horse any better than a barstool, yuppies who take dude-ranch vacations, etc. I saw this from time to time when I lived in Texas, though not being a cowboy myself, I couldn't say how the actual cowboys took that sort of thing. Probably either stoically, or as the excuse for drunken bar brawls, since those are pretty much the only emotions permitted to cowboys.
Of course none of this is the plant's fault. It's being as authentic as it can.
Photo credits: in text.
1 One of these reasons has to be that some cacti, for example Carnegeia gigantea, the saguaro cactus, are very slow-growing, and consequently hella expensive: about $100 a foot for saguaros. And of course we have to bear in mind that the people who dress movie sets are not generally botanists by training, even if they clearly ought to be.
2 It remains something of a mystery to me how insect pests like whiteflies and mealybugs manage to process Euphorbia sap so that it doesn't harm them. Why they do it is less of an issue: obviously Euphorbias have been very successful plants, found all over the world, and any insect that found a way to get past the plants' defenses would have a huge amount of energy, food and water available to it, and that would obviously be beneficial to the species (the insect's species, not the Euphorbia – it'd pretty obviously be kind of disastrous for the Euphorbia). But how do they do it?
3 Anybody interested in the whole evolution / creation "debate" really should know this already, but many people seem not to, so: the word "theory," in everyday usage, means something that's just a plausible guess, as in, "That's just your theory." In science, a "theory" is a whole network of evidence that points to a single conclusion: nobody thinks that gravity is just a plausible guess but the jury is still out. Nevertheless, gravity is still a theory as scientists use the term. Certain creationists have taken advantage of the difference in meanings to misrepresent evolution, and pretty much all of them continue to do so even when it is explained to them that the word means something different to scientists than it does to the population at large. "Oh, but even you say it's just a theory" does not constitute an argument against evolution. If you still need convincing, consider: the big goal in the physics community is to come up with an idea that explains all the different intensities and types of interactions between matter and energy, and this goal is referred to as the Grand Unified Theory of Physics. Physicists do not mean by this that they're in search of some wild but plausible guess: they actually have plenty of those. What they want is a plausible guess which is supported by all the evidence and observation already collected. So if someone pulls this "just a theory" crap on you, know: they're twisting words around on you to mean things that the words don't mean.
4 Another support for the idea that these are from different ancestral plants: the Cactaceae are, with one exception, found only in the Americas. The single exception is the genus Rhipsalis, which has one African species (R. baccifera) and two closely related Madagascan species.
Euphorbia, by contrast, has a wider distribution, including some species native to the Americas (the best-known example being Euphorbia pulcherrima, the poinsettia). American Euphorbias tend not to resemble cacti nearly as much as the African ones do; I suspect that this is because the "job" of being cactusy is already taken in the Americas, so Euphorbias have had to find other niches in order to survive here.