Weirdo Loner-Type (Leuchtenbergia principis)

Leuchtenbergia principis is a lonely guy. For one thing, it's the only species in the genus Leuchtenbergia, what botanists call a monotypic species.¹ For another, in its native habitat (pretty much everything in the northern half of Mexico, except the coasts), it's typically found as single individuals, often growing next to a clump of Agaves and tall grass. (I guess they're not comfortable in crowds?)

Leuchtenbergia doesn't look the way you'd expect a cactus to look, which is a lot of the charm. The body of the plant bears multiple green or blue projections (called tubercles) which are triangular in cross-section, and stick out about 4-5 inches / 10-13 cm from the center. In strong light, the tubercles can even develop red or purple edges, which echo the differently-colored margins on a lot of Agave leaves. The shape of the tubercles and overall Agave-like appearance lead to the common names: agave cactus, prism cactus,² and (more inexplicably) cob cactus. The tubercles end with several long, soft, papery spines of varying thickness and length, which point in all directions. The overall impression, then, is of an Agave with a bunch of dead grass caught in its leaves.³ Which is just the thing you'd want to look like, if you were trying to hide next to a small clump of Agaves and dead grass.

Though that may be giving the plant too much credit. Given that they're often found as single individuals, near Agaves, I suppose there might be an element of peer pressure involved in them looking the way they do. Or maybe bullying. (Agaves can be terrible bullies, as you know if you've ever had to work with a lot of them.)

The IUCN⁴ lists Leuchtenbergia principis as a "least concern" species, on account of its wide range and large number of wild individuals (estimated at about 500,000). Regardless of what the IUCN says about it, Mexico is free to protect whatever species they want, and has placed L. principis on its national list of protected species.⁵ The main risks are from illegal collectors and livestock grazing.

The root is thick and long, to about 8 inches long and 3 inches in diameter. (Metric equivalents: 20 cm and 8 cm.) Absolutely everyone describes it as "parsnip-like," so I will too, even though I'm not all that familiar with parsnips. As plants age, they'll shed tubercles (usually when dry and/or during the winter) and grow a trunk, though that's a slow process: neither of my plants have, as far as I can recall, ever dropped a tubercle spontaneously. The white or tan wooly hairs around the base of the tubercles drop with age as well: the trunks are naturally bare and corky-looking. Even plants that have been pampered and coddled for their entire existence still end up looking like they've had this terrible, brutal life,⁶ though they do generally look better than the ones that actually have lived in the wild.

Mine, unfortunately, aren't old enough to have much of a trunk yet: this is about as good as the pictures get.
It's hard to come by any exact measurements for the maximum height of a Leuchtenbergia -- 24 inches (61 cm) is popular, and one source said 30 inches (77 cm). They grow so slowly that I doubt it matters much to anybody growing one as a houseplant: it's not one of those plants that's constantly going to be outgrowing its location.

Mature specimens may also develop branches, though they have to be pretty old first, and even when old, this doesn't seem to happen very often.

As a houseplant, Leuchtenbergia principis is very easygoing, with only a few non-negotiable aspects of care.

LIGHT: Because they're adapted to grow in the shade of other plants, Leuchtenbergia is perfectly able to survive without a lot of direct sun. The tubercles can move around a bit, depending on the light and humidity: they'll lie flat if there's too little light, and pull closer together and more upright if the air is exceptionally dry. So if you notice that your plant's lying flatter than it was when you bought it, well, that's one possible reason why.

Both my specimens lived inside year-round for their first three or four years, and although they didn't grow much, and the tubercles did splay quite a bit, they survived. It's better, though, if you can provide a lot of direct sun: they'll grow faster, the color will be better, and you stand a better chance of getting them to bloom. The best solution for people who live in an area with cold winters (like me) is to let the plants spend summers outdoors and winters in a bright spot inside.⁷

If you do let your plants summer outside, remember to start them off in a shady, protected location and gradually move them into a brighter spot: they can sunburn if you try to take it too quickly. There's a popular opinion that the plant actually looks better if grown in bright shade or part sun, as opposed to full sun; I don't have enough experience growing Leuchtenbergia outdoors to have an opinion on that yet.

WATER: Lots of conflicting information on this one. Until this summer, I watered my two Leuchtenbergias every two weeks unless the soil still felt moist (which happens occasionally in the winter), winter spring summer fall. This won't work for everybody, and temperature is a factor there, but I've never had a tubercle drop spontaneously,⁸ so I think I must be doing something right.

The long root will require a deeper pot. Clay is preferable to plastic, both because it will dry out faster (less chance of rot) and because it'll keep the center of gravity a little lower than plastic, so the plant won't tip over as easily. I'm ignoring both of those suggestions with one of my plants, though: it's in a plastic azalea pot.⁹ The most likely reason for this is that I probably didn't have any appropriately-sized clay pots around the last time I thought about repotting it.

Soil should be fast-draining, with a lot of coarse sand, gravel, or similar material. Avoid potting soils that retain a lot of water (peat moss is not your friend). Some people add eggshells, limestone, or other sources of calcium to the soil when they pot plants up: as best as I can tell, this is more natural, but doesn't affect the health of the plant much. What matters is that the potting mix dries out quickly. The root should not be above the soil line after you repot, and you shouldn't have to repot very often in the first place.

Some of my sources say that the tubercles will turn yellow if the plant is too dry; I don't know if this is true. The first sign of too much water is rotting and death, so if you're not sure whether to water, don't.

TEMPERATURE: Leuchtenbergias, if kept dry, can supposedly tolerate freezes with no problem, and can even go down to 15F / -9C for short periods. I have no intention of ever verifying any of that, and I don't recommend that you test this either, but it's nice to know that the possibility theoretically exists.

One caution about cold: some people overwinter their plants in very cold rooms (or garages, or what have you). If you do this, keep in mind that the colder the temperature, the more likely you are to kill the plant by watering it. Either go cold and dry, or warm and not-very-wet, during the winter. Most sources recommend a temperature between 40 and 60F (9 to 16C) during the winter, with no water at all. (In nature, they get moderate rainfall in the summer and virtually none in winter, which is presumably where this idea comes from.) Other people water just enough to keep the tubercles from dropping, but I don't know how they know when that is.

Being from the hot Mexican desert, L. principis obviously doesn't have much in the way of a maximum temperature, at least not one that you're likely to reach in cultivation.

My personal plants stayed at about 65-75F / 18-24C year-round, for the first few years I had them. Since they've been out for the summer, I think they've had a night or two in the upper 40s F (about 9C), but I'm probably going to bring them in for good once we start seeing low 40s (about 5C).

HUMIDITY: Pretty much irrelevant, though very high humidity might encourage rot, especially if the soil is cold and wet. The plant is naturally adapted to dry air.

PESTS: Leuchtenbergias are primarily affected by scale and mealybugs. Both scale and mealybugs can be hard to see: scale looks like scarring or part of the woody trunk, and mealybugs could blend in with the wool around the base of the tubercles. Watch for shiny, sticky spots on the leaves: I've found that I usually see the drops of honeydew before I spot any insects. When I've seen honeydew and ignored it, I almost always regret it later, even if I couldn't actually find any insects.

Fungal and bacterial infections are also possible, especially if the plant is poked with something sharp or damaged during transplanting or something like that, but I didn't get the impression that it's a common problem, and I've never encountered it personally.

PROPAGATION: If you want a bunch of small Leuchtenbergias, the way to go is to get seeds. They're easy enough to find on-line.

Seedling at about 3 1/2 months old. It'd been indoors under artificial lights for all that time, so it's maybe a little stunted and/or etiolated, compared to a natural seedling.
When I started about 60 seeds at once, in May, I got about 60-65 percent germination; I would probably have done better if I'd tried to start them outdoors, but I'm not very well set-up for starting seeds outside.¹⁰ Of the 38 or so that germinated, about 30 were still alive and healthy three months later. I transplanted a batch of 15 on 25 August, of which 12 out of 15 were still alive as of 19 September.

Photo from 27 August.
Other propagation methods are rumored to exist, but I can't vouch for them.

• One can supposedly take cuttings of plants that have branched: I'd assume that one should let the cut piece dry and callus over before planting it, as for other desert cacti.
• I also hear that it's possible to take cuttings of individual tubercles and root them, but I was unable to find anybody on-line who claimed that they had actually done this successfully. (If you have, let me know.)
• One variety of L. principis essentially grows plantlets at the end of each tubercle, which could be rooted and grown if they were normal plantlets. But they're not. More on this later.
• There are also claims out there that normal plants will, rarely, produce offsets at their base, which could be separated and potted up on their own. There's also a variety which offsets really heavily. Later on that one too.
• People also sometimes start seedlings and then graft them to other cacti, which allegedly gets them to grow bigger faster. This may be true, but I'll never try it myself, because it looks wicked stupid to me.¹¹

GROOMING: There's no grooming in the usual sense -- they don't need to be pruned or dug up or anything -- so I'll talk about flowering instead.

The flowers are yellow, and about 3-4 inches (8-10 cm) across. Very occasionally, one will find an individual that blooms yellow with pink edges, or pink, but those plants are rare. The flowers allegedly have a scent, but nobody describes it and I couldn't smell it on my own plants, so it must be pretty faint or nondescript.

My own plants bloomed simultaneously once this summer, and I got a not-great picture to compare the color:

The smaller plant, in the clay pot, consistently produced very vividly yellow flowers that got slightly lighter over time; the larger plant in plastic made pinkish buds that opened to pale yellow flowers, which stayed pale yellow.

The stamen colors were also different: the small plant's stamens were the same yellow as the petals, while the large plant's were sort of pink-orange. Which was sort of cool, I guess.

The weird part is that I'd seen the smaller plant bloom before, when I first saw it for sale, and it was producing pale yellow flowers then, so apparently the flower color is at least partly determined by growing conditions. I'm not sure which conditions are the important ones, since for all intents and purposes these two plants got the same treatment -- watered and fertilized at the same times, same light, temperature, and humidity.

The 2009 bloom. Granted, the color's off, but you can at least tell that it's not the intense Crayola-type yellow of the 2013 bloom.
The flower buds appear at the tips of the tubercles, on the current year's growth. I think it would be fair to characterize the speed of development as "excruciatingly slow," though my perceptions are probably not to be trusted. (I got really excited when I first saw the buds on my plants, and then checked them semi-obsessively every time I went outside.)

Just one of the many, many pictures I took this summer of slowly-developing Leuchtenbergia flower buds.
Plants need to be about four or five years old in order to flower,¹² and can produce two rounds of blooms in a single year if conditions are right, between late spring and early fall.¹³ Strong light seems to be necessary: it's probably possible for a plant to bloom indoors, but I'd think the conditions would have to be pretty exceptional. Once a plant is old enough to produce flowers, it can bloom every year thereafter.

Supposedly a single heavy drench will spur bud development about three weeks later: I couldn't tell you if that's true or not, since my plants pretty much get heavy drenches every time they're watered. According to one source, plants that are kept indoors for the winter then moved out in the late spring are likely to produce their first buds in June, and then bloom continuously until September. I didn't notice buds on my own plants until 12 July, so that's at least approximately correct. Individual flowers don't last long. My longest-lived flower lasted three days; the shortest for about a day and a half. Buds may drop if a plant is moved while they're developing.

None of my reference sources would say whether the flowers were self-fertile. (Nobody even seemed to feel like it was an interesting question, which was frustrating.) My guess is that they are not, though that's based on the first flowers from the small plant not producing fruit on their own. I didn't think I needed to do anything, since the black bees seemed pretty thrilled with them --

OH HAI IM IN YR LUKTEN- (*hic*) LUKTIN- (*hic*) LUKTENBIRERGERA DRINKN YR NECTAR
-- but the bee's presence didn't lead to self-fertilization, whether it's possible or not, and I didn't attempt to pollinate the flower myself, because I naively trusted the bees to take care of it. (I'll have to try self-pollination when one of the remaining buds opens.) I did take a paintbrush outside when both plants had flowers simultaneously, and that seems to have worked.

The fruits are said to be green or greenish-blue, about 1 inch (3 cm) long and 3/4 inch (2 cm) in diameter. Most of which is right on the money so far. They may or may not turn red when ripe. One of the photos available through Google image search shows a purple fruit, even.¹⁴ Each ripe fruit will contain "several hundred" seeds. Seed collection apparently goes better if the fruit is slightly overripe.

Cleaned and dried seeds can be stored. Nobody was specific as to how long, exactly, one can store seeds, but once you have a hundred or so seedlings, are you really going to care that much about how long the remaining seeds will remain viable?

This seedling is roughly a week or two old.
FEEDING: Doesn't appear to be a huge thing. Feed during the growing season with a regular balanced houseplant fertilizer. It's not a particularly needy plant as far as fertilizer goes.

Although Leuchtenbergia is a monotypic genus, it's sufficiently related to Ferocactus and Astrophytum to be able to hybridize with them occasionally. Hybrids with Ferocactus are called Ferobergias, and look more or less like Leuchtenbergia except with smaller tubercles and stiffer spines: the overall color is still green to green-blue, the flowers are still yellow or pink, etc. Oddly, Ferobergias are also often variegated, though the variegation is not necessarily attractive. (To me.)

Leuchtenbergia / Astrophytum hybrids aren't common enough to have an official name; one source suggested Astrobergia, which . . . sure, I guess we'll go with that. They look more or less like Ferobergia (short tubercles, green or green-blue, yellow or pink flowers, etc.), but with the little white dots (trichomes) all over the surface like Astrophytum. They're not unattractive on their own, if you like Astrophytum (for example), but they're also almost always grown as grafts.

Astrophytum appears to be less compatible genetically with Leuchtenbergia; a lot of what I found about them was just people complaining that their attempts to make viable crosses weren't paying off. (The only specific suggestion I saw was that transferring pollen in the early morning might help; I have no idea if it does.)

There are at least two cultivated varieties of Leuchtenbergia principis: L. p. var. trachythele, which has the same overall shape but with rougher, sandpapery skin, and one or two monstrose varieties. Trachythele is either very rare or very unloved; I only found one reference to it, and all the photos that come up in Google either look exactly the same as the species, or dramatically uglier than the species. And it's not like it's the world's most gorgeous plant in the first place, you know. Plus, when you're trying to make something more appealing to consumers, one's first impulse is usually not "let's give it more of a sandpapery texture!" So if you see a trachythele for sale somewhere, snap it up if you're interested, 'cause you probably won't get many other chances to buy one.

There appears to be more than one monstrose variety, but they're all called the same thing. One of the monstrose ones just offsets a lot, which isn't necessarily unpleasant if you don't mind the regular species. Not sure that should qualify as "monstrose," but whatever.

The other monstrose form is more deserving of the name. It produces scaly buds from the tip of every tubercle, which develop not into flowers but instead into entire new plants, each of which also immediately sets about budding at each tubercle. It's so insistent about doing this that it has to be grafted, I gather -- I don't know if the roots fail to develop properly, if they just can't keep up with the above-surface growth, or what, but they won't survive on their own roots. I'm not a huge fan of monstrose cacti in the first place, and I've already told you how I feel about grafts, so I find this one pretty revolting and would just as soon not think about it anymore if that's okay with you.

Leuchtenbergia principis holds a special place in my heart, and both purchases are sort of pleasant memories for me: the first one I bought was a rare case of non-buyer's remorse. Only one lonesome plant at the ex-job one day when I went in for something. I noticed it, of course, and thought it was interesting, but I didn't necessarily want to buy it or anything. Took some pictures, got home, uploaded the pictures to the computer, looked up the plant on Google, and then realized I NEED THIS PLANT. So I called the ex-job and asked them to hold it for me, then went back and bought it a couple days later. Usually, these things work the other way: I buy on impulse, then look at the pictures and at Google and wish that I hadn't. So that was a nice change of pace.

The second, slightly larger plant I bought just because I couldn't believe it was so cheap. Frontier, in Cedar Rapids, had had this single specimen for a while and nobody wanted to buy, so they'd marked it down to $5. And still nobody had been interested, apparently. I'd had my first specimen for a little over a year at this point, and I was pretty happy with it, but $5 for one this big and this old was such a great deal that I might well have bought it even if it had been a species I hated. Some deals are just too good to pass up.

Plus: the first specimen needed a friend.

References (in something resembling, but not quite, alphabetical order):

Cactiguide.com
Cactus art
Cactus and Succulent Society of America¹⁵
The Central Ohio Cactus and Succulent Society (article by Bruce Brethauer)
CoronaCactus Nursery LLC
Dave's Garden (monstrose)
Dave's Garden (species)
Dave's Garden (var. trachythele)
Desert-Tropicals
The Garden Forums¹⁶
Gardening With a Not-So-Angry Redhead
GardenWeb forum (1)
GardenWeb forum (2)
IUCN page
Kara Nursery
Lophophora
The Lovely Plants
Plant of the Week
Tucson-Gardener.com
Wikipedia

-

Photo credits: all photos are my own.

¹ Because most of the fun in being a botanist is having a specialized term for every little thing. Or at least that's how it looks from here.
² Probably a reference to the shape of the tubercles -- glass prisms are often triangular -- but it might also have something to do with the range of colors the plant is capable of producing, depending on its conditions: green in lower light, blue in higher light, red, red-violet, or violet in very strong light.
³ When I got my first one, I spent a while fighting the impulse to clean it up by trimming the spines down to something more sensible-looking. In the end, my plants kept their natural look, but only because I was too lazy to trim them. If you're wondering, no, it probably wouldn't have hurt the plant if I had trimmed them: as far as I can tell, their only real function is to hide the plant, something which isn't really relevant in cultivation, so if it really bothers you, grab the scissors and go nuts. Though people might, you know, judge you for it.
⁴ (International Union for Conservation of Nature and Natural Resources)
⁵ Crazy people are always wandering into Mexico to steal plants. And non-crazy people sometimes steal plants to resell, especially plants like Leuchtenbergia that take a long time to get big. Why sow seeds and wait thirty years for specimen-sized plants when you could just drive to Mexico with a shovel, right? So I think Mexico's doing the right thing here.
⁶ I also considered "Gangsta Rapper" for the Leuchtenbergia principis "person" because of this -- I don't know if it's still going on, but there was a time when all the rappers seemed to be inventing much rougher childhoods for themselves than they could have had in reality.
⁷ At least one of my sources, don't remember which, said that Leuchtenbergia would never thrive or look good if it was ever kept indoors at all, and that if you can't give it full outdoor sun all the time, you may as well not even try to grow it.
This is obviously one of those people for whom plants are only worth growing if you can grow them to their full genetic potential, a sentiment I addressed in this post (scroll down until you hit item #4).
People are of course free to set their own standards for what counts as "successful" cultivation of a plant, and there's nothing wrong with the "full genetic potential" standard if that's the sort of thing you're into, but . . . people should be free to set their own standards for what counts as successful cultivation of a plant, and not expected to adopt the standards of someone else. I mean, if you're not entering it in a show or something, the only thing that ought to count is whether or not you're enjoying the process. Don't let other people make you feel bad about enjoying your own hobbies, that you're spending your time and your money on. For fuck's sakes.
⁸ The one tubercle that's come off either of my plants did so because it was broken off, when the plant got knocked over.
⁹ Standard pots have the same height and width; azalea pots are 3/4 as tall as they are wide.
¹⁰ Watering is the main problem: the sprayer we had with a "mist" setting broke, and anything stronger than mist is likely to knock plants around, bury them under soil, etc. What I did was, I filled a plug tray with soil, wet the soil, put a seed in each cell, and then covered the tray with a plastic dome. This is probably more moist than they would have liked, but I was worried that without the dome, the seedlings would dry out and die before I noticed that they needed water.
¹¹ Though I'm not a fan of grafted cacti (or Euphorbias) in general. If you like that sort of thing, you might think a grafted Leuchtenbergia looks cool. You'd be wrong, but you might think that. (Readers upset by this should read footnote #7 again.)
¹² Plants grown indoors are likely to take longer than this. In Arizona, or Southern California, you could maybe go from seed to bloom in four years, but in Iowa, where it has to endure lower light from October to April, I'd expect closer to six or seven.
¹³ In my particular case, the small plant produced I think five flowers between 21 and 31 August, just boom-boom-boom-boom-boom, and the large one started on the 31st and still has three buds left.
¹⁴ (This isn't a case of my sources disagreeing with one another. Leuchtenbergias are apparently just really inconsistent about fruit color. It's weird, since you'd think that any plant that depends on animals to distribute its seeds would want the fruit to be as visible as possible, but maybe the impulse to hide is just that strong.)
¹⁵ The CSSA link no longer works for me; I make no promises that it will work for you. I don't think it required registration when I read it previously, but it does now. I would be delighted to register, but the site didn't provide anywhere to do that, the last time I checked.
I'm including it on the list anyway, because 1) some of you might still be able to read it, and 2) it was part of the research I did, and I figure it's better to give credit nobody can verify than it would be to pretend like I didn't read it and have someone come after me for not giving appropriate credit. Though I don't think it contained any unique information anyway, so probably I should have just pretended that I hadn't read it.
¹⁶ This one may or may not work either; they seem to have changed the way their URLs are constructed since I visited last. Either that or I bookmarked the wrong thing previously. If all else fails, you can go to their search page and search for "leuchtenbergia attemps" [sic] and that should bring you to the correct conversation. This one is more critical than the CSSA link, because the discussion in question is about creating Ferobergias and Astrobergias, and was the only place I found much of anything about this subject at all.