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

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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.

Engineer (Ficus elastica)

Ficus elastica, usually called "rubber plant" or "rubber tree," hails from the northeast corner of India, south and east into Malaysia and Indonesia. It really has been used in the production of rubber, despite what some websites will tell you, but natural rubber currently comes from a different plant, Hevea brasiliensis,¹ in the Euphorbia family (Euphorbiaceae).

Hevea brasiliensis, the "real" rubber tree. Photo credit: KENPEI, via Wikimedia Commons, with some cropping and resizing.
A surprisingly large number of plants turn out to contain substantial amounts of isoprene, the small molecule which is polymerized to form rubber.² The two plants you're most likely to have met already are Ficus elastica and dandelions (Taraxacum officinale). People have tried making natural rubber from these, but rubber from Hevea brasiliensis is cheaper and of higher quality.

So Ficus elastica is not the rubber tree, but it's a rubber tree.

Another "rubber plant" is the one sold as "baby rubber plant." This is Peperomia obtusifolia, and "baby rubber plant" is an unfortunate common name that got attached to it at some point, no doubt by marketers.³ Peperomia obtusifolia is a great houseplant, but it will never grow into a rubber tree, no matter how well you care for it, because it's a different species. (An appallingly large number of websites don't know this. You'd be dismayed, and possibly even sickened.) It doesn't even contain meaningful amounts of isoprene, as far as I can tell.

Peperomia obtusifolia. This is the variegated variety, which in my experience is more commonly sold than the plain green one.
So now that we've gotten the mistaken-identity stuff out of the way, let's talk about our actual subject.

Ficus elastica has been introduced to tropical and semitropical areas as a landscape plant, though it's only reliably hardy in USDA zones 10 and 11. (One commenter at davesgarden.com has planted a rubber tree outdoors in Charlotte, NC, zone 8, which s/he says is cut back every year, mulched heavily with "pine straw"⁴ and covered with a tarp, and returns every spring for the last 3 or 4 years. It gets about 4 feet tall by the end of the growing season. So with certain allowances, one can grow it in colder zones than 10.)

A young Ficus elastica being used as a landscape tree. Image credit: Louise Wolff (darina), via Wikimedia Commons. Resized.
As a landscape tree, F. elastica has many of the same drawbacks as Schefflera actinophylla: it's a large, vigorous, fast-growing tree⁵ which drops a considerable amount of leaf litter and barely-edible fruit, which is a pain to clean up.⁶ It also produces extremely deep shade, under which few other plants can grow. Outdoor Ficus elastica plants tend to grow wider faster than they grow taller, so the area they take over can be considerable. They also have large, robust roots, which can ruin water lines and sidewalks, and tend to break apart in high winds, both of which make them unsuitable for planting near houses. Which doesn't stop nearly as many people as it should.

On the slightly more positive side, F. elastica appears not to be invasive. Different websites give different information, but as best as I can tell, it's been deliberately cultivated in Hawaii and other Pacific islands without becoming an ecological threat (except as a habitat-loss issue; any introduced plant growing in place of a native is an ecological problem to some degree). The general feeling appears to be that it's behaved itself so far but should still be watched closely.

The reason for calling F. elastica an "engineer," though, is that it builds bridges. More accurately, people build bridges with it.

Big whoop, you're saying. People build wooden bridges all the time. Nothing special about that.

*sigh*

Okay, first of all -- damn, you're jaded.

Second, what's special and engineer-like about Ficus elastica bridges is that, one, they're built using the roots, not the trunk and branches, and two, they're built while the tree is still alive and growing; i.e., the bridges are alive.

And not with the sound of music, neither.

The people who figured out how to do this are the Khasi of Northeastern India, particularly in and around the city of Cherrapunji, in the state of Meghalaya, where there are dense natural forests and an annual monsoon season during which truly ridiculous amounts of rain fall.⁷

The process requires some patience, but it's also elegantly simple. A Ficus elastica tree is located near the spot where one wishes to place the bridge. (If you can find one on the opposite side too, so much the better, but you only need one.) A betel nut tree⁸ is cut down, and its trunk hollowed out. The hollowed trunk is then placed across the gap one wishes to bridge, and a root from the Ficus is guided into the trunk. The root grows along the trunk until it reaches the soil on the other side, at which point it roots into the soil. Over time, the Areca trunk rots away, the Ficus tree grows, its root thickens, and eventually you have a solid, walkable bridge, albeit one without a smooth walking surface.

The catch, of course, is in the word "eventually." Though it'll speed the process some if you start multiple roots at the same time, and especially if you start roots from a second tree on the opposite side of the gap at the same time, these bridges still aren't strong enough to be usable until about 10-15 years after they're started, so some long-term planning is necessary. On the other hand, once present, they will continue to get stronger for the life of the component tree(s).⁹ Hell, there's even a double-decker bridge made from living F. elastica roots, somewhere around Cherrapunji. (Photos here.)



From the video, one can see that material is added to the top of the bridge to create a walkable surface, and it also appears that people stretch aerial roots from the trees to points along the edges of the bridge, creating what are more or less suspension cables and railings.

Ficus elastica does more than bridge design and construction, though: it's also heavily involved in the arts. It's been grown indoors since the early 20th century, but I always associate it (and Monstera deliciosa) with the 1950s. So I googled to figure out why that would be so, then got derailed when I found this 1952 painting by Soviet Realist painter Aleksandr Laktionov, called "New Apartment" or "Moving to the New Apartment" or something like that (the translation depends on who you ask):¹⁰


And then I kind of lost interest in the question, which is the sort of thing that happens to me a lot when I try to read art/literature criticism.

Ficus elastica and paintings of Ficus elastica also recur throughout the Margaret Atwood novel Cat's Eye, large parts of which are set in Canada in the 1940s and 1950s, which you should totally read.¹¹ And people are still painting them (do a Google image search for "rubber plant painting" sometime). Here is one from Lynne Parrish:

Image credit: Lynne Parrish. Used by permission. (This painting is also available for sale at her website, should the reader be so inclined.)
So all of that is . . . um, pretty cool, if not terribly relevant to your questions about how to grow a Ficus elastica, right? So let's talk about how to care for Ficus elastica.

LIGHT: The best location for a rubber tree is one that gets bright indirect light or filtered sun. Full sun can also work well (it's how they grow outside, after all), but plants sold for indoor use are usually pre-acclimated to lower light, and may require an adjustment period. Sunburn manifests as bleached spots on the leaves; leaves that are getting more light than they're used to may curl under, and new leaves will be smaller. In both cases, the plant is doing what it can to minimize the amount of leaf surface that's directly exposed to the sun. Excess light may also embolden any spider mites that are present, which is best avoided. (See PESTS for more on spider mites.)

Many books and websites will tell you that rubber trees do great in low light. Don't listen to those people. Although it is true that rubber trees will survive a few months in a dark corner if they have to, there's more to growing plants than merely failing to kill them.¹² Plants receiving inadequate light will drop their lowest leaves, grow larger new leaves, and stretch weak, floppy stems toward whatever source of light they can find.¹³ Leaves of dark varieties like 'Burgundy' may also turn lighter and greener in color.

Ficus elastica 'Burgundy.'
Rubber plants will benefit from summers outdoors in a shady spot, but if you let your plant vacation outdoors, you should keep in mind that it will probably drop some leaves when you bring it in again. This is just part of being a Ficus, and doesn't mean that your plant is dying, so don't panic: leaf drop should be over after six weeks, possibly well before. Variegated plants are likely to burn in full outdoor sun even if you try to introduce them to it gradually: keep them in all-day shade, or brief morning sun at most.

Ficus elastica 'Tineke' (or similar: several nearly identical variegated varieties exist and I have no idea how to tell them apart).

WATER: As with basically all houseplants, F. elastica needs a potting mix that will drain quickly, and a pot with drainage holes. Soil that stays water-logged for long periods won't necessarily kill your plant, but it may cause the lowest leaves to drop prematurely. (This can even happen if your rubber tree is merely too close to another plant that's being overwatered.¹⁴)

Indoor F. elasticas do best if allowed to dry out almost completely between thorough waterings. I know, you'd think that a plant from the (alleged) "wettest place on Earth" would be hard to overwater, but that's not the case. Go figure.

The rootballs of large pots, especially nonporous ceramic or plastic ones, can stay wet for a lot longer than you'd think, so be particularly sparing with plants in really large pots. The "thorough" part of thorough waterings is important: the rootball will accumulate minerals if not flushed out periodically, and the mineral buildup can affect the plant's ability to take up water. One doesn't need to flush heavily at every watering, but doing so about every six months will help to keep the plant healthy.

My Ficus elastica 'Burgundy' as of 23 Aug 2011.
TEMPERATURE: It's probably best to keep rubber plants above about 60-65F (16-18C), though they will tolerate lower without complaint if the change is gradual and they're otherwise healthy. Sudden changes in temperature, in either direction, can result in leaf drop, as will a drafty location.¹⁵

HUMIDITY: High humidity is definitely preferred, but they will tolerate dry air when they have to. Don't place rubber plants where they'll have hot, dry air from a heat vent blowing directly at them, though. Misting is not a good idea (see PESTS); if you want to increase the humidity for your rubber tree, it's probably best to group it with other plants, move it to a more humid room, or use a humidifier.

Spider mites on a Ficus elastica.
PESTS: Personally, I've only ever seen spider mites on F. elastica, but one should also watch for mealybugs and scale. Spider mites are especially likely on plants in direct sun, dry air, or both, but familiarize yourself with the signs and check your plant regularly, wherever it is.

F. elastica may also develop fungal leaf spot diseases under the right conditions; this is especially likely when the leaves are wet from watering or misting. It's not really treatable, and spotted leaves can't be un-spotted, but it won't spread if the leaves are dry, and is unlikely to be a major problem indoors. (It's a bigger headache for the producers, since a spotty plant isn't going to be sellable.)

Ficus elastica leaves usually have small white dots near the leaf margins, which are regularly spaced, don't rub off, and may resemble bugs to the uninitiated. These are special cells called lithocysts, which are enlarged cells containing crystals of calcium carbonate. Some other Ficus species have them too, and they're especially easy to see on dark F. benjamina and F. maclellandii leaves.¹⁶ I tried to find out what purpose lithocysts might serve, but Google was unhelpful.

Ficus elastica 'Burgundy' leaf, showing the lithocysts (yellowish). Bonus vocabulary: lithocyst refers to the cells that contain the calcium carbonate crystals; cystolith refers to the crystals themselves. Sample usage: Trish Smith's sixth wish with lithocysts' cystoliths: pith Phish with fifth dish myth.¹⁷
PROPAGATION: Most sources advise propagating F. elastica by air layering. I've never air-layered anything, but it's easy enough in theory. Check Google.

I have, on the other hand, pretty consistently gotten ordinary stem cuttings to take. Get a piece of stem about 4-6 inches (10-15 cm) long, take off all but the last leaf, plant it in damp perlite, place in a warm, bright but sunless spot, and make sure to keep the perlite from drying out. It's best if you can keep the cutting from moving around, too (movement disrupts developing roots). The process is slow, but will go faster with some gentle bottom heat. When the plant has roots (evidenced by roots growing out of the drainage holes and/or new leaves), transfer it to soil.

When I knew the 'Tineke' cuttings had rooted.
Cuttings don't necessarily need to have a growing tip in order to work; my F. elastica 'Tineke' came from a single cutting I received in the mail. I cut it in half and planted both pieces in perlite. The piece that had been the bottom grew a new growing tip, and is now actually taller than the piece that had a growing tip already.

My 'Tineke' cuttings, rooting in perlite shortly after I received them. The piece without a growing tip is on the left.

Rubber trees can be propagated from stem cuttings containing only a single leaf; a good post from someone who's done this can be found here.

Some people claim to be able to root Ficus species in water, but this has never, ever worked for me (and I've tried it a lot), so I don't recommend it. I've also seen people recommend planting cuttings directly into soil, but that tends to lead to fungal problems and kill the cutting, at least indoors.

Though planting directly into soil has worked for me at least once.
Most commercial production of rubber plants is now from tissue culture, which is either impossible to achieve in the home or merely very, very difficult, depending on whom you listen to.

My NOID Ficus elastica as of 23 Aug 2011.
GROOMING: Growers treat rubber plants with hormones to keep them compact and bushy. When the hormones wear off, as they will after a few months, the plants will begin to stretch. This is particularly an issue if the plant is being grown in low light and/or overfed. Short of treating the plant with growth retardants yourself, there really isn't a lot to be done about this. Placing the plant in a bright spot will reduce the extent of the stretching, as well as improving the plant's health in general.

The developing leaves emerge from the top of the stem, clothed in a protective sheath. In some varieties, the sheath is bright red just before the leaf unfurls, then dries up and turns brown. I've seen people on-line mistakenly refer to the sheath as a "flower:" this is understandable, because of the color, but wrong.

(Not a flower.)
Plants that are about to hit the ceiling and are otherwise healthy can be cut back and will sprout a new growing tip. If you're very lucky, they'll resprout two, but let's face it, you're probably not that lucky. (Plants that get to spend some time outdoors, and have been properly acclimated, are much more likely to branch when cut back.) If you're just trying to bring one unruly branch under control, you can cut it back at pretty much any time; if you need to do more radical pruning, it's best to wait until late spring or early summer.

There's no real trick to making the actual cut: just use a sharp, clean knife or pair of shears or whatever, and cut. The new growing tip will emerge pointing in the same direction as the new topmost leaf, so avoid cutting above a leaf that's pointing out away from the plant unless you want that stem falling over all the time. Don't prune plants that are doing poorly, as this will stress them further.

The sap is capable of staining fabric and furniture, so people usually recommend putting down newspapers or plastic or something before pruning. Rubbing alcohol is rumored to remove dried sap from skin and/or fabric. As with Euphorbias, rinsing cut ends in cold water will slow or stop sap-bleeding.

Even without cutting a plant back, though, one can still see occasional spontaneous branching. This plant has branched a second time from the base since this photo was taken.

Like all Ficus species, F. elastica has a vigorous root system (hence the bridge-building). Although plants will survive being extremely potbound,¹⁸ if you want your plant to keep growing, it's best to move your plant to a larger pot when roots start poking through the top of the soil, or you notice your plant drying out more rapidly than it used to. If you want to keep your plant in the same pot, you can root prune: take a sharp, clean knife and cut the root ball from top to bottom, about an inch (2.5 cm) deep, then repeat about every six inches (15 cm) around the outside of the root ball. Work the roots free with your fingers, discard any root pieces that have been severed, then re-pot into the same container with new soil around the root ball. This carries some risk of fungal infection (for the plant -- if you get a fungal infection from this then you've done it incorrectly), but my experience at work was that all species of Ficus breezed through the ordeal, then began a new flush of leaves shortly afterward.

Aerial roots (small, threadlike roots that emerge from the stem above the soil line) are a fairly common development on indoor plants, but not so common that I could find some in the house to take pictures of. (Sorry.) In dry air, aerial roots tend to wither soon after they appear, and it's no big deal to remove them. Plants in moister environments may hang on to their aerial roots, which some people find unattractive. If that's you, yes, you can clip off aerial roots whenever you see them, and the plant will cope just fine.

FEEDING: F. elastica doesn't have very high nutrient needs under normal indoor conditions. So yes, you should feed, but no, you don't need to get carried away with it. (You might even want to hold back on fertilizer to try to stunt your plant's growth, if it's threatening to hit the ceiling.) I've seen nothing that leads me to think that nutrient deficiency diseases are particularly likely in rubber trees.

Ficus elastica 'Shrivereana.' Image credit: Frank Vincentz, via Wikimedia Commons. Resized, color slightly tweaked.
Ficus elastica cultivars have many, many names, which is not surprising for a plant that's been grown for over a century, but they fall into a handful of broad categories: there are the green ones (e.g. 'Decora' and the species), the red/black ones ('Burgundy'), and the white/pink/green ones with blotchy variegation ('Tineke,' 'Sylvie'). I've seen pictures occasionally of varieties with speckled cream/green or green/green variegation, like 'Shrivereana' above, but I've never seen one in person. I also only know a yellow-margined green variety from photos. All varieties grow pretty much the same regardless of color, though the blotchy variegated ones are a bit slower and are more inclined to sunburn.

Around here, most of the Ficus elasticas I see are relatively large specimens of 'Burgundy,' generally in 8-10 inch (20-25 cm) pots and about three feet (0.9 m) tall. They're ridiculously popular choices for condolence plants. 'Tineke'-style variegated plants are usually sold in 4- to 6-inch pots (10-15 cm), and I almost never see green or speckled rubber trees for sale, even though I'd love to have a speckled rubber plant.

Ficus elastica 'Decora Honduras.' Image credit: Forest & Kim Starr, via Wikimedia Commons.
Rubber plant toxicity is difficult to get a handle on; I'd always assumed that it wasn't that bad, since it was so commonly-grown and so rarely warned about, but it actually is a little dangerous. Allergies and skin irritation appear to be the most common problems, and persons with life-threatening latex allergies or extremely sensitive skin should maybe not try to grow this plant.¹⁹ Like Ananas comosus, the sap contains a protein-digesting enzyme (a protease, in the jargon) called ficin-E, which may be responsible for some of the skin irritation.

The sap is more dangerous by ingestion; stomach upset (not specified, but I assume we're talking some combination of nausea, vomiting, and diarrhea) is the most likely outcome. As a rule, toxic reactions are worst in organisms with smaller body weight, so I would definitely not put this plant within reach of a bird, cat, or small dog, and the leaves bleed sap readily enough that I wouldn't trust the plant with young children either, at least not until they outgrow the put-everything-in-the-mouth stage.

'Tineke' again.
Leaving aside the toxicity, F. elastica is a decent houseplant. I certainly have more respect after learning about this whole bridge business. They do seem to place a high value on consistency, like other Ficus species do: if you don't make any sudden changes (especially with regard to light and temperature), you should be fine. You're not going to grow any bridges, probably, but unless your house is a lot bigger than I think it is, you probably don't need any, either.

Selected References:

Cherrapunjee.com (root bridges)
Atlas Obscura (root bridges)
Wikipedia (general)
BellaOnline (general)
Ray's Horticulture Site (propagation from single-node cuttings)
davesgarden.com (general & landscape)
Mgonline.com (landscape use)
University of Florida (landscape use)
Forest and Kim Starr's photo page for Ficus elastica (images, fruits, landscape use, naturalized)
ScienceDirect (abstract for paper describing the discovery of ficin-E)
Plant Care Guru (general indoor care, some specifics about fungal problems)
Tropical Foliage Plants: A Grower's Guide, 1st Edition, by Lynn P. Griffith, Jr. (Ball Publishing, Batavia IL, 1998)

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Photo credits: my own, except as otherwise noted in text. Isoprene structure is public domain, from Wikipedia.

¹ Hevea brasiliensis is in the Euphorbia family. As far as I'm aware, it's not used as a houseplant in temperate climates, though in Googling around, I did find evidence that they're grown in containers in Southeast Asia (see KENPEI's pictures at the bottom of the linked page), so probably someone out there has tried it.
More commonly, Googling "hevea brasiliensis houseplant" gets you pages where Ficus is discussed under the name Hevea. Like for example this one, which is kind of depressing. Sometimes I think people don't even try to get the details right.
² Isoprene:

Polymerization is when multiple molecules of isoprene react with one another to form a long chain of atoms with a repetitive structure. It's sufficiently technical and complex that I can't really take the time to explain exactly how it works, but that's the basic idea: a bunch of small roundish molecules (isoprene) become single, long, chainlike molecules (rubber).
(EDITED:) Isoprene can be obtained and polymerized without involving plants at all (which is delightfully referred to as "synthetic natural rubber"), but actual natural rubber's polymerization is all done within and by the plant itself, from a slightly-modified version of isoprene. (Read about IPP and DMAPP if you're interested.) The human contribution to natural natural rubber lies in extracting the tiny rubber globs from the sap of the plant, then washing, filtering, pressing, and stretching them into a useful product.
³ (I have problems with marketers.)
⁴ "Pine straw?" Does that mean pine needles? And if so, why not just say pine needles? Is it a regional thing?
⁵ I'm not even going to try to put a number to the maximum height. I ran into websites claiming a maximum height of anywhere from 1-2 feet (0.3-0.7 m), which is clearly wrong and possibly refers to Peperomia obtusifolia, all the way up to 200 feet (61 m). No doubt the correct value is somewhere in between there, but damn.
⁶ Like other species in the Ficus genus, F. elastica produces figs, which are hollow and lined inside with the flowers. They can only be pollinated by a specialized species of wasp; the seeds are then spread in the droppings of birds and other animals who eat the figs.
Figs produced by indoor rubber plants will therefore never produce viable seeds, and outdoor plants will only produce viable seeds if the necessary wasp species lives in the area. I was unable to find either the actual species name of the F. elastica wasp or its current range: all I know about it is that it would have to live within the plant's natural range. (NE India and the Himalayas, south and east to Malaysia and Indonesia)
⁷ Wikipedia says that the average rainfall for the month of July in the city of Cherrapunji is 327.2 cm, or 128.82 inches. In one month. Over ten feet / three meters of rain. If you do the math, that works out to about 0.2 in / 5 mm of rain every hour, twenty-four hours a day, for 31 days. And that's on average: some years would be wetter than that. Consequently:

⁸ Technically there is no "betel nut tree." "Betel nuts" are the fruit of the palm Areca catechu, wrapped up in a leaf from the betel vine (Piper betle), generally with lime (mixed calcium oxide and calcium hydroxide) added. From what I read on-line, both the vine and the palm fruit are stimulants, but the Areca fruit is responsible for most of the psychoactive effects; the betel leaves seem to be there mostly as flavoring. I didn't examine this particular aspect that closely, though, since it's not directly relevant to the Ficus, so if you're interested in this you should probably look elsewhere. My point is that "betel nut tree" should be understood to mean the palm Areca catechu, even though the fruits of A. catechu by themselves are not sufficient to create a "betel nut."
⁹ (I found speculation that the bridges might last 500-600 years, though as far as I know nobody has managed to precisely date any.)
¹⁰ This site's interpretation of the scene, which swings from prickly to pretentious and back again:

The room is cheerfully lit, although the source of light is hidden from us. It is the natural light of the Socialist Realist bright future. In the center is a middle-aged woman with a war medal, proud mistress of the new apartment, who seems ready to break into a Russian folk-dance. Nearby is her son, an exemplary boy and young pioneer. A portrait of Stalin takes the place of a father. The gazes of this Soviet family do not meet; the mother looks into the audience as if inviting our approval, the son looks up to his proud mother, and Stalin looks in the opposite direction, as if watching us through the half-open door, guarding the limits of the visible. The scene appears to belong to some familiar totalitarian sitcom: the characters wear appropriate Soviet uniforms and freeze in the established theatrical poses known from films and paintings, as if waiting the predictable prerecorded applause. A few neighbors with whom the family will share the communal apartment gather at the door, jolly smiles frozen on their faces. The furniture in the room is very sparse and the private objects are limited to books, a radio set, toys, a political poster, a globe with the largest country of the world usually colored in bright pink, a balalaika, an a sickly-looking rubber tree plant (Fikus) in the foreground.

I totally get the sitcom aspect (the woman in the center has a very Rhoda-Morgensternish quality to her, I think), but I think calling the Ficus "sickly" is uncalled for. (One yellow leaf is all! That's not "sickly!") It is essentially a propaganda painting: Wikipedia notes that Laktionov was known for portraying happier images of Soviet society than most people would have actually experienced on a day-to-day basis, which may have something to do with why he had a better career than most artists of the time: telling the powerful what they want to hear is a time-honored and well-trod path to success.
¹¹ It's one of my five favorite books, in fact. This is at least partly because I read it at when a young and impressionable college student, but I re-read it recently and it holds up a lot better than some of my other favorites, so I'm left to conclude that it's just a good book.
¹² (At least sometimes. With some plants in my collection, yes, I settle for not killing them.)
¹³ There are other reasons one might see leaf drop (see WATERING) or stretched stems (see GROOMING), but if you're seeing both at once and the plant isn't in a bright spot, inadequate light is likely your problem.
¹⁴ How does that work, you may be wondering. Well, one of the gases released from decaying potting soil is ethylene, which happens to be a plant hormone responsible for, among other things, fruit-ripening and leaf-dropping. Ficus elastica is more sensitive to ethylene than a lot of other houseplants, so being too close to a plant with soggy soil may cause the Ficus to drop leaves even when the Ficus's own soil is fine. (Ethylene is also a minor component of natural gas, so being too close to a gas-powered appliance may also lead to leaf drop.)
¹⁵ If you're getting the impression that pretty much everything causes rubber trees to drop their leaves: um, you're not wrong.
¹⁶ According to my source, all Ficus species have cystoliths, but when I examined my own plants, I couldn't find any on F. lyrata, F. microcarpa, or F. pumila.
¹⁷ Interpretation: Ms. Smith is apparently a sorceress of some kind who dislikes Trey Anastasio and co., and has utilized cystoliths in the casting of a spell which gives her at least six wishes. One of the wishes is that the members of Phish be told an untrue story (number five in a series of stories) involving a dish, a story so incomprehensible or outrageous that it will destroy not only their brains but also their spinal columns. The moral, obviously, is to keep Trish Smith away from the Ficuses and/or improvisational musicians.
And yes, I know this isn't as good as Nana Anna's anise 'n' Ananas nanus cookies, but it's at least longer and harder to say.
¹⁸ Seriously, some of the plants we sold at the garden center had rootballs that were like 90% roots and 10% soil. They needed water all the time, but as long as they got that, they were fine.
¹⁹ I'm erring on the side of caution. It's not clear to me that people with latex allergies (who are, I'm assuming, allergic to natural rubber from Hevea brasiliensis) would automatically also be allergic to the sap of Ficus elastica, and as I said above, there are a lot of websites who think that Hevea brasiliensis and Ficus elastica are the same plant, so it's hard to put too much stock in some of the warnings. However, Ficus species do tend to be more allergenic than plants from other genera (F. benjamina is a problem for me personally, though it's not so bad that it keeps me from growing them.), and if you've had a severe reaction to natural rubber before, you may as well not chance it. It's not like you don't have other options for indoor plants.