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).
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.
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.)
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.
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:
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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.
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.
Cherrapunjee.com (root bridges)
Atlas Obscura (root bridges)
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)
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.
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.