Hypoestes phyllostachya, or "polka-dot plant," has two main distinguishing features. One is the small pink dots all over the leaves, which are both sort of adorable and the main reason why people bother to grow the plant, and the other is its deep, burning desire to find out what's going on over there, with over there defined as wherever the plant is not already growing.
So far, this latter tendency has brought the plant out of its native Madagascar and into homes and gardens all over the world. As well as, alas, quite a few wild areas.
Yup, that's right, it's another invasive species. I found reports of it becoming invasive in Australia, Costa Rica, Hawaii, Puerto Rico, Zimbabwe, and India, though details are hard to find. The most specific information I could find about Hypoestes' invasiveness was this short article from Invasive Species Weblog, which tells us that Hypoestes isn't doing anything very dramatic in Costa Rica, but it is crowding out native species, and it's confusing the local butterflies,1 some of whom are trying to use it as a host species to lay their eggs on. This doesn't work; for whatever reason, the caterpillars in question can't, or won't, feed on polka-dot plant, and so starve, and then there are that many fewer butterflies the next year. This sort of quiet ecological disruption is likely also happening in the other areas Hypoestes has taken an interest in, because that's how these things usually work.
According to one site I ran across, Hypoestes phyllostachya is that rare creature that began its interactions with humans as a houseplant, and then moved out into the garden as an annual. I doubt this, though I suppose it's possible. Certainly it's become a lot more potentially useful to outdoor gardening in recent years: for quite a while, the regular species form, green with pink dots, was the only variety available. It's nice, and apparently not hard to grow, but not everybody's garden has space for pink polka dots. That's changed recently, with the addition of new, white- or red-spotted varieties, as well as varieties where the dots have gotten so large that the leaves are basically just pink (or white, or red) with green margins and veins. These all have variety names, but the names are wildly inconsistent from seller to seller and are best ignored.
I don't know how outdoor gardeners actually feel about the plant. We had it both springs I worked in the greenhouse, and it didn't sell well either year, though there could be a lot of reasons for that. 2008 had the historic flooding, and 2009 had the overall collapse of the economy, so neither one was a particularly good or typical year.
I have been surprised to learn, though, that some people really hate this plant indoors. I recently checked a book out from the Iowa City library (House Plants, by Andy Sturgeon; it's unclear whether it's the exact same edition as the Amazon link, though2) where Hypoestes was, first of all, categorized as a "pariah" plant,3 and then also repeatedly insulted: "this objectionable little thing," "There's one [variety] called 'Splash' which is particularly nasty," "they aren't as easy to kill as they should be." This seems to me like an overreaction, though I'll admit it's not my favorite plant either.
I've personally only ever grown Hypoestes indoors, and I find it kind of unsatisfying as a houseplant, mostly because it seems to need constant grooming in order to look nice. Since I have very little time for plant-grooming, this means that my Hypoestes will rarely if ever look nice. But let's move on to the care information, since we're practically there already. I'll try, in this profile, to give care information for the plant indoors and out, since a lot of people do in fact move from one to the other through the year, but I've personally only ever grown them indoors, so bear that in mind.
LIGHT: Outdoors, there's more or less general agreement that these should not be placed in full sun, which is too intense. After that, opinions diverge, with some websites saying that partial sun is okay and others saying they should get no direct sun at all. I personally would go with partial sun, if I were planting some outside, but it may not matter that much. Either way, the plant will let you know whether it's happy: the pink spots will shrink and/or vanish if the plant isn't getting enough light, and the leaf will bleach to a lighter green if it's getting too much.
Indoors, the story is surprisingly similar: partial sun, very bright indirect, or very bright artificial light works just fine. I've grown Hypoestes with and without natural light, and the plant doesn't appear to care which.
WATERING: I'm sure Hypoestes could be overwatered if a person were really trying hard, but it's not easily done, in my experience. All the normal cautions apply indoors: don't make the plant stand in its drainage water, water thoroughly when you do water, etc. Outdoors, I'm really not sure, but my impression of these in the greenhouse was that they always needed water, and even if they didn't need water, it was good to water them anyway because they were about five seconds away from needing water. If your plant should happen to dry out, it will usually come back, if you catch it soon enough, though you will generally have to pay for your mistake with a little defoliation.
TEMPERATURE: I hear these will not grow well below 60F/16C. They will, however, survive light freezes (they'll die back to the ground but will resprout), and are perennials in USDA zone 11. Possibly. Allegedly. Plants which are too hot will wilt, because they transpire a lot, but this doesn't necessarily mean anything's wrong with the plant: usually it will go back to normal when it cools down.
HUMIDITY: Indoors, this is potentially a problem, though I'm not sure how one knows when to blame humidity specifically. Outdoors, there's only so much you can do, of course. Indoors, you have the options of putting the plant in an enclosed space, like a terrarium (which it will outgrow almost instantaneously), setting the plant on a pebble tray (which I'm not convinced does much to increase humidity, but people keep recommending it), adding a humidifier to the room, grouping it with lots of other plants, or doing a hell of a lot of misting.4
PESTS: This plant is surprisingly pest-resistant. Outdoors, the only problem everybody mentions is powdery mildew, though depending on the source, root rot, mealybugs, aphids and whitefly could also be an issue. I've not personally ever had any pests on my Hypoestes, nor do I remember any such problems at work.
The plant is also not attractive to deer, though it's uncommon to have a deer problem indoors.5
PROPAGATION: The primary purpose in life of any Hypoestes is: to get somewhere else, so it is an extremely agreeable propagator. The flowers are self-fertile, and if you permit the flowers to develop will produce seeds, which have a tendency to fall into a neighbor's pot and sprout (hence the "person" for this profile: Hypoestes phyllostachya is possibly the single best example of a plant that is always trying to get all up in another plant's business). This self-seeding, indoors at least, happens just often enough to be annoying, but not so often that one can rely on it for propagation purposes.
Polka-dot plant is much more reliably propagated from cuttings. In fact, you can treat cuttings pretty much however you want, and they'll work. Planted directly into damp soil, cuttings will look dead as hell for about three to ten days, and then all at once they will stand up, dust themselves off, and start looking for neighbors to seed into. Rooting cuttings in water is even easier: water-rooted cuttings don't even play dead, and are looking for new territory before they even have roots.
GROOMING: And you will have cuttings to work with. Oh, will you ever have cuttings to work with. The reason for this is that Hypoestes needs to be pinched back ruthlessly, all the time. Which is okay if you have that kind of time and focus, but it does, seriously, require a bit of dedication.
The flowering stalks are also not nearly as attractive as the regular foliage, and the latter turns into the former in a gradual, sneaky way: the new leaves start out large and then steadily get smaller and more vertical as the stem lengthens. Eventually, short-lived flowers start popping out of the spaces between the leaves and stems. Ideally, you will want to pinch back any branch that starts looking like it's going to turn into a flower spike as soon as it starts to happen, but if you don't, all is not lost. The stems can be cut pretty much anywhere at pretty much any time. If things turn really dire, you can even cut all the stems back to the ground: the plant will (usually) start over again by sprouting new stalks. With a plant that's gotten tall and leggy, this is often the best way to start over.
I'm not sure how much of the grooming stuff is applicable to people growing this plant as an annual outdoors: they may be less leggy and weird under more ideal conditions. No experience with this.
FEEDING: As you would expect, a plant that grows this fast needs to be fed pretty regularly too. I don't think they're terribly particular about the formulation you use, though since you want to promote leaves and not flowers, a high-nitrogen fertilizer (like 24-8-16) is probably better than a 20-20-20 or whatever.
I have reluctantly concluded that polka-dot plant must be toxic to children and pets (which probably explains why deer won't eat it). Nobody is willing to identify a particular toxin or provide any specifics about how dangerous it is, but it does tend to be on all the lists. A few sites seem to really emphasize that Hypoestes is toxic to cats, though I don't know why, whether the listmakers just think cats are really important or whether it's significantly more dangerous to cats than other animals. You know how it can be with toxicity lists. I expect it's probably toxic to a lot of things, though, which might explain the Costa Rican caterpillars.
The poor, confused things.
Photo credits: all mine.
1 It turns out that butterflies actually are the airheads you've always secretly, deep down, suspected that they were.
2 And I don't recommend it anyway: the pictures are beautiful, but there's remarkably little information in it. Each plant he discusses gets maybe 200 words, if it's lucky, and a lot of that isn't care-related: it's just, like, a verbal description of this plant that there's a gorgeous picture of on the same page. The book might be decent for finding out what you have, because the pictures are generally large and clear and pretty, but it would be very close to worthless for trying to figure out how to take care of it. The one positive (besides the photography) is, there's a bit of snark here and there: although I believe that all plants have their good points, I like a plant writer who's not afraid to stake out positions against certain plants occasionally for the sake of being interesting.
3 Part of the category description: "Some plants are unquestionably vile. Laws should be passed to prevent their sale in the shops and they should be made unwelcome in our homes. There are plenty to choose from: brash gaudy things that totally lack style and plain ugly things without any obvious merits. Many were popular in a bygone era and quite frankly should have been left there."
I agree with Sturgeon about some of the plants he calls pariahs (Begonia rex-cultorum, Ficus elastica, Codiaeum variegatum), disagree about others (Saintpaulia ionantha, Epipremnum aureum), and have mixed feelings about, or no experience with, the rest (Hypoestes phyllostachya, Peperomia caperata, Impatiens walleriana, Solenostemon scutellarioides, Aphelandra squarrosa).
4 Of those options, the terrarium is the most effective, followed by humidifier, grouping, pebble tray, and misting, in that order. Misting sounds like a good idea, but if your home has any kind of air circulation in it at all, the humidity gain you get from spraying water around is going to be lost the moment the moistened-air has a chance to drift away. You either need to prevent the wet air from leaving (terrarium, grouping) or continually replenish the moisture (humidity, grouping, pebble tray). Misting doesn't really accomplish either.
5 And if you find that you do have a deer problem indoors, usually you can manage it by spraying plants with soapy water: this interferes somehow with the deer's natural egg-layinga cycle, as well as killing deer larvae.
a Look it up.