Practical Joker (Alternanthera dentata 'Purple Knight')

This plant is the oddball of the large and motley group of plants in the greenhouse at work. For one thing, nobody knows what it is: I get asked on a pretty regular basis about it, and by employees as well as customers. (I've even been asked on more than one occasion by the same employee, which I would make fun of, but alas, I had to ask twice myself.)


What it is, is Alternanthera dentata 'Purple Knight,' also known as "joyweed."¹ The story is that it was brought in in the summer of 2006 as an annual, and when everything got brought in for the winter, they brought this in too, and it's been living in the greenhouse ever since, getting huge and changing colors and confusing people.

The above picture is a plant I produced from cuttings and brought home: you can see that the stems and main veins are a dark reddish purple color, but the leaves are green. This is because the plant apparently needs ridiculous amounts of light in order to maintain the purple leaves for which the variety is named. This is the main one from work, which looks more maroon than purple²:


The white bits in the second picture are flowers, which are tiny clover-like things that eventually drop seeds everywhere: I've found quite a few Alternanthera seedlings growing on the greenhouse floor. I haven't tried to plant any seeds deliberately yet, so I don't know how easily sprouted they are, but I'm guessing they're not difficult, considering the common name.

My own plant came from cuttings, which is sort of an entertaining experience with Alternanthera: when you first take a cutting, it will completely collapse within a few hours. And I do mean completely. Then it will remain that way for about a week, leading you to think something along the lines of crap, I killed it, at which point you will pull it out and throw it in the trash and hope for better luck next time, unless. . . .

You forget about it and go on about your business, in which case about 75% of the time, it will pop right back up, yell "Surprise! Fooled ya!" and begin to grow. That remaining 25% of the cuttings really are dead, but you have to wait until they go black and start to rot before you can be sure.

I don't know why it's considered an outdoor plant more than an indoor one; obviously it can perpetuate itself indoors for a while in greenhouse conditions, and mine hasn't shown any real signs of decline since bringing it home,³ which makes me think it can handle normal home conditions too. It's possible that they get scraggly-looking or something after a while: the one at work got really wild and leggy all at once when it decided to bloom, and so far I haven't really managed to get it under control. It seems very focused on building long, mostly leafless stems for the flowers, and cutting these back just makes it try harder.


Care seems to be remarkably easy: I keep it pretty evenly moist – it doesn't like to be dry – and give it the brightest light I'm capable of (which is apparently not enough to maintain leaf color, but at least the leaves are still reasonably large, and it continues to grow more of them), and pinch back the growing tip when it looks like it's getting too tall. It grows fast, befitting the "joyweed" name, and doesn't seem to have slowed down any yet, so it may be the case that it's one of those plants like Tradescantia pallida that you can count on to keep you busy during a long winter. Though it could just be playing with me.


UPDATE: I didn't have a picture or a cutting handy when I wrote the original post, but I had my plant collapse on me, so there is now a small follow-up post available here with a photo of a collapsed plant. If you wanted to know what that looks like.

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Photo credits: all me.

¹ [puzzled look]
² It also looks that way in person, though like I say, the color changes a lot: bright green in lowish light, maroon if it's hanging up near the top of the greenhouse in full sun, and sort of a dark purple if it's getting bright light but not full sun.
³ Granted that I also have it in greenhouse conditions here, kinda. I'll have to do a post about my jerry-rigged mini greenhouse sometime.