Wednesday, February 16, 2011

Widower (Persea americana)

One of the fundamental problems long-lived plants have to solve is where to propagate themselves. When is not as much of a problem -- you live a long time; if you don't reproduce this year, well, there's always next -- and both why and how are pretty much decided for you already: it's going to be a seed,1 and you do it or else your whole species is going to go extinct. But where?

If you just let the seeds drop, your next generation is in a decent spot -- you know this because you have managed to live there for however long it's been. However, this also means that your children are going to be competing with you for water and minerals, and you're likely to shade them so much that they won't be able to get any light, and will struggle to get going at first, which will lead some of them to early deaths. Or maybe they'll outcompete you and lead to your early death. Either way, not terribly appealing.

You could set your seeds free to float on the wind, or drop them into a body of water, to float around until they hit a suitable spot: this ensures that at least some of them will wind up far enough away from you that they're not going to compete with you. However, most wind-borne seeds will land in places where they can't grow (too salty, too wet, too rocky, too dry, etc.), and water-borne seeds are mainly going to travel in a single direction -- either around the edge of the lake or downstream. Water travel is nice in that you know that there will be water waiting for the seeds when they get wherever they're going, but it's also very limiting, in terms of total land space: sooner or later, you're trying to drop seeds around the same lake shore that your relatives are already circling, or down the same stream that's nothing but root-to-root you up and down the banks already.

Two varieties of Persea americana fruit: the black one is I think a Hass; the green is a "Slimcado," a variety that's supposed to have 1/3 less fat than . . . something. I don't think the sticker actually said what it has 1/3 less fat than.

Finally, you can take advantage of something more mobile than yourself, which is what the avocado (Persea americana) decided to do. The fruits are calorie-rich, so anything that can eat them2 gets a good reward for doing so. They even fall off the tree before they're ripe and then ripen on the ground, which is more convenient if your seed distributor doesn't climb. And the seed is (relatively) huge, hard, and rounded, so it easily survives passage through a large animal's gastrointestinal tract, which as a bonus means that the seed starts its life in a moist, nutrient-rich pile of dung.

If you're an avocado, then, it would seem that you have life all figured out, and you can just lay back and enjoy the rewards of your cleverness while your partner distributes your seeds for you. Right?

Well, you could, if only you hadn't picked this guy to be your seed-carrier:

That's a ground sloth,3 and they went extinct in North and South America about 10,000 years ago, give or take.4 But that's okay, 'cause you've got a back-up:

Photo by Leonard G., from Wikipedia.

Oh, shit.

That's a gomphothere, the New World answer to the elephant,5 which also started to go extinct around 10,000 years ago,6 though the gomphotheres stuck it out quite a while longer -- possibly lasting until as recently as 6000 years ago.

As far as I know, there is no hard evidence to show that either of these animals was definitely the distributor of Persea seeds, but there aren't a lot of candidates: whatever distributed avocados had to be a pretty big animal, because all indications are that they're supposed to be swallowed whole. There are only so many animals big enough to do that at the right spot in the fossil record. It's an educated guess.

Whatever it was, if not the sloths or the gomphotheres, went just as extinct as they did, so the central point remains unchanged. So the widow part of the "person" for this plant is because Persea has lost an animal partner, so to speak. The reason why he's a widower, and not a widow, is because the Nahuatl people of Mexico, who stepped in to take the gomphotheres' and sloths' place,7 chose the name for the plant that wound up sticking: ahuacatl, which means, in the Nahuatl language, "testicle."8

Fruits on a tree outdoors. Photo by B.navez, via Wikipedia.

No two sources agree on exactly where or when human domestication of the avocado first began, but Wikipedia says the first evidence of avocado use dates to 10,000 BC, and organized human domestication began roughly 8000-9000 years ago. There's also some confusion about the geographic origin of the species, with some sources putting the original range from Central America north to the Rio Grande River and south to Peru, and Wikipedia locating it specifically in the state of Puebla, Mexico. I'm not going to go into the history of avocado cultivation or the peculiarities of cross-pollinating avocados or any of that, nor am I going to talk about how wonderful and delicious they are,9 or how nutritious, or how easy to grow outdoors, or any of that, because dammit, this is a houseplant blog and we have to get to the houseplant stuff.

I suspect that most indoor gardeners eventually wind up trying to grow an avocado from seed. Seeds are easy to get, and easy to sprout, so it's sort of a logical thing to try.

Nobody, as a result, tries to sell them as houseplants, though I have my suspicions that a clever enough marketer could probably pull it off: the leaves are a nice, slightly metallic green, and they're not particularly difficult to grow. A new name, maybe a slightly dwarfed habit (either a small-statured variety or a regular variety that's been chemically dwarfed), some kind of made-up legend about them being lucky,10 and I think a grower could have a hit. The only thing that keeps them from being grown more indoors, I think, is that indoor-grown plants tend not to look that great. But mass-production in Florida and proper pruning of the young plants would take care of that problem, too. I really do think there's money in this idea somewhere.

PROPAGATION: The usual way one starts an avocado is, one removes the seed from the center of a supermarket-bought fruit, washes it off in running water (no soap), suspends it pointed-end-up with toothpicks over a glass of water, puts the whole thing in a warm, sunny spot, and waits for it to split open and begin to sprout.

Seeds suspended in water with toothpicks. Photo by KVDP, from Wikipedia.

This method is fairly straightforward, though there are ways to do it wrong. You should change the water regularly, lest the seed start to rot. Opinions differ on exactly how often to change the water: I'm inclined to say at least once a week, though some sources say once every few weeks. More often is better than less often. You should also maintain the water level more or less constant at about halfway up the seed: don't submerge it completely, and don't let all the water evaporate so the seed dries out.

But you don't have to do it like that. You can also start it directly in soil. If it's decent soil and you keep it moist (not waterlogged: moist) for long enough, in a warm, sunny spot, just under the surface of the soil, that's supposed to work just as well. I've started one in soil before, but that sort of doesn't count: the seed had already sprouted while in the fruit. (I don't know why I didn't get a picture. Sorry.)

Either way, as long as the seeds never dry out, germination is usually pretty reliable. If I remember correctly, although the online advice is usually to wait 4-6 weeks before the sprout emerges, the one we started in water was much faster than that. And the second one didn't even wait on us to take it out of the fruit.

LIGHT: Well, the websites all say that you should give your plant as much light as you can, up to and including full sun, but the plant doesn't necessarily have to have that much: my personal plants get some sun for a few weeks in the summer, but only bright indirect light the rest of the year, and they've done fine. They're not gorgeous, and they might grow faster if they were getting more light, but the point is that they're not that particular about how much direct sun they get, so long as the light they're getting is bright.

Plants grown under lights may grow fast enough to run into the lights, which can bleach and burn the developing younger leaves, causing them to fall off. Keep some distance between artificial light and the top of your plant.

The better of my two plants, at about 11 months old.

WATER: The usual on-line advice is to keep them from ever getting really dry, but not so much that they get sopping wet either. My avocados dried out much more slowly than I was expecting when first potted up a year ago, but lately they're needing water every couple weeks. I don't feel like they're particularly fussy about when they get water, so long as I water thoroughly.

One site said that the leaves of plants which are too wet will curl under, while plants that are too dry will drop leaves. I've never seen either behavior on my plants, and it's not like I'm super-consistent with the watering, so I'm not sure I believe any of this. I have seen curled leaves on other people's plants, but I'm not convinced that this means they're too wet. (I guess if you suspect that the plant's too wet already, this might count as confirmation.)

HUMIDITY: The usual advice is to do something to boost the humidity near the plant as much as possible. I can't say my plants have ever complained about the humidity in here, but then, the humidity here is rarely all that low. If air is too dry, you may get tip burn or dropped lower leaves, though tip burn can also be caused by mineral build-up in the soil, and occasional leaf drop on an older plant is normal.

TEMPERATURE: There's a lot of debate on-line about whether or not established avocado trees, outdoors, can take a freeze or not, but I'm hoping that figuring out who's right isn't important if you're growing an avocado as a houseplant.

PESTS: Nothing in particular. I haven't had any spider mite problems on my plants, and spider mites are the pests I'd have if I were going to have pests.11 Persea species are especially susceptible to a particular mite called the persea mite, but unless you live in an area where avocados are grown outdoors, persea mites aren't likely to be a problem on your plant.

The outdoor growers seem most concerned about beetles, caterpillars, and weevils, none of which are likely to be an issue inside either. Fungal diseases are more of a concern (especially if you're misting to keep humidity up, or if you water from overhead in a shower or something, like I do), and scale and mealybugs are always something to watch out for.

GROOMING: The recommendation for pruning young avocado plants is usually to wait until a stem gets about 6 inches (15 cm) long, then cut it back to 3 inches (8 cm), over and over, until the plant has branched out well. I have a hard time doing this to plants, even when I know they need it,12 which is one problem, but the other problem is that the plants aren't always co-operative even when you do cut them back. This one, after getting cut back, grew one humongous leaf and one medium-sized leaf, didn't branch, and then stopped doing anything at all for several months:

My stubborn non-branching plant.

You'll also want to check your plant regularly to see whether it needs to be repotted: Persea doesn't like to be cramped. There seems to be general consensus that you shouldn't repot an avocado in the winter; if you can, wait until spring instead.

You're also not likely to get fruit on a plant grown indoors, though it happens occasionally on older plants, especially if they get to summer outdoors. If you do get fruit, it won't be the same kind of fruit you originally purchased.

FEEDING: Feed with a regular houseplant fertilizer at quarter-strength with every watering, or feed every three months at full-strength. The former method is more hassle for you, but probably better for the plant. Older, established plants may need more fertilizer than younger plants of the same size.

Persea is sensitive to mineral buildup and overfertilization; either can result in burnt leaf tips. (Tip burn is also a sign of low humidity, though.) Flushing the pot with a lot of distilled water or rainwater is one way to deal with this; you could also remove as much soil as possible from the roots and replace it with new soil, though that's more traumatic for the plant.

One of my references, the New World Encyclopedia, had this to say about the avocado:
There is an important interdependency between avocados and people. The plant lacks a seed dispersal technique outside of humans. It is hypothesized that it originally co-evolved with large mammals that are now extinct, such as the giant ground sloth, with these ecological partners vital to seed dispersal. New mechanisms have not evolved, but the effectiveness of human intervention has allowed the plant to prosper. Of course, in exchange for this benefit, the avocado provides a nutritional and desirable fruit for people.

This bugs me, because of course a new dispersal mechanism sure as hell has evolved, and we're it. We don't eat the fruit whole and then pass them in our . . . er, "dung," true, but Persea americana doesn't produce the same kind of fruit it used to, either. It's shuffled its genetics around and come up with a different fruit than what existed in the past, and if you don't believe me check this out:

"Criollo" avocados, the fruit of uncultivated Persea americana. Photo by Nick Saum, Released under the GNU Free Documentation License (CC-BY-SA-3.0). Via Wikipedia.

Sure, we had a more direct and deliberate hand in the reshuffling than the sloths did, breeding them purposely to get the traits we wanted, but it's still evolution. We are part of the environment of the avocado plant, and it's changed to reproduce itself better in this new, slothless environment. Change in successive generations of an organism, over long periods of time, is what evolution is.

This also means, to extend my earlier metaphor, that the once-widowed avocado has remarried, and we're the bride. I hope this works out better for Persea than its previous marriages did.

References in no particular order:13

Photo credits: Mine unless otherwise identified in the text.

1 Unless you're a moss, fern, or liverwort. But ew. Who'd want to be one of them?
2 (EDITED:) There seems to be broad agreement that the whole plant, including the leaves and fruits, is toxic to mammals, fish, and especially birds. A lot of people reportedly feed pieces of the fruit to their pets without any problems, and it probably is safe for cats and dogs in small amounts, but I don't recommend feeding any part of the avocado to any animal, especially especially especially birds, because you can't know how your particular animal will react or how much might cause a problem, and death is definitely a possible outcome.
I didn't see much suggesting that avocado plants or fruit are dangerous to humans as a group, though some individuals have allergies to the fruit. Humans don't appear to be very sensitive to the actual toxin in avocados (called persin, from the genus name Persea), but children would probably be more strongly affected, so do treat the plant like it's poisonous if you have kids around.
One particular variety of avocado, which depending on the source is either a subspecies (P. a. drymifolia) or species (P. drymifolia), is used in cooking in Mexico, where it is said to impart an anise-like flavor to dishes. The leaves of drymifolia are apparently safe in the amounts one would normally use for cooking, but as far as I can determine, only that particular variety is safe and useful. Drymifolia is distinguished from the others by having fruits with thinner skins, and the leaves smell of anise even before being cooked. Plants being grown as houseplants from supermarket avocados are probably not drymifolia (at best they might be a hybrid of it), so I do not recommend trying to cook with the leaves of a plant you've started from seed. (Even if they aren't toxic, the flavor is not likely to be what you're looking for anyway.) Instead, I'd recommend that you track down a source to sell you a whole plant with a confirmed identity. The only such source P. drymifolia I found on-line was Rolling River Nursery, out of Northern California, but their website doesn't seem to be updated very often ("available spring 2007"), and I have no personal dealings with them at all so I don't know if they're a good place to buy stuff or not. Caveat emptor.
3 Specifically, that's "Rusty," a reconstruction of the giant ground sloth Megalonyx jeffersonii. Rusty lives (or "lives," rather) at the University of Iowa Museum of Natural History, where he is apparently one of the major draws. I mean, they have him on the t-shirts and everything:

In person, he's actually kind of terrifying. You maybe don't get this from the photo, but he's enormous -- maybe 8-9 feet (2.4-2.7 m) tall and at least as long. And there are claws, which didn't show up that well in the above photo, but here he is from another angle:

4 I'm sure it's just a coincidence that human beings are thought to have arrived in the Americas around 14,500 years ago.
5 Technically, the picture above is of a skeleton found in China, but it made the whoops-it's-extinct joke better so I used it. One North American species was Gomphotherium angustidens, which looks pretty darned elephanty with its skin on:

Public domain illustration, from Wikipedia.

As for the relationship between gomphotheres and elephants: the terminology is confusing, and nobody seems terribly inclined to spell the interrelationships out clearly. My understanding is that the order Proboscidea of mammals contains the family Elephantidae (modern elephants), and the extinct families Mammutidae (mastodons), Gomphotheriidae (gomphotheres) and possibly the Stegodontidae (no common name) if you're the kind of person who believes the Stegodontidae should be a separate family. (Not everyone believes this; it's controversial.) So they're . . . fairly related to elephants, and they look quite a bit like elephants, but they aren't technically elephants.
6 Coincidence! (It actually might have been: there was an ice age ending around 10,000 years ago. It really might not have been our fault. I would be really surprised if we hadn't found some way to make the situation worse for the gomphotheres, though, because it's what we do.)
7 The Nahuatl among others, probably; I have a less than comprehensive understanding of how many distinct groups of people have lived in southern Mexico and Central America during the last 15,000 years, or what most of them would have called Persea americana. You don't know either, so don't go judging me.
8 In fact, for quite a while, people assumed that since it resembled a testicle, it had to be good for fertility, and maybe also an aphrodisiac, because people are easily convinced to think about sex and collectively very suggestible. The avocado thus became associated with lewd and promiscuous behavior, to the point where the early commercial growers and marketers had to launch an aggressive PR campaign to convince everyone that it was possible to eat avocados without turning into a huge slut.
9 I actually wouldn't know: to the best of my recollection, I've never eaten an avocado. Which you'd think I'd eat one while I was working on the profile, just to be able to say I'd done it, but no. (The avocados in the house are all bought and consumed by the husband. Who has thus far not turned into a huge slut, just in case you had lingering doubts about that.)
10 Or maybe this would be a good moment in history to bring back the aphrodisiac rumors?
11 I worry that maybe I'm tempting fate by saying so, but I haven't seen a mealybug or scale insect in here in a long time. And it's not like I stopped checking for them.
12 I don't trust them to resprout. I'm getting better about doing it anyway, but it's still difficult for me.
13 (Also I think it's probably an incomplete list. I try to document, but I lose things sometimes. Sorry.)


Ginny Burton said...

Wow! Wonderful profile! But I'm astonished that you've never eaten an avocado. What about guacamole? You lived in Texas, so surely you've had guacamole.

What is it that you have against the fruit? Is it an association with all the avocado colored kitchen appliances of the 1970s?

Anonymous said...

I was shown a good trick for producing multi-stemmed plants. Let the avacado sprout get on its way and then take either a scalpel or an old fashioned razor blade and carefully split the terminal bud (actually it only has a terminal bud which is why you try this). I saw that it worked; in fact you can do it repeatedly if you want a bushier plant. Takes a steady hand an a relaxed attitude toward mistakes. Much nicer looking plant though when you succeed.

El Gaucho said...

Fascinating post, thanks. I never knew the modest avocado had such a history. Coming from Southern California I'm used to seeing avocado trees only growing on semi-arid hillsides, so it's hard to imagine that they once had a widespread distribution.

Paul said...

Very thorough! I'll have to link to this whenever questions emerge again elsewhere.

And I would try to grow one, but I'm on a tropicals moratorium...water and humidity demands are too much for my current setup.

Geoffrey said...

It's so funny that you're posting this now. I actually just failed at this during the past couple of weeks (I wasn't paying enough attention and the water evaporated ...). I'll have to try again with my next avocado ...

Jordan in Oregon said...

That was a fantastic piece. Seriously, a lot better than other sites 'grow your own avocado! All you need is toothpicks!' etc etc. Great job.

Pat said...

Great profile. You did leave out the one thing we all want to know. Why have you never tried an avocado when your husband eats them? Don't you share food?

Jeane said...

I don't like eating avocados at all, but I finally managed to grow some into plants! It took me three tries to get one to sprout and leaf out. I feel bad cutting the tops off to make them branch so mine are kinda spindly-looking. You can see them all here.

Dozer said...

Great post! I love all the details about the history of the plant, and the extinct critters. Too bad we don't have any more gomphotheres running around anymore.

I had heard that getting them to germinate was difficult, so I tried a batch of four seeds. Every one sprouted - oh boy! So I gave them away, and kept one. It is about 2 years old now, and can be seen here.

mr_subjunctive said...

Ginny Burton:

As far as I know, I haven't eaten them in guacamole form either.

The problem is the texture. I don't, as a rule, like heavy, creamy anything (ice cream is a notable exception, though even that gets to be a problem pretty quickly), plus I sort of have a conceptual problem with a fruit that isn't, y'know, fruity. Which I am told avocados aren't. The unfruity-fruit thing is silly (though I don't like tomatoes either, so at least I'm consistent), but the texture truly could be a problem for me.

The husband actually asked me if I wanted part of one he was having, a couple days ago, and the conversation went sort of like,

ME: No, I can't.

HIM: But there's a lot here. Just a taste? And shouldn't you have tasted one if you're going to write about them?

ME: But I already wrote a footnote in the profile that goes up on Wednesday saying that I'd never eaten one. If I taste this then I'd have to edit.

HIM: .

At some point, no doubt I will break down and try it, but we're not there yet.

If I do it right, maybe I can get a blog post out of it.


To a degree, but not absolutely. He eats all or most of the cheese, avocados, tomatoes, oatmeal, (non-green) beans, avocados, bagels, fruit juices, lemonade, coconuts, beer, tea, salad dressing, soy milk, and papayas in the household; I eat all or most of the cereal, eggs, meat, bananas, fish, milk, raisins, cookies, rum, candy, canned fruit, cherry Pop-Tarts, pears, pickles, coffee, and crackers.

Don said...

Mr_subjunctive: You left out the most important oddball fact of all: In France, they're known as lawyers (avocat).

Emily said...

Weirdly, I opened PATSP earlier this afternoon and I thought the score was like a 1.7 or so and then opened it again just now and saw it was a 4.4. But perhaps I was hallucinating.

As far your being willing to eat ice cream in small doses - Asian places (Vietnamese, Chinese) will sometimes have avocado milkshakes, which I ordered once on a whim and then repeatedly thereafter because they are delicious. You could try it in that form as I'm sure there are some online recipes.

Kenneth Moore said...

I'm slightly allergic to avocado. Doesn't stop me from eating them. But it saddened me briefly that I was slightly allergic to something with such a wonderful name-origin.

I think I tried sprouting some of these as a child, but my impatience got the better of me, I would assume. I never got one to sprout, but heck, I'll try anything a dozen times! Gonna go buy a papaya and an avocado at the store this weekend. :P

Mae said...

Widower is indeed an apropos term for these. I did research in an avocado orchard in California and at one point in the year, after they had all fallen and all the meat had disintegrated, you're left with the skin intact with the seed rattling around inside. When you step on them they pop like a pressurized chip bag and then turn your ankle on the huge seed. Their dead carcasses combined with this popping sound is creepy to say the least (because you're walking through an orchard 6"+ deep in dead leaves and other dead...animals...I know I at least tripped on a rabbit and later a deer carcass...I don't want to know what else was hiding under the detritus)

Here in California they do sell them often as a potted indoor plant. Not often enough to be called common but not rarely enough to be considered unique or rare, either.

And in case you're wondering, seeds that germinate whilst still inside the mother fruit are called "viviparous".

If the leaves become acclimatized to bright sunlight (which most houseplants aren't used to so they burn, as you stated) they will develop a thick cuticle which gives the plant a really gorgeous shape because of its firm, deeper green leaves. Thicker cuticles also contribute to leaf-curl (this doesn't indicate any health-problems, it's just a matter of physics. The upper leaf surface develops a thick cuticle while the bottom layer does not,leading to sideways displacement).

I will testify that avocados barely handle a light freeze. In 2008 in San Luis Obispo we had a sudden light freeze (rare for these parts) and hundreds of acres of avocado orchards burned. The whole tree was just dead with the brown leaves still attached. Happily, after heavy pruning and even stumping, the trees rebounded a few years later, albeit smaller and more oddly shaped than before.

Sorry, but can you tell I've been spending most of my time out in an avocado orchard for the past two years? Not actually tending the plants, mind you, but taking data from them. Because that's what we plant scientist do, take useless data!

Unknown said...

I don't know if you've already come across it but there's this book called The Ghosts of Evolution by Connie C Barlow. It's about all sorts of plants that are missing their partners. Not a scientific analysis or anything (which worked for me as a student in a non bio field at the time) but still interesting enough to borrow from a library for a bit.