Wednesday, April 21, 2010

Repotting Questions (With Answers!), Part I

Today we have assorted answers to repotting-related questions that I've seen either on other sites or as search terms leading people to PATSP. Part II will be up on Friday.

Is it true that some plants prefer to be rootbound?

I really don't think so, though 1) it is maybe sometimes the case that the stress of being rootbound might encourage blooming for some plants, and 2) some kinds of plants mind being rootbound a lot less than others.

This is what we mean by "rootbound."

Do you have to repot new plants when you bring them home right away, or can it wait a while?

Depends on the plant. I always used to repot everything when I first got it home, because that's what my mom did. I don't do that anymore, because it's not always the case that they need it.

That being said, it is frequently the case that new plants will be potted in a mix which is mostly peat, and would benefit from being switched to something a little quicker-draining. So changing the soil and putting the plant back in the same pot is often a good idea, though I don't usually do that unless the original potting soil is terrible.1

How do you know when a plant needs to be repotted?

Plants should be repotted when two of the following apply: they dry out quickly even though you saturated the root ball with water when you watered, the plant's roots are wrapped around and around the inside of the pot (you will have to knock the plant out of the pot to determine this), or there are roots growing out of the drainage holes.

Rootball of Callisia fragrans. This could probably go a little longer if it needed to, but if a rootball looks like this then you're probably good to go ahead and repot.

With plastic pots, sometimes trying to squeeze the root ball lightly through the pot can tell you something about how rootbound the plant has gotten: this is particularly the case with plants that have underground runners like Stromanthe sanguinea cvv., Maranta leuconeura cvv., Sansevieria trifasciata, or Rhapis excelsa.

Another indicator, which is not the most reliable but is another thing to watch for, is if you see exposed roots above the soil line. This is not entirely reliable because some plants naturally have aerial roots (Monstera deliciosa, Epipremnum aureum, Philodendron bipinnatifidum, Anthurium andraeanum cvv.) and others actually push themselves out of the ground over time, exposing roots (Pandanus is kind of famous for it, but also Chamaedorea metallica will do this), but still -- it's an indication that soil is breaking down and/or washing away.

Some exposed roots on a Breynia disticha; this might actually be normal for Breynia (I'm not sure), but it's what the kind of exposed roots I'm talking about would normally look like.

Even if older plants haven't outgrown their pot, they will generally need to have their soil replaced at least every couple years. This is because soil particles break down over time, and as the particles get smaller and smaller, they compact more and more tightly around the roots. Eventually, the particles can prevent air from getting to the roots altogether, suffocating them and leading to rot.

Every time I pick up the pot of a particular plant, there's a small pile of dirt at the drainage holes. It's composed of weirdly uniform-sized particles. Is this bad? Is dirt leaking out of the soil? Do I need to replace this? What's going on?

The saucer of my Araucaria bidwillii.

It's probably either an earthworm or a centipede in your potting mix.

Ohmigodohmigodohmigod what do I do?

Um . . . nothing?

But it's a BUG!

Yeah, so? It's not hurting anything.

But! They're GROSS!

I'm telling you, you don't have to do anything. The presence of an earthworm or centipede might cause the soil breakdown to happen faster than it otherwise would, and obviously if you're so squicked by the idea it's there that you can't sleep at night, then maybe you should dump the soil out and stick the plant back in the same pot with new soil. However, earthworms are after edible decomposing bits in the soil, and centipedes are looking for insects to eat: neither one is going to go after the plant itself. It'll be fine even if you leave them alone.

But I want them deeeeeaaaaad! And I don't want to touch them! Can't I just dump in some poison?

[rolls eyes] Okay, well, it's your karma.

If you must kill them, you can wait until the plant next needs to be watered, place the plant in a bucket, and slowly add water until the water level outside the pot is up to the soil level inside the pot. Anything that's living in your plant's soil will either claw its way to the top, where someone less squeamish than yourself can remove it, or it will fail to do so and eventually drown.

Don't make the plant sit in water any longer than it absolutely must: once you've dealt with the soil problems, get the plant out of the bucket and draining in a sink or bathtub or something. Plants can drown too.

When I repot, how do I know what size pot to use?

The usual rule of thumb is to go up by increments of one inch (2.5 cm) until you reach a five- or six-inch pot (13 or 15 cm), then go up in increments of two inches (5 cm) thereafter. So if your plant is in a 3-inch pot (7.6 cm), move it to a 4-inch (10 cm); if it's in a 6-inch (15 cm) pot, move it to an 8-inch (20 cm). This basic rule of thumb applies most of the time; there are also some special circumstances when you can jump more than two inches at once, which let's not get into those right now.

And how do I know what size a pot is?

[puzzled look]

Well, you measure it. Or look at the tag, or the underside of the pot. There's usually some indication. For round pots, the pot size is the diameter across the top. For square pots, the pot size is measured across the diagonal, not the length of a side like you'd expect. This means, among other things, that 3-inch square pots are slightly over 2 inches per side, and a 5-inch square pot is 3.5 inches on a side,2 which even confuses people in the business sometimes.3

On triangular or otherwise oddly-shaped pots, I would assume that the longest side or diagonal you can find is the technical "size" of the pot, though this doesn't actually come up often enough to be worth spending a lot of time thinking about.

When repotting, what kind of pot is best? Should I use clay, plastic, ceramic, or what?

This depends a lot on the size and species of plant you're talking about. Clay pots dry out faster, because water can seep through and evaporate from the pores in the clay. Plastic and ceramic pots, on the other hand, tend to retain water longer. So plants like cacti and succulents, which are prone to rot if they stay too wet for too long, tend to be better off in clay, everything else being equal, while more drought-sensitive tropical plants are safer in plastic. Ceramic pots are more decorative, but are also more likely to lack drainage holes; they otherwise work like plastic.

For plants which are very small, both plastic and clay pots dry out quickly, so I generally use plastic regardless of which plant is involved. For very large plants, the center of the root ball tends to dry out very slowly, and this can be very bad for certain plants, so I try to use clay whenever I can, to speed drying. The disadvantage of this, of course, is that large clay pots are very expensive, easily broken, and extremely heavy.

Plants that are very tall and top-heavy for their pot size will benefit from the extra weight of a clay or ceramic. Plastic pots can be weighted down by adding a layer of rocks to the bottom or using a potting mix with added gravel, coarse sand, or "aquatic soil." (See Friday's post re: "aquatic soil.")

What's the difference between a standard pot and an azalea pot, and how do I know which one to use?

Standard pots are the same height as their diameter. A four-inch round standard pot is also four inches tall. Azalea pots are only 3/4 the height of their diameter, so a four-inch azalea pot is only three inches tall. The difference between them is really fairly minimal for most purposes.

I tend to favor azalea pots for their slightly lower center of gravity. (Plants get knocked over a lot here in the Subjunctive Botanical Gardens.) I do not recommend using pots that are taller and narrower than standard pots, or shallower and wider than azalea pots: the first are so top-heavy that you're just begging the plant to fall over, and the second are so shallow that tall plants like Sansevieria or Dieffenbachia may never be able to root themselves stably. One strong breeze and the plant falls over the pot edge and uproots itself.

Standard 4-inch plastic pot (orange, on left) and a 4-inch plastic azalea pot (green, right). The heights are not precisely 4 and 3 inches, but they're a little closer to it than this picture makes it look: part of the issue here is the angle from which the picture was taken.

Is it ever acceptable to put a plant in a pot that lacks drainage holes?

I don't know how to answer this question.

What do you mean? Yes or no, right?

I guess. But "acceptable" is a tricky word. I can't decide whether or not it's acceptable for you to do that. It doesn't actually matter to me. It's your plant. It'd be acceptable to me if you poured liquid nitrogen on it, if that's what you really wanted to do. Given the choice between a pot that has drainage holes and a pot that doesn't, the one with drainage holes is considerably more likely to still be home to a happy plant in a year. It's kinda up to you.
COMING UP ON FRIDAY: Choosing and amending a potting soil, adding clay shards for "drainage," tips for repotting particularly awkward (large, floppy, sharp, fragile) plants.


1 Some of the more recent plant purchases were succulents, which are fairly rot-prone as a group, and need quick-draining soil that isn't going to hold a lot of water or stay wet for a very long time -- and they were potted up in a mix that was mostly peat, which holds a lot of water and stays wet for a really long time. So I did repot them more or less immediately on arrival.
2 More or less. Different plastics companies appear to define it differently. This 3-inch pot, for example, is only 3 inches across the diagonal at the fill line inside the pot, not across the actual top of the pot:

I also found one square pot in my collection that was 4.5 inches according to the code on the bottom, but 5.5 inches across the diagonal and 4 inches on a side, so I'm not sure what measurement they were using. But in general. Strict mathematical precision is not the point; the point is not to use a much larger pot, because a much larger pot is going to stay wet for a really long time, and this may affect the plant badly.
3 During my last purchase at my ex-job, the boss herself rang me up, and although I told her I was buying 3-inch cacti, and she saw the pots right there, she only charged me for 2-inch ones ($3.50 each instead of $4.95 each). And it's not because she was being nice to me or cutting me a deal. She's just never believed me about the diagonal thing, even though she can read what's stamped on the bottom of the pots, or get a ruler and measure, just as easily as I can. I didn't mention it at the time because I didn't know that's what she'd done until I was looking at the receipt in the car. Though if I'd noticed at the store, I probably still wouldn't have said anything.


AJP said...

This might be a dumb question, but what about plants that have graduated into the biggest size pot I want to put them in... do they still get rootbound? Is that a bad thing for the plant? Should they still get their soil changed periodically?

mr_subjunctive said...


They'll still get rootbound, it's still a bad thing, they'll still need to have soil refreshed occasionally (because of the soil-breakdown thing I talk about in the post). If the plant is otherwise healthy, you may have to take it out of the pot and cut off part of the root ball from time to time, to make room for new roots. It depends to some degree on what the plant is, and what kind of conditions you're growing it in.

If it's the kind of plant that can be divided, dividing part of the plant off every so often and refilling that area with soil would also work.

Liza said...

mr_s, this is a great post! You are awesome.

Ivynettle said...

This is interesting, apparently you do measure pot sizes differently in the US. Here, square pot sizes are given as, for example, 10 x 10 cm (or even 10 x 10 x 11, including the height) - at least that's how I learned it during my apprenticeship.
Never heard of Azalea pots, either. The only differently-proportioned pots I know are the higher-than-wide ones of the Dutch houseplant growers. (We get most of our houseplants from Dutch nurseries).

Anonymous said...

There are a few plants that don't mind - or possibly prefer - to be somewhat potbound. When moving an african violet into its final pot I choose one to be its lifetime pot. That doesn't mean that soil never needs to be replenished, and of course they will be divided from time to time, but they seem happy not to be moved.

On the other hand, some plants will actually break pots as their roots grow and they create offsets. As you say, plants are the strangest people.

On the pot subject - I really like the dark brown clay bonsai pots (not glazed) for all kinds of plants. All the benefits and none of the drawbacks of plastic and ordinary clay. Cost is scary but they do last forever.

Anonymous said...

Gee, the suspiciously regular particles coming out of the drainage holes of my banana could have something to do with the night-crawlers left over from a fishing trip that we introduced last year.

I assume earthworms still provide the same function in a pot as they do in the ground, and if that's the case, I'm happy to have them.

I keep hearing that bananas (and Musa mini varieties in particular) like being rootbound, but I'm not sure how much I believe it. Mine puts on a serious growth spurt every time we pot it up.

mr_subjunctive said...


I think I've seen some pots that listed their dimensions both ways, as 10 cm x 10 cm and as 4" square. Your way seems a lot clearer, though ours is easier to text.

I'm very surprised about the azalea pots, though: almost everything is sold in grower pots with azalea-pot dimensions here. (The exceptions are the things in 1- and 3-gallon pots, which tend to be a little taller than they are wide.)


Well, okay, but African violets don't exactly get rootbound, from what I've seen. Not in the way that a Schefflera or Agave will.

I'm not familiar with the bonsai pots you're talking about; could you point me in the direction of a picture?


They do and they don't. Given the choice between a pot containing soil and worms, and a pot containing just soil, I'd go with the wormless one. Although they churn up and aerate the soil outdoors, for an indoor or container-grown plant, your soil should already be pretty well-mixed and aerated. If it's not, earthworms aren't going to help, and if it is, their main effect will be to break it down faster. That said, a few of my plants do have worms or centipedes or something, and don't appear to be any the worse for it. I don't advocate (as the post hopefully makes clear) poison or panic in the event that a plant appears to have some, but you may want to repot sooner rather than later, or check on the roots more frequently than one otherwise would, just to make sure the potting mix is still drying out in a timely fashion.


I realized after writing this that I left out the "Eco Pots," biodegradable pots made from rice hulls. This is mainly because I've never used one. They look like they're more or less equivalent to plastic pots, in that they're pretty lightweight and I don't think they probably breathe, at least not as much as clay. Part of the reason why I've never tried one is because the ones we had where I used to work looked really unstable: they were mostly taller than standard pots, with narrow bases, and I worried about them getting knocked over all the time. They were also considerably more expensive than plastic or clay, which was further disincentive.

I don't know whether rice-hull pots are any more environmentally responsible than clay: clay takes a lot of fuel to fire and transport, but I don't know what the manufacturing process might be like for rice-hull pots, or whether there might be chemicals involved in their processing that makes them less of an environmental good deal than they first appear. Both are probably preferable to plastic, environmentally speaking.

EBC said...

First, thanks for this post. I live in central Alaska where the air is incredibly dry, especially in winter, and could not figure out why all my plants were absorbing water so incredibly fast. It turns out, those clay pots had everything to do with it. I'll only use them on succulents and cacti from now on until we move at least. Do you know if painted clay pots act more like plastic or ceramic pots since the outside surface is sealed?

About the Eco Pots: I've used several and will never buy them again. I originally got them because I liked the look a lot, but every pot I've had has cracked within a few months. I don't believe it has to do with the cold up here since I've only used them indoors, unless they froze when being shipped up here, and are cracking in those areas.

mr_subjunctive said...


I would guess that painted clay winds up somewhere in between clay and plastic in terms of breathability, probably closer to plastic, but I don't have direct experience with it so I wouldn't put too much stock in my guess.

(Technically I've used painted clay, because I sometimes get painted clay pots via garage sales or whatever, but I try my best to get the paint off before using the pot.)

EBC said...

Do you normally take the paint off because of aesthetics, because you want the drying performance of the clay, or for some other reason?

mr_subjunctive said...