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.

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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.