Repotting Questions (With Answers!), Part II

Now for the thrilling conclusion to my two-part repotting series. Those who missed part I (primarily about when to repot, and types of pots) may wish to check it out first, but it's not necessary.

What kind of potting mix is best to use, when repotting a plant?

Ball Professional Growing Mix, package. Ball also sells (at least around here) bags that are one-half and one-quarter this size, which have way more colorful packaging; the mix appears to be basically the same either way, though.
Well, I've told you before what I use: Ball's Professional Growing Mix. (I recently found a source that's 25% less expensive than the store where I used to work: the ex-job charges $24 for the 2.9 cubic foot (79 L) bag, and Washington GardenDecor and Greenhouses, in Washington, IA, only charges $18! This is incredibly exciting for me.) This is primarily composted bark (about 45-55%), with some peat moss (looks like about 30%), and a few miscellaneous other things. Often, and especially for rot-prone succulent plants, I mix in some perlite (a white expanded rock used to lighten and aerate soil mixes) or "aquatic soil" (smallish pieces of fired clay, which improve drainage).

If there is no Ball Mix available in your area (you're better off to try independent garden centers, as opposed to large chains like Lowe's or Home Depot), look for something with more composted bark and less peat -- the breakdown of what's gone into the mix should be listed somewhere on the package, though actual numbers may or may not be present. In a pinch, mixes saying they're good for cacti and succulents tend to be okay for houseplants. (The actual houseplant soils tend to be too wet, but the faster-drying, faster-draining cactus mixes are better, and sometimes that's as good as you can get.)

Epiphytic plants like bromeliads and orchids are harder to generalize about, but Ball mix plus some unchopped sphagnum moss works pretty well for bromeliads, and for orchids I'm probably not the person to ask, but the pre-bagged orchid mixes containing lots of large chunks of bark seem to be adequate, at least.

Do not use topsoil (more or less equivalent to garden soil). Even a little. Topsoil is extremely bad for container-grown indoor plants. Even if all the other gardeners are doing it. Even if they say they won't be your friend anymore unless you do it. Just say no.

Packaging of the "aquatic soil" I use. One may also use other brands (some are made of volcanic rock instead), other crushed clay products like Turface, aquarium gravel, very coarse sand, or certain "bonsai soils." Anything that's not going to dissolve when you water, can't hold a lot of moisture, and is made of particles which are at least 3-4 mm (about a tenth of an inch) across. The main advantage of this particular brand is that it's pretty cheap, and a lot of places carry it.
What about something like Miracle Gro?
I do not recommend Miracle Gro potting mix for anything, though if it's what you have already, it can be made somewhat more useful by mixing in perlite and/or aquatic soil. 2 parts Miracle Gro to 1 part perlite works reasonably well for most tropical plants. (Tropicals which are sensitive to fluoride do better with something besides perlite added: perlite generally contains some fluoride.) For plants which are prone to rot if they stay wet too long, like a lot of cacti and succulents, or tropicals which are fluoride-sensitive, I'd recommend something more like 3 parts Miracle Gro to 1 part aquatic soil to 1 part perlite, or 3 parts Miracle Gro to 1 part aquatic soil. Those are not tremendous improvements, however, and if you're buying aquatic soil and perlite already then you may as well just break down and buy some soil that will work, rather than spending the money and labor trying to fix the Miracle Gro.¹

Does potting soil go bad? Like, if I have a bag of potting mix I bought three years ago and never used, can I still use it, or do I need to throw it out and buy a new bag?

You can probably still use it. Soil that has been stored in a dry location should be fine. I wouldn't use soil that's been flooded or rained on (the plastic bags in which potting mixes are sold almost always have holes in them, and soil that has cycled between wet and dry will break down faster), but if it's stayed dry and looks and feels more or less normal, you should be fine.

Can you keep recycling soil forever so long as you continue to add nutrients (fertilizer), or does it eventually go bad and need to be thrown out?

It eventually breaks down to the point where it needs to be thrown out. Organic material like peat and bark slowly deteriorates into small pieces that pack tightly around roots. Inorganic material, like gravel and sand, can be saved and re-used almost forever, in theory (it does break into smaller pieces, but it takes a lot longer), but I think this is probably not worth the time it would take to separate.

You should suspect soil breakdown when a plant you've had for a long time without repotting, which always did well previously, starts going yellow, or yellow with green veins. There are nutritional deficiencies that will cause this color change as well, but soil breakdown has been the cause of yellowing in a few of my plants now (Strelitzia nicolai, Dracaena deremensis 'Warneckei,' Asplundia 'Jungle Drum'), and the odds are good that a repotting will also fix whatever trace element deficiencies are present.

I've seen products in the garden centers that advertise "gel crystals" that hold water extra-long in soil. Are these good to use in the soil for houseplants?

No.

That's it? Just, "no?"

Um. No, no, no?

They don't serve any real purpose for houseplants. They won't help, and they might hurt houseplants by keeping them too wet. They're more useful in outdoor container plantings, which dry out quickly in the wind and sun, but indoors, potting mixes are usually too water-retentive as it is. It really doesn't do you any good to keep the plants wetter longer.

Growing plants in nothing but these "crystals" is also done sometimes, and I admit, the effect can be kind of interesting, in a glass container. This is best suited for plants which can be grown indefinitely in water, like Tradescantia pallida, Syngonium podophyllum, Dracaena sanderiana, etc.

Still, there are some concerns about long-term toxicity of these products: the polymer used (polyacrylamide) is safe on its own, as far as anybody can tell, but it does decompose, and some of the decomposition products may pose long-term environmental problems. (.pdf) This would be forgivable, in a risk/benefit kind of way, if there were some huge benefit to growing plants in polyacrylamide gel as opposed to a regular potting mix, but it doesn't seem to. And it's not as though there's a shortage of things to plant plants in. After all, potting soil is cheaper, also holds water, plants seem to like it, and doesn't turn into poison in 5-10 years. So.

When repotting, do you have to put rocks, clay shards, or some other material in the bottom of the pot first, for drainage?

No. I used to do that, but now I don't think it serves any useful purpose beyond making one feel frugal for recycling broken pots. I'm told that there are hints that it may actually impede drainage in container-grown plants, though I haven't actually tried to find any such evidence.

How much soil should I put in? How do I, you know, actually do a repotting?

If you're moving a plant up a size, from a 6-inch azalea pot to an 8-inch azalea pot, start with about an inch and a half of new soil in the bottom of the pot. Set the plant's root ball down in the center of the new pot, and then fill in soil around it.² Pack it down with your fingers -- if it's too loosely packed, the plant may tip sideways later, when the potting soil settles, or the new soil may compact, leaving you with the raised circle of the old root ball sticking half an inch above the rest of the soil in the pot. (If that happens, just add more soil until the level is even. It's not a big deal.)

The procedure is similar for root balls that are irregularly-shaped: you try to dig a hole the approximate size and shape of the rootball, set it in, and then pack dirt around it.

Usually, the final soil level should be half an inch to an inch (1.3-2.5 cm) below the rim of the pot, though nothing particularly bad will happen to you if it's lower than that; it'll just look weird. If the final soil level is higher than that, you'll have trouble watering from the top in the future, so you may want to do it over.

Finally, in most cases, you'll want to water the plant in. This is different from watering a plant only insofar as regular watering is usually faster -- a newly-repotted plant's soil is sometimes a little water-repellent, and needs a bit of time to moisten; water may also percolate through the soil now that there's more of it. If the soil settles significantly, you may need to add more. Two situations in which you do not necessarily want to water in right away: 1) if the plant's old rootball was already really wet when you repotted it, 2) if you think you might have injured the plant in the process of repotting (especially an issue with succulents and cacti).

Stenocereus thurberi. They're pointy and un-fun to repot.
Do you have any tips for repotting very large or sharp plants?

Indeed I do. My method for large, floppy plants is here (using a Boston fern as the example plant), and for large, sharp plants, you can try this post, which is similar but uses a screw pine (Pandanus veitchii) as the example.

When repotting cacti and spiny Euphorbias, gloves are often very helpful, though I have only encountered one or two pairs in my lifetime that long, sharp spines couldn't still poke through, and sadly, neither pair belonged to me personally. In a pinch, a newspaper, towel, or paper towel can be used to hold the cactus steady with one hand while the other firms the soil around the roots. I also hear that people can do wonderful things with tongs sometimes.

Euphorbia trigona.
Similar stuff applies to Opuntias, though because of their evil, evil glochids, I wouldn't even attempt repotting them without pretty hardcore gloves. And long sleeves might not be a bad idea either.³ Hate, hate, hate Opuntias.

For extremely large plants, it sometimes helps to have a second person around to tell you when the plant is properly vertical, and/or to help with the lifting and maneuvering. And avoid the ceiling fans.

What about for very fragile plants like Schlumbergera or Sedum morganianum?

Sedum morganianum. The leaves are pretty loosely attached, and fall off easily. Detached leaves can be used to start new plants, though, if set on top of a lean, fast-draining potting mix in a sunny spot and watered occasionally.
Not really. I have so far managed to get through life without ever having to repot a Sedum morganianum, and with Schlumbergera, I think it's best to just go slowly, assume in advance that some pieces will fall off, and be ready to try rooting them as new plants. (If any readers have suggestions on this one, leave them in comments and I'll edit the post as necessary.)

So what have I left unanswered? Anybody?

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¹ Why is Miracle Gro bad? The main reason is that it's got a high peat content, and peat absorbs a lot of water, so it dries out very slowly. Good for an outdoor container, maybe, but not a quality you want for a houseplant. Also, when peat finally does dry out, it starts to repel water, and then is hard to get wet again. So for houseplant purposes, often the wet is too wet and the dry is too dry, is basically what I'm saying. Some plants would grow okay in Miracle Gro; I've used it myself in the past for a few things just because I didn't know any better, but the water-repellency annoyed me even then.
A second problem is that Miracle Gro, unlike the Ball potting mix, is not sterilized before shipping, which means that it may contain mold or mushroom spores, insect eggs, or other undesirable things. Mold and mushroom spores are not as big of a problem as you might think, at least not for the plant. (Some molds look really repulsive, and some people may be allergic or whatever, but in most cases it's only an aesthetic issue, not a health issue, and the plant itself usually doesn't care.)
As for insect-eggs, Miracle Gro always seems to bring me fungus gnats. Which, fungus gnats are frightening for novice houseplant growers, but they typically don't harm the plants. More of an annoyance than a problem. Still, given the option, most of us would choose not to have fungus gnats, and they're very nearly a sure thing, in my experience, if you buy Miracle Gro. So I don't recommend MG for anything.
Miracle Gro makes a number of variations on the basic mix; I don't know whether any of them are any better for houseplants. But I would be surprised.
² In some cases, I also try to loosen the roots from the root ball before planting, though I don't always. If I can pull apart the mass of roots, in a badly rootbound plant, I try to do so, but if the situation is bad enough, that's often not possible, so I just plunk it in and hope for the best. With some plants, like Ficus benjamina and Spathiphyllum, I do get kind of rough trying to rip the roots apart from one another, because in my experience they're able to take it, and at least in the case of Ficus benjamina it appears to be good for the plant, stimulating new root growth.
³ Long sleeves are also a pretty good idea whenever you're repotting Euphorbias; many species have poisonous sap that can cause serious skin irritation, and it's a common enough thing for plants, especially plants like Euphorbia trigona (its shape is such that stems are very likely to poke one another in the process of repotting). Eye protection wouldn't be a terrible idea for Euphorbia tirucalli, either.

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.

I Get [Imaginary] Letters

Imaginary Reader¹ writes,

Dear Mr. Subjunctive,

I've noticed that even though you claim to be learning about annuals and perennials and all that, you never do any plant profiles about them. Houseplants are nice and everything, but let's see this education you've supposedly been getting already.

Imaginary Reader

P.S. Despite my insinuations above, your blog is the Bestest Blog Ever!!!1!!1!1!1!!eleven!!1!1, and if you ever wrote a book -- which, by the way, you should do -- I would totally buy, like, five hundred copies. Of the hardback. And then read each copy one at a time. And buy five hundred copies for each of my friends.

Gerbera jamesonii.
Well, thanks, Imaginary Reader. I'm blushing, I am. The reason why I continue to neglect the outdoor stuff is the obvious one: I still have nothing to say beyond ooooo, purty flowers. Don't misunderstand: I'm developing enthusiasms and attachments and interests, but I'm still not trying to grow much new stuff at home, by myself,² which is a different enough thing from growing it at work that I think my work experience kinda shouldn't count yet. So I'm not getting any experience that strikes me as bloggable. I mean, I trust most of the readers who could possibly care already know as much as I know about Impatiens, which is that you don't put it in sun. Unless it's the kind that's supposed to be okay in sun.³ In which case maybe you can put it in sun.

And, while it's true that doing a plant profile almost always results in me learning something, and therefore, maybe I don't actually need to know much before beginning the profile, with a lot of these plants, I already think about them a lot more than I would really like to. To return to the Impatiens example: it's everywhere at work; I can't get away from it. I hate it. Fucking Impatiens. Maybe after it's all gone and I can look back nostalgically, maybe then I'll be interested in thinking more about it, but for right now, I'd rather have some kind of complex dental surgery.

Thanks for writing,

Mr_Subjunctive

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Photo credit: me. "I Get Letters" inspired by the same feature at Cactus Blog, who apparently don't have to resort to making letters up.

¹ As in, I made up this letter myself so that I could answer it. I hope this doesn't disillusion anybody.
² I did buy some coleus (Solenostemon scutellarioides), though I think I intend to keep it inside, and I also got three six-packs of Portulaca for the outside planters. WCW sold me on the Portulaca by saying that they re-seed themselves every year. I would, no doubt, grow more than that, if my "yard" wasn't 2.75 ft² (0.26 m²)spread over three different planters.
³ We have some newish varieties of a New Guinea-type Impatiens, that are claimed to be okay for full sun. I can't really get a read on whether we believe this to be true or not; WCW seems maybe a little skeptical. They are, in any case, doing okay in the greenhouse so far, in full sun, so maybe.