Choosing Correct Container Size. How? Why?

aharriedmom(8B FL Sunset 28)August 16, 2012

** I am aware of potential issues with potting into a too large pot (see 3rd paragraph) - my primary question is in regards to knowing if a pot is/will be too small for a plant. This is information that I've searched the forum for but I'm either not using the right search terms or there just isn't a lot of information about it. I feel pretty confident that I'm just not using the right search terms, lol. I can't be the only one who has wondered about this.

As you might know, I'm new to houseplants. I've made up some gritty mix & 5-1-1 (waiting for my coarse perlite to make up more) and have repotted a few plants into the different mixes. I have several more that I plan to put into one of the two mixes but I'm unsure about pot sizes. Most of them are in the same size pot that I bought them in - 4". A few are in bigger pots (either that they came in or that I put them in) the largest is 6", not counting my peace lilies which are in larger pots.

Will I do the plants a disservice if I leave them in the 4" pots? How do I decide what size pot to put them in? I know that with the gritty or 5-1-1 I can put them into larger pots with no issues (except, perhaps, initially more frequent watering). I just happen to have a lot of cute small pots or cache pots and not as many bigger ones.

Right now I believe that if the plant's roots are filling the current pot, it ought to be potted up into a larger size. But what about the plants which might grow rapidly? Of course, except for a couple, I'm not entirely sure which of my plants would be/are rapid growers - but still...

I repotted the Dark Pittsburgh Ivy into gritty mix today. It had a little root system so I felt okay about putting it back into the 4" pot. We'll see how it goes!

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Container size matters. First of all, I know of no reason a large container would be an issue (except for cost or space) unless you want to deliberately keep the plant small. That is why bonzai tends to use undersize containers. In general, the larger the better, as keeping them in small containers actually increases watering and fertiliing frequency because of reduced buffering, and will tend to slow down or dwarf plants. If the plant isn't showing excessive roots or root circling, you are probably ok for now. Some say to judge container size based on matching size of above ground foliage, and I believe this is a good rule of thumb when you have fast growing plants, but plants that are mature will not need quite so much. Best test is still to pull out of the container and check root growth. Always tend toward overpotting rather then underpotting.

    Bookmark   August 16, 2012 at 11:26AM
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aharriedmom(8B FL Sunset 28)

Okay, thanks. So simple, really.

So.... one more silly question - how would I use the foliage size to judge a pot? For instance, I have an arrowhead vine, pink butterfly Nephthytis, that is almost two feet tall but not very thick. Do I just imagine the top growth all "smooshed" down into a ball?

-- I have a few plants that are potted into clear containers which are sunk into cache pots. I'm starting to lean towards believing that's much easier because I'll be able to see root growth without disturbing the plant. :D

    Bookmark   August 18, 2012 at 8:23AM
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When your plant out grows your pot size. you can tell as the pot will not be able hold sufficient water. The entire pot is full of roots and the water runs right through it. You notice the foliage wilting within hours of water being applied,instead of days. To keep it in the same size pot, the root mass will have to be reduced, as well as replacing the mix. Al

    Bookmark   August 18, 2012 at 9:24AM
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dickiefickle(5B Dousman,Wi.)

You need to check for growing conditions for each individual species of plant .Generly a plants pot size increase only by 2 inches at a time ,4 inch to a 6 inch.
However some plants need to be rootbound in order to bloom or cast out new shoots .
If you dont know the name of a plant , posting a pic will usually get a correct reply.

    Bookmark   August 19, 2012 at 4:56AM
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Here's what I do generally. I think of the plant having about as much of a root system as it does top growth. So I try to provide enough space for that many roots. This can vary depending on what you are trying to do, and what kind of plant you have, what soil you are using, and so on. Some plants like tall containers, and some do better in wide squatty ones. It might even depend on if the container is say, terracotta or if it is plastic or glazed ceramic. Sometimes there are things that just have to come with experience. I just changed a lot of my plants to some of tapla's mixes, and I am having more learning to do even if I have been growing house plants for a long time. There's no shame in questions and none in just trying things and keeping notes on what happens. I just put a rex begonia in a pot that was "too big", it was doing poorly where it was. My thought was that the big pot would provide extra humidity for it, and since it grows kind of along the ground (rhizomatous), would give it room to grow if it should snap out of its funk and want to do that. It is doing much better than it was, and is growing now instead of dropping leaves. This was probably a plant that I shouldn't have bought, as it needs an environment that will be hard for me to provide. A really good book on house plants, (and probably you could check one out at the library) has a wealth of cultural requirements for plants. You can kind of get an idea if you need to provide something special for a plant by researching it, or if you shouldn't buy it in the first place, as in my case. Anyway, the rex will be okay for now, Not sure how it will like winter at my house.

    Bookmark   August 19, 2012 at 8:34AM
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I agree.

I hear that is a "horticultural myth" from the "pros" in the container forum. ;)

What plants would like small pots to induce blooming?

Maybe some could learn from this.

    Bookmark   August 19, 2012 at 10:53AM
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For some reason I thought this was a different forum then the container forum. :)

I think cacti like smaller pots?

    Bookmark   August 19, 2012 at 12:38PM
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tapla (mid-MI z5b-6a)

Possibly pertinent to the conversation is why it's a myth that this or that plant "prefers" to be grown under root bound conditions.

Here is a cut/paste job about what determines "appropriate pot size":

How large a container can or should be, depends on the relationship between the mass of the plant material you are working with and your choice of soil. We often concern ourselves with "over-potting" (using a container that is too large), but "over-potting" is a term that arises from a lack of a basic understanding about the relationship we will look at, which logically determines appropriate container size.

It's often parroted that you should only move up one container size when "potting-up". The reasoning is, that when potting up to a container more than one size larger, the soil will remain wet too long and cause root rot issues, but it is the size/mass of the plant material you are working with, and the physical properties of the soil you choose that determines both the upper & lower limits of appropriate container size - not a formulaic upward progression of container sizes. In many cases, after root pruning a plant, it may even be appropriate to step down a container size or two, but as you will see, that also depends on the physical properties of the soil you choose.

Plants grown in slow (slow-draining/water-retentive) soils need to be grown in containers with smaller soil volumes so that the plant can use water quickly, allowing air to return to the soil before root issues beyond impaired root function/metabolism become a limiting factor. We know that the anaerobic (airless) conditions that accompany soggy soils quickly kill fine roots and impair root function/metabolism. We also know smaller soil volumes and the root constriction that accompany them cause plants to both extend branches and gain o/a mass much more slowly - a bane if rapid growth is the goal - a boon if growth restriction and a compact plant are what you have your sights set on.

Conversely, rampant growth can be had by growing in very large containers and in very fast soils where frequent watering and fertilizing is required - so it's not that plants rebel at being potted into very large containers per se, but rather, they rebel at being potted into very large containers with a soil that is too slow and water-retentive. This is a key point.

We know that there is an inverse relationship between soil particle size and the height of the perched water table (PWT) in containers. As particle size increases, the height of the PWT decreases, until at about a particle size of just under 1/8 inch, soils will no longer hold perched water. If there is no perched water, the soil is ALWAYS well aerated, even when the soil is at container capacity (fully saturated).

So, if you aim for a soil composed primarily of particles larger than 1/8", there is no upper limit to container size, other than what you can practically manage. The lower size limit will be determined by the soil volume's ability to allow room for roots to 'run' and to furnish water enough to sustain the plant between irrigations. Bearing heavily on this ability is the ratio of fine roots to coarse roots. It takes a minimum amount of fine rootage to support the canopy under high water demand. If the container is full of large roots, there may not be room for a sufficient volume of the fine roots that do all the water/nutrient delivery work and the coarse roots, too. You can grow a very large plant in a very small container if the roots have been well managed and the lion's share of the rootage is fine. You can also grow very small plants, even seedlings, in very large containers if the soil is fast (free-draining and well-aerated) enough that the soil holds no, or very little perched water.

I have just offered clear illustration that the oft repeated advice to 'only pot up one size at a time', only applies when using heavy, water-retentive soils. Those using well-aerated soils are not bound by the same restrictions.


    Bookmark   August 19, 2012 at 7:20PM
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Now I see why I was mixed up about "stress blooming". The plant will grow many many tiny little blooms if in a small pot. Funny thing is I forgot I even tryed this first hand by accident never thinking it related to this :) I had a house plant do this, when I planted it in a bigger pot and now it is bigger and way more healthy. So you could say I tryed stress blooming and seen what it did, nothing good for the plant at all. Tiny weak little flowers. Plants that have the right root space will grow the right sized blooms and the plant will be over all healthy. Same thing with a cherry pepper plant- it was in a 2 gal container growing fine and giving my nice sized peppers. Then a month later I noticed it was giving many little flowers and growing small little peppers that were pointless in size :)

^So, is that stress blooming? Because as far as plant health and production goes, stress blooming is not good. I was wrong for ever thinking it was good, of course :)

    Bookmark   August 19, 2012 at 9:34PM
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Thanks for clarifying that Al. My assumption that a larger container is better was certainly based on using a properly draining soil. Using a smaller container because a soil retains too much water is not a scenario I would consider, and to me would be the result of a bad soil choice.

    Bookmark   August 20, 2012 at 10:45AM
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tapla (mid-MI z5b-6a)



    Bookmark   August 20, 2012 at 1:28PM
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