How to Choose Correct Container Size for Different Plants

newbie_in_nj(6b E/Central NJ)May 25, 2009

I don't think the search function is working as I tried a number of times to find an old threads on this topic and others.

I'm putting some firt year perennials and many annuals in containers.

Last year I just guessed and now realize some were jam packed but since they were annuals didn't stop plants from blooming but a few pots had plants with stems at all angles and I think were stunted by being so packed.

I know choice of appropriate size container is more important for perennials as they will be staying in first pot for this year. Don't know what's too big even if it drains well and what's too small and will constrict root development.

I realize I can just "try and see what works" but I don't have an endless supply of containers so have to choose carefully what goes where to start.

Is there a web site anyone knows of that provides some guidelines?

I didn't want to be cheeky and post a list of many plants I'm potting up for someone to go through and make suggestions. I'd like to but that's a lot to ask someone else to do the thinking for me if there's some online site/article/video anyone can suggest.

Thanks in advance if anyone has guidelines they use. I would think anticipated root mass is important but I have no idea of how I'd find that out short of year of experience, which I don't have, or a horticultural degree.

I found a video online that stressed size of plant "above soil" as key so plant is aesthetically in harmony with the "decorative" pot. Other than having the proper room for root development in first couple years I'm less concerned with aesthetics like seeing bare soil around perennial. I can always put appropriate amount of Sweet Alyssum seedlings into pot to fill it out and compliment perennial bloom colors. I just don't want to choose too small and inhibit first year growth and risk compromising plant's proper development while still a little tyke.

I've got a couple Container Gardening books and have looked through some at box stores but they don't tell you "how" to choose correct pot size other than tell you to take into account height and width of plant above ground.

Something like Agastache Rupestris will grow to 36-42" high and 18" wide. I have no clue as to the root mass size through years 1-3. Obviously I want a pot wide enough so it doesn't tip over with more than passing breeze. Found that out with Salvia Coccinea and tall Zinnia last year.

Sorry to ramble but I've got pots all sanitized, both regular and gritty mixes in the making with lots and lots of seedlings just dying to get out of their second growing on containers.

Thanks again for any input, advice or reading recommendations.

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tapla (mid-MI z5b-6a)

Maybe someone will come along with a website that lists approx soil volumes for individual plants. You were so considerate of us that it makes me want to help you even more, but it's difficult to make suggestions w/o knowing what plant material we're talking about. I will say though, that in the 5:1:1 mix, and especially the gritty mix, you can get away with erring on the large side for container choices. Over-potting is a major consideration when using water-retentive soils, but when using soils designed to hold little or no perched water it is much less a factor - almost none, with the gritty mix. You can grow very small plants in very large containers (if you want to - I'm not recommending that you grossly over-pot everything because making soil takes time & $) when using well-aerated & free-draining soils.


    Bookmark   May 25, 2009 at 10:27PM
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newbie_in_nj(6b E/Central NJ)

Dear Al, you came very close to offering to review a list of the plants I have to put in containers...LOL.

If you are considering making that offer just let me know.

Don't know if it's appropriate and/or would help others facing same choice with first year plants to do it on the forum.

What do ya think?

If you're offering, I'm willing to supply you with a list. It's not long compared to the number some people grow.

My Sedum and Echinacea are happy plants now that they're in the gritty mix. They came through winter outdoors and look great. I just put them up against the house foundation with bags of leaves/pine needles all around for insulation. I don't have to shelter them from "normal" rains as they flow freely enough on their own. Amazing that a tall Sedum could make it through winter in a pot and look better than the sad top heavy thing I bought on sale in Fall.

Let me know if you're willing and able to take on this "small by comparison" container selection. I know I don't have to get this selection perfect as I can always go up in size next year but I just don't want to restrict the root mass development and compromise the plants.

For the record, that's as cheeky as I can be but if you're offering your experience/recommendations I'm in for the taking. :)

    Bookmark   May 26, 2009 at 6:10AM
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tapla (mid-MI z5b-6a)

Go ahead and post it. I'm sure (the collective) we can figure something out. ;o)


    Bookmark   May 26, 2009 at 9:08AM
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What was the outcome of this post? I am faced with the exact same dilemns (i.e. new to container gardening & unsure of "proper" pot size). I have already purchased some small starter plants from the nursery and am all ready to go but want to pot them correctl the first time. I have.....

Hens & Chicks
Pachysandra Terminalis
Japanese Painted Fern
Moonbeam Coreopsis
Plantain Lily Hosta (aureomarginata)
Sentimental Blue Balloon Flower
Bath's Pink Dianthus
Palace Purple Coral Bells
Autumn Joy Stonecrop
Sweet Woodruff
Ajuga (a purple leaved variety & one with more greenish brown leaves)

    Bookmark   June 10, 2010 at 1:42PM
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jenn(SoCal 9/19)

I found this while searching this forum about Agastache. I would dearly love to grow many of the southwest scented Agastache varieties in our rich clay soil -- not just for one year. But after reading the above about gritty mix in large plants, I have new hope!

    Bookmark   April 21, 2011 at 11:35PM
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tapla (mid-MI z5b-6a)

There are tons of info about the gritty mix and soils in general on the forum, so if you stick around, I'm sure you'll find a conversation you can join, or just start a thread. If you have questions, you can ask them at the link below, which has sort of become a gathering place for discussions about the gritty mix and other soils, like the 5:1:1 mix.


Here is a link that might be useful: More on soils and the gritty mix

    Bookmark   April 22, 2011 at 12:35AM
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As an occasional container gardener I also struggle with "what size for what plant".

Right now it is hollyhock that I am going to plant in a pot, cuz to plant them in my garden is to feed the critters.-

Any suggestion as far as what size container?
I'd like to plant two of them in a container - if that would not require too huge a pot.


    Bookmark   April 22, 2011 at 1:30PM
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jenn(SoCal 9/19)

Thank you, Al. I did post a new question at the link you posted above.

I'm just astounded at how much I've learned from reading some of the posts in this forum starting last night! Unbelievable.

    Bookmark   April 22, 2011 at 7:41PM
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tapla (mid-MI z5b-6a)

Ellen & Jenn - A copy/paste job from a reply I wrote on another thread:

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 (like the gritty mix) composed primarily of particles larger than 1/16", 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   April 22, 2011 at 8:41PM
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thank you Al for the info.
I see your point.
But since I am only potting up one or 2 hollyhocks, (most of my gardening is in the ground) I'm not getting into the soil properties.
I'll be using one of the better commercial potting mixes, pro-mix if I can find it.
So given that, I'd appreciate if anyone has planted a hollyhock, or other tall plant, in a container, if you can give me the idea of size.


    Bookmark   April 27, 2011 at 11:02AM
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tapla (mid-MI z5b-6a)

If you're going to use a peat-based soil, then container size can be very important because over-potting is a real consideration. Some knowledge of how well the soil drains and how much perched water it supports is required to make an informed advisement about how large you CAN go in container size. Also, how large the plant material is currently, has a bearing on what container size is the best choice.

If you're planting from seed, start in a 4" pot & pot up incrementally at about the time the root/soil mass can be lifted from the container intact for best growth. If you wait longer than that, you'll lose growth potential, and if you over-pot, you'll also lose growth potential because of the cyclic death & generation of roots that coincides with your watering rhythm.

FWIW - using a fast (draining) soil that doesn't support significant amounts of perched water would allow you to plant in much larger pots w/o worries about root issues or the tight roots that restrict growth if you're not right on top of potting up at the appropriate time.

Best luck. Questions?


    Bookmark   April 27, 2011 at 11:43AM
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thank you very much.
This is more complicated than I thot, but I appreciate the explanation.
Definitely more than I knew before!


    Bookmark   May 4, 2011 at 5:56PM
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I've been known to positively stuff large patio planters full of annuals and perennials for that full look... but by autumn, I'm tearing out the annuals, which are destined for the compost pile, and re-planting the perennials in the garden... or in pots of their own. If you're only growing the plants for one season, I don't think size of container is overly important... but if the plants must remain in them for more than one season, you'll need to consider size a bit more closely.

I agree with Al... the more durable and free draining a medium is, the less worry there is about over-potting. You're giving yourself a much wider margin for error.

If I'm using a regular bagged soil, I use a pot that gives the plant a few inches of extra room for growth, but not too much, because the soil will take a long time to dry out. If I'm using a rendition of the Gritty Mix or 511, I opt for a larger container, because I want the roots to have plenty of growing room, and there's very little worry about any perched water... or excess saturation. I take into consideration the size the plant will become, and worry less about the size the plant is now.

Agastache is a gorgeous perennial... but I've personally found that the more common varieties are rather invasive. I've been pulling out seedlings and spreading roots for three years, now... from only one plant!

The Palace Purple Heuchera is the largest of the Heucheras, I do believe. It can spread quite nicely, but isn't overly aggressive.

Various Dianthus varieties are lovely, but they tend to die out within a few years, and seem to be a short lived perennial... at least, that's how they behave in my gardens.

The ground covers you mention are nice ones, and I've grown most... the Pachysandra has taken a few years to form a nice clump, the Sweet Woodruff has spread like wildfire, and the Hen & Chicks behave nicely, forming a pretty little colony. If given the right conditions, Ajuga will form a nice colony, spreading by way of runners, rather like Strawberries.

Autumn Joy is wonderful for late summer color, and leaves an interesting form through winter. The clumps I have haven't been overly aggressive, spreading slowly.

Ferns don't seem to do well in my gardens... I think the critters like them too much, or the soil simply isn't to their liking. I'm not really sure... they're not my area of expertise.

I probably should have prefaced all that with the fact that I garden in zone 5b, Central Illinois.

I do think you've chosen some nice plants to grow, whether you place them in containers or in the garden. I hope you enjoy them as much as I do, even though most of mine are in the ground. :-)

    Bookmark   May 5, 2011 at 12:09PM
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