How large a container should be, or CAN be, depends on the 3-way relationship between plant mass, container size, and soil type, assuming correct watering habits. 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 3-way relationship noted, which should logically determine appropriate container size(s).

It's often parroted that you should only move up one size in containers when "potting-up". The reasoning is the soil will remain wet too long and cause root rot issues, but it is the size/mass of the material and soil type/composition that determines both the upper & lower limits of appropriate container size - not consecutive volume progression.

Plants grown in slow 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. This (smaller soil volumes) and the root constriction that accompanies it will 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 you 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 trees 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.

We know that there is an inverse relationship between soil particle size and the height of the 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 (saturated).

So, if you aim for a soil (like the gritty mix) composed primarily of particles >1/16", there is no upper limit to container size. The lower size limit will be determined by the soil volume's ability 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 root have been well managed and the lion's share of the rootage is fine.

Al

Ok, so just want to clarify:.. I will use my slightly adjusted version of your pine bark fines mix you had suggested last year, and I found last year my potato plants and a few others seemed to repsond well to the fabric pots.. I know that at one point you had had something that a more vertical pot was better drainage-wise (I also used the synthetic shammy strip for a wick at the bottom)..

so. basically you don't feel that a shorter pot is a disability for a plant that has a large, spread out root system, as long as the soil drainage is just fine?

"... basically you don't feel that a shorter pot is a disability for a plant that has a large, spread out root system, as long as the soil drainage is just fine?"

Taller pots will always have a higher % of unsaturated soil when you're using a soil that supports perched water. Illustration: Imagine a soil with a 3" PWT. In a 3" deep (cylindrical) pot, the o/a soil volume will be 100% saturated after a thorough watering. If the soil was 6" deep, the o/a volume would be 50% saturated, and if it was 24" deep, only about 12% saturation would occur.

IF the drainage is 'fine', which for our purposes I'll take to mean there is no PWT, then the issue is moot. You can grow in extremely shallow containers with no concern for perched water, and it becomes impossible to over-pot. In well-aerated soils, you can grow the tiniest of plants in very large containers, if you prefer.

I regularly grow very healthy sedum and houseleeks in containers less than 1" deep when using the gritty mix. That would be very difficult when using a bagged soil, because such a high % of soil would be saturated after a proper watering or notable rain.

Some very shallow containers with healthy plant material:

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In this last picture, the houseleeks are in less than 3/4" of soil, yet they are very healthy:
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Al

OMGOSH!!! Those pics are fantastic, tapla!!! Okay, I understand what you're saying regarding the perched water table, and that it is not so much container choice as the soil therein, etc.! Thanks so much! Just wanted to make sure I was grasping the information and I think I can make an intelligent choice based on this!

:)

Okay, I am now rereading your original message for the third time and have one more question.. you had talked about managing the roots so that the larger ones dont block out the water/nutrient capability of the fine roots.. and this also makes a difference.. I have heard that every other year, one should root prune 1/3 of the roots (I'm assuming the large ones) and then replace in the container.. is this what you mean by "managing the roots?".. And the smart pot claims to "air prune" the roots as a benefit.. is it claiming to automatically keep the roots at a nice, healthy size because the porosity of the container is stopping the roots from circling?

I don't think I talked about actively managing roots in this thread - that I can find. What I usually mean by managing roots is the periodic removal of a fair portion of the largest roots of woody plants, along with varying %s of soil, which makes room for fine roots to grow and replenished old soil that might be collapsing with fresh, structurally stable soils.

You still need to do this (root prune), even when using containers that automatically air-prune roots to help prevent them from encircling. The reason is that roots in the center of the root mass still continue to increase in diameter and have the same potential to choke each other. These roots, as they get larger, also diminish the available volume of soil suitable for colonization by fine roots. In the same way that plants left too long in conventional containers will become extremely root-bound and permanently stunted, those plants in containers like 'smart pots' will follow suit. Pots that air-prune roots only help to prevent them from becoming encircling roots, and don't relieve you of the need to manage roots of plants you intend to grow long term in containers.

I hope that covered your question?

Al

That totally coverd my question! Sorry.. I always end up writing a million things just to make sure I understood. I'm totally satistfied, now.. hope I didn't drive you nuts getting to that point! :)

I'm so glad Al is back. Outstanding information Al, thank you for going into such detail.

Thanks, guys! I wasn't gone long, but it was long enough that I missed all of you, too! I always have plenty of other things to keep me busy, but DO have fun here.

Take care.

Al

Yea, what Al said...lol. :) Tomato plants, for instance, can grow in something as small as a 5 gallon container, but come a big storm, it might break the thing in half. I prefer a bit of depth to my containers also to keep the plants sturdier. I pot up the stem about an inch from wee seed started seedling to mid-stage. Then, from mid stage to final home, I pot up about another 6 inches. This helps the plants become sturdier in a way & they also grow new roots off the buried parts of the stem. Like Al says though, it depends on a few things like plant mass, etc.. For instance, I have an Aloe plant. They prefer somewhat shallow, but very wide containers.

Hmmm - You're much safer planting aloe in a deep container than a shallow container for the reasons I noted upthread.

If you are using a bagged soil that supports perched water (virtually all do), it wouldn't be wise to pot in a shallow/wide container because such a high % of the soil remains saturated after watering. E.g., a soil that supports 3" of perched water in a 4" deep container, will be 75% saturated after a thorough watering, a condition at which aloe would surely rebel. If, OTOH, you are using a coarse mix that supports no perched water, the depth of the container hardly matters at all, and you CAN grow w/o worries in a shallow pot.

It's very unlikely that the plants in the shallow containers pictured upthread would be so healthy if they were in a water-retentive soil.

Al