Beginner Gardening: Good Growing Practices - an Overview for Beginners, 1 by tapla

tapla wrote:
Peg - you probably won't get any benefit from the root stimulant, but ..... What are the NPK% of your fertilizer?

TR - I'll end with some thoughts on what REALLY determines what size pot is appropriate for what size plant. You'll be surprised to learn it's the soil - not how big the plant is or the size of the pot it was last in.

2 things determine whether or not a repot with root pruning is in order is 1) is it an appropriate time to do a full repot. Almost all houseplants, unless winter growers, are best repotted in the summer, in the month prior to their most robust growth. 2) how congested the roots are, and how congested they are GOING to be when the next appropriate time to repot rolls around. Growth becomes affected at about the state of root congestion where the root/soil mass can be lifted from the container intact. A reasonable guideline would probably be found in asking yourself this question at the appropriate time for repotting, "Will the plant reach that point of root congestion before it's less than half way through its most vigorous growth period?" If the answer is yes, it should probably be repotted. If the answer is no, it can probably wait another year.

In many cases, repotting (as opposed to potting up) allows you to use the same pot for a number of years before the plant outgrows it. You remove the old soil, reduce the roots by 1/3-2/3, and put the plant back in the same pot in fresh soil. After only a few repots, you'll know intuitively when the plant needs a larger pot. Which is the perfect lead-in to what determines appropriate pot size.

Here's a copy/paste job of something I wrote a while back:
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 and zombie-like 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 just 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.

Al