Overpotting Bonsai

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"What happens if the limits are exceeded? If you are using an organic amendment such as bark, you will experience accelerated soil composting. This means that you will lose your effective soil particle size more quickly than if you used a smaller pot which is wicked dry daily. This is the most common effect. You use a pot that is too large and stays too wet. The organic amendment quickly decays in this wet environment, particle size decreases, soil collapses, saturated level increases, even more water is retained, roots eventually remain in standing water, root failure occurs with or without the presence of a pathogen."

"Even if the above doesn't occur, what kind of root growth occurs in a volume that is not wicked dry daily? When you water properly, a new charge of air is pulled into the pot by the volume of water draining from the drain holes. CO2 and other gases are purged from the soil. The longer you leave these gases in the soil, and the longer you wait to introduce a fresh charge of oxygen, the poorer the roots will be. If you create a situation such as over potting that doesn't require daily watering, then you don't obtain optimal soil growing environment.

The BEST environment is a soil that dries out daily. The best potting practice is to shift to the next larger size pot after each time the plant becomes root established as evidenced by forming an intact rootball. UC Davis studies have proven this, and I have conducted my own studies with Acer palmatum which have verified it to my own satisfaction. It is not a marginal effect, the resulting growth improvement is significant."

"The best way to achieve fastest growth is to shift (repot) just as soon as the plant produces an intact rootball. This is standard nursery practice and a well established principle. If you do this, you don't have to disturb the rootball or prune the top, thus there is little or no shock and it can be done at any time of the year. Bonsai practices somewhat complicate this, since we want specific root configurations, but for plants in training it still holds.

Ok, what's an intact rootball? An intact rootball is when you can knock the nursery can or pot off the root ball and it won't fall apart."

"Even after a plant 'apparently' occupies all the soil spaces with roots, it may still grow normally for some period of time. This is probably due to two factors that I can think of. One is that tiny hair roots are still growing, exchanging gases, absorbing nutrients, etc. Secondly, the somewhat larger roots are not yet 'lignified', or woody, and thus are still also fairly active.

I think it is better to determine 'rootbound' by both the symptoms of growth (or lack thereof) and the physical density of the roots. For our purposes (bonsai), trees should be rootpruned and repotted LONG before they reach rootbound conditions. This doesn't happen overnight. There is a long gradual procession of slowing growth over time, usually several years before all new growth stops. It is clearly evident what is happening if you stop to look."

As a conclusion; I would strongly advise regularly potting on trees into larger and larger pots as and when the root mass demands it.
Whilst it is good practice to find the right size pot for a particular tree the exact size is not absolutely essential; just don't be fooled into using greatly oversized containers and occasionally check that your trees in development (potentsai) have not become rootbound.