Overpotting

by Brent Walston

edited by Robert Potts

Introduction

Overpotting a bonsai tree into a container that is too large is a mistake that many bonsaists make unknowingly. The following is an explanation of why this is a problem. It is taken from several posts on the Bonsaisite Forums, by Brent Walston.
RP

Let's start with the physics

Water will drain from a pot until the lowest level of saturated soil (that can be supported) is reached. At this point drainage stops and this saturated layer remains saturated, no more water will drain out, ever. The height of this column of soil depends on the nature of the mix. A coarser soil will have a lower (shallower) column or layer of saturated soil than a finer mix. The total retained amount of water is less for a coarser soil.
Water can be removed from this saturated layer in two ways: evaporation (the water will be wicked upward as water evaporates from the surface), or by the absorption of water by the roots (powered by foliage transpiration). Of these two, removal by transpiration is by far the most effective. To prove this to yourself, just place two pots of identical soil next to each other, one with an established plant in it, the other with no plant. Water them thoroughly and then compare the weight of the pots over the period of one hot summer day.
If the plant is not root established, it cannot remove very much water by transpiration. This leaves too much water in the parts of soil without roots. In the short run, this is not much of a problem. In a proper environment, the plant will grow and will root establish quickly so that the saturated level is wicked dry in a day or two after a few weeks or months of growth.
However, if the pot is so large that the saturated level cannot be removed by normal root colonization, problems begin. This is not dependent on the soil type. With coarse soils a larger pot could be tolerated, but there are still limits to the space that can be quickly root colonized.

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 it stays too wet. The organic amendment quickly decays in this wet environment, particle size decreases, soil collapses, the 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. Using only stable inorganic amendments would avoid this scenario, but there are other problems.
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. Carbon dioxide 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 overpotting that doesn't require daily watering, then you don't obtain an optimal soil growing environment.

And finally

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 shown 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.