Of course your choice of pot has a lot to do with the aesthetic vision you have for your tree, and I’m certainly not going to get into a debate about ‘feminine’ and ‘masculine’ trees and pots (hint – I’m not a fan of gendered bonsai!) Or glazed/unglazed (etc).
The pot for your bonsai is more than just its physical receptacle, it is also a life support system, holding water, soil & microbes and providing physical support. There are some physical attributes of pots which promote or inhibit a tree’s health, including materials, volume, width/depth ratio, drainage and even colour.
To start with materials. I had quite an unsatisfactory experiment with concrete in the form of hypertufa, for a while I was trying to save money by making bonsai pots using this material. Hypertufa is a combination of cement, sand, an organic material like moss or coir, and perlite/vermiculite. It worked well for making pots, but I found that they dried out really quickly; further reading told me that cement and particularly hypertufa is very porous. It also can leach calcium & silicon, which may or may not harm/benefit your tree depending on how much comes out.
A study on tomatoes showed better results for plastic pots in winter, and clay pots in summer, which related to the temperature of the pot.ref Clay/ceramic pots did not heat up to the same degree as plastic. As an illustration of this is below – a thuja occidentalis has suffered heat stress damage on one side of the pot.
This comes down to the principle of temperature buffering – or, the ability to withstand temperature variations without transmitting these to the roots. Buffering is improved when the pot is larger, and when the surface area at the top of the pot is reduced. On the other hand, some species respond well to having warm (not hot) roots.
Be aware that a dark or black pot will get hot out in the summer sun. In one study, a black pot caused the growing medium to be up to 10 degrees C higher than the air temperature.ref In a sunny or hot locale, this could prove deadly to roots if maintained for too long. Where the pot is positioned and the foliage of the tree in it will affect how much sun hits the pot. One study found that plants grown in white pots had 2.5 times the root density of those grown in black or green pots.ref
The geometry of the pot affects evaporation rates, since more surface area provides more space for evaporation to take place. You can see the differences in the table below. Yellow highlights show two pots of similar volume. The 7cm radius pot with 5cm of depth has a similar volume to the 9cm radius with 3cm depth. But the second pot’s surface area is 1.7 times larger than the first. This will significantly increase evaporation. It’s probably no great surprise to anyone who has bonsai that a shallow wide pot requires more careful attention to watering.
Another aspect to consider in choosing a pot is the geometry relative to the tree being blown over. Whilst a heavy pot can compensate for geometry somewhat, a tree wired into a pot is effectively a giant lever with the fulcrum at the edge of the pot. Wind coming sideways onto the tree will push the lever and if the pot is too narrow relative to the height of the tree, the tree will fall.
You can calculate the force needed to turn a tree over based on the fulcrum position of the edge of the pot. The larger the difference between the pot radius and the centre point of wind force on the tree, the less force is needed to push it over. I’ve done some calculations on a 2.5kg tree with a 30cm-ish diameter foliage canopy, and you can see below that once it gets to 50cm tall with a 20cm wide pot, the wind needed to push it over becomes much lower.
If the centre point of wind force on the trunk moves upwards, the surface area of the foliage increases, or the pot width decreases, you can soon end up with instability. My suggestion is to test this when you’ve repotted, push the tree at the point where you think the wind will be centred, and see how much force is needed to push it over.