Just like a nursery, a bonsai artist growing material for himself will need to periodically take care of it. This will mean trimming roots and up-potting. Depending on the size of the material, an assortment of pots must be available for the plants growing needs. I have become extremely fond of the terra cotta pot. I understand they are not for every climate and they will freeze and break, but my climate is temperate and I can get many years of use from them.
I like the bulb pans. They are getting very difficult to find since bulb planting seems to be on a decline. Most nurseries bring them in around Halloween since winter blooming bulbs are planted in late Sept. Paper white narcissus and crocus are traditionally planted in this type of pot, and displayed thru the chilly winter months in anticipation of the spring to come.
I shop all year looking for the pots at every nursery I stop at. I travel around the state quite frequently and always keep a sharp lookout for the pots. Many nurseries will have them leftover from the previous year, and many nurseries are not restocking if they did not sell well. This economy does not allow merchants to carry over stock like in years past so they need to cash out and rebuy what sells.
I purchase mine in the six inch size, eight inch, ten inch, twelve inch and fourteen inches across. The large ones are very difficult to find, as well as the six inch. They are the perfect size to start the second season cutting in from the previous year. I can keep tridents and elms in the six inch untill the trunks are about 3/8 inch. The I move them up to the eight or ten and grow them out untill about an inch or more. This year I plan on moving some large tridents into the 14 inch size, since I found some this year. The terra cotta seems to hold a great deal of moisture due to the clay being low fired and pourus. Regular highfired frost proof bonsai pots are more vitreous and so do not retain the moisture reserve a terra cotta pot can. They also breath better and allow for more air exchange within the potted soil.
If one can find then I highly reccomend growing out material in these clay pots.