We have spring-time summer at present at my location. Hot and wonderful weather that demands a need to cool down. The small mame-bonsai are in very shallow pots with a limited space for roots and soil. The sun and warm weather heats up pots, roots and soil rapidly. This happens faster than it is possible to cool them down with water. No matter what, the soil is heated to an unhealthy level if precautions are not taken. The space is so narrow and limited that normal transpiration isn’t enough to keep the inner temperature down st the trees. And roots are harmed by a soil heated up too much.
Of course it is possible to place the trees in the shade and this will cool them down to some extent, but this will also have the negative effect that growth elongate and a compact tree is ruined. Some sun is necessary to keep internodes (distance between each leaf pairs) short, to develop a compact tree needed for bonsai. Especially with tiny trees like mame (bean) bonsai it is essential to achieve a compact and detailed implied image of a tree.
Double potting is the answer to your dreams.
The normally heated area close to the pot can be cooled down by placing the pot in another pot filled with soil. The extra border of soil will make a small wall that keeps the heat from being to strong. When the moist soil surrounding the inner original pot evaporates in the heat, it helps cooling down the center part. It acts like a termo protection or isolation layer helpful in the summer heat.
Roots from the tree in the center pot also will be able to growth through the bottom holes and into the extra pot and soil, keeping it healthy growing during the growing season. Any time needed, it is possible to take up the inner pot with three, and cut expanded roots at the bottom of the pot. Double potting will also work with larger trees, if necessary.
The two mame-bonsai shown are in training period and not fully developed.