A friend and I published a study that we did about protecting the bonsai root mass from solar heating in the ABS Journal a couple years back (American Bonsai Journal Volume 53, number 3), . It can be accessed here:
The bottom line? Of course the first choice is moving to shade and repeated watering, but if you are unavoidably away from your trees during the daylight hours, the most effective strategy that we found was covering the entire pot and soil surface with white cotton t-shirt material. Covering with other non-permeable or insulating materials appears to impede radiative, evaporative, and/or convective cooling, which is crucial for the pot, soil, and root mass to give up any absorbed heat. The cotton reflects a good deal of sunlight but allows the pot to breathe.
Nice article, thank you!
What I found usefull is keeping the trees closer together, that helps them cool down easier, and painting the benches white, there is about 15 degrees C difference in the surface temp between the one painted white and the other that is coated with brown stain.
This is a great study and paper! Along with an important conclusion that just covering the top has a negative impact on heat loss through the substrate.
I had the privilege of talking with Larry Jackel a couple weeks ago at the bonsai display at the Denver Botanic Garden. In addition to covering the part or all of the sides of a pot that get afternoon sun, he also uses coir floor mats under the trees. Despite their brown color, I assume that they benefit the space through evaporative cooling?
To Jayhawk: I’m guessing that the coir mats do at least three things: first, provide more physical space beneath the pot for air to circulate and water to drain, facilitating convective cooling; second, to thermally isolate the pot from direct contact with the stone platform, reducing conductive heating from stone to ceramic; and third, provide evaporative cooling for the stone platform after watering (the coir holding a small amount of water that evaporates slowly instead of just running off the table immediately). The stone platforms are stunning visually but also absorb and later radiate a lot of solar heat to the pot and tree. I’m guessing that adding a thin porous cotton layer over the soil (in addition to the white paper(?) covers over the pot) would further reduce direct solar heating of the root mass while allowing evaporative cooling from the soil to continue. The coir would not have as important a role if the platforms were thermally insulating (e.g. wood) instead of conductive stone.
To Kenez: yes, grouping trees is also an interesting strategy, if physically possible for a particular group of bonsai. Their foliage helps shade the neighboring pots and soil surfaces, keeping the solar heat load down. White benches can help, but if they are wooden, the conductive heat transfer from them to the ceramic pots is much smaller compared to stone. Plus, each container shades the area beneath it so the white on nearby bench surfaces would seem to be a secondary effect. But every little bit might matter, depending on the particular arrangement of pots on benches or platforms. Placing rigid insulating foam “feet” or risers under each pot could accomplish a similar goal.
