Bonsai – The shading of the vessels
There is also some information on the effects of shading on root growth of the vessel. A study was conducted where the containers were shaded at various levels four woody species were used, the roots were examined, counted and classified before the study and after 6, 12 and 18 days. The study found that the greater the exposure the higher the% root damage, and that most damage occurred during the first six days.
Junipers were the most affected have lost 88% of the roots after the first 6 days, the Hollys were 72%, 48% of the Austrian Pines and Mugos by 40% after six days. The author did not indicate what% of exposure these numbers came from. (Whitcomb, Carl E. George Mahoney WA 1984. “Effects of temperature in containers in the plant root growth” Oklahoma Agri. Exp Sta Res Rept. P-855 :46-49). Whitcomb shows some striking pictures of the effects on the roots of this type of exposure. On the sides of containers exposed roots there are none, the shadow side, there are plenty of roots.
According to Whitcomb, this study shows that the thermal stress in the roots of plants in containers is a serious problem. The rapid loss of roots after exposure correlates with the abrupt plant stress frequently observed when the plants are spaced in the vase adapted during the summer. Heat-killed roots are prime locations for the entry of disease organisms through the death and root rot, high temperatures can be an important factor in providing an easy entry for disease at the roots.
Colors of the vessels
There are some studies on the replacement of black plastic with white plastic, but as the white containers reduce soil temperature, the white polyethylene becomes brittle on exposure to UV light and collapses.
Damage caused by heat
I’ve been thinking about this question many times and have noticed that some trees that I have on my workbench seem wither under the hot summer sun, where the same species that have piled on the floor “seems” more healthy. I always suspected that the damage from the summer heat may have been responsible for the loss of some important plants, especially juniper, after repotting in the spring.