From time to time we offer baby pine tree cone plantings for sale. These are little trees that sprouted from Japanese Black Pine cones that Eric picked. Sometimes the seeds fall out and grow next to the dried cones, but a few will grow right out of the cone into cute little baby trees. The trees in these plantings are about one year old. In our growing grounds we noticed that while they still had green true needles, the immature baby needles or bracts that young black pines make turn an interesting blue-green color in cooler weather.
Changing color in coniferous bonsai can be quite delightful in fall and winter; from larch having yellow needles that drop in fall to some junipers that turn purple. In Japan, the most popular type of juniper foliage is called "Itoigawa" and while it is a bright vibrant green in spring summer and early fall, when temperatures drop the foliage turns a copper brown color, sometimes with hints of red. Older pine bonsai and other conifers sometimes also turn shades of yellow-green in winter.
We can attribute the color change distinctly to temperature. In preparation for shows in Japan, bonsai professionals keep junipers and pines that will be shown during winter exhibitions in greenhouses or indoors during the night to prevent the bronzing or yellowing of the foliage.
Similarly, In our nursery when junipers are taken from a greenhouse in January and set next to clones (genetically identical!) of the same species in the field, they are distinctly different shades of green. Below Junipers on the left were just brought out, while the ones on the right have been outside through three months of cold weather.
Conifers have a couple different "anti-freeze" mechanisms. From creating sugar inside tissue to creating wax coatings on the outside of needles, and both are interesting and beautiful ways to survive colder temperatures.
In research on some spruce species, scientists found a phenomenon they dubbed the "glass state." They found that trees could dehydrate their cells to the point that instead of forming damaging ice crystals the contents super-cool similar to the way that window glass is a super-cooled liquid. This prevents tiny ice crystals from damaging the cells, protecting the tree until temperatures warm again in spring.
We've sent a few trees off with some testers to climates outside northern California. One of our testers was worried that their juniper was turning brown. We've reassured them that this is winter protection, not a health problem. Color changes in conifers are actually a sign of good health in cold weather - they are a visual sign that the tree has the ability to protect itself.