Our patches of the evergreen conifer, Microbiota decussata ‘Prides’ have switched from their summer coats of green to their winter coats of bronze. The same occurs on a number of plants in the genus Thuja, Cryptomeria, and Juniperus. I have a few folks each year ask how to keep conifers from going bronze in winter, which, unfortunately defies the natural botanical processes and needs of the plant. Instead of fighting nature, let’s understand why the plants turn bronze.
As part of the acclimation to the cold winter temperatures, the pigment chlorophyll in many conifers disappears. It is replaced with the red brown xanthophyll pigment, rhodoxanthin. Many of the plants’ internal processes, such as converting sunlight into sugars don’t function as well in cold weather and intense sunlight, so the plant came up with a way to reduce those processes via internal shading. The leaf chloroplasts, which normally produce the green pigment, chlorophyll, undergo a transformation into chromoplasts, which instead of producing chlorophyll, synthesize the yellow-orange group of pigments known as carotenoids. It is in these chromoplasts, that rhodoxanthin (a carotenoid) pigment accumulates. The chromoplasts also release excess oxygen from inside the leaves, which can accumulate in winter, and cause significant cellular damage (think bloated stomachs) in the foliage.
The higher the intensity of the winter sun on the foliage, the higher the accumulation of orange pigments, for more shading. This is also why the undersides of the leaf usually remain green. Once the sun intensity lessens in spring, and the temperatures warm, the chromoplasts switch back to chloroplasts, and chlorophyll production resumes. I’d say Mother Nature is pretty darn intelligent, so don’t be a chromophobe.

Thank you for explaining a complex natural phenomenon. If you have the information available, could you also discuss the reasons why some flowers (dahlias are a good example) occasionally change their colors from one year to the next? Regards, Maryann
Good explanation. Yet the wintergreen mutants of Juniperus, Thuja, and Cryptomeria with no purpling or bronzing don’t seem to show any major disadvantages. Perhaps the wintergreens are just preferred in warmer, snowless climates where they are more visible and the temperatures are not as brutal. Looking at hundreds of juniper cultivars north to south there seems to be no advantage either way in terms of vigor and overall health.
That was a most interesting essay.