The sight of a green tree turning brown in winter can cause alarm, especially when the tree is expected to remain evergreen. This change, often called bronzing, is common in many conifers. Whether this color shift is normal depends entirely on the specific type of tree. In many cases, this shift is not a sign of damage but a natural, protective response to harsh cold conditions. Understanding the tree’s identity and its biological mechanisms is necessary to determine if the winter coloration is a healthy adaptation or a serious problem.
The Many Trees Called Cedar
The term “cedar” is a common name applied to many trees, causing confusion when diagnosing winter changes. True Cedars belong to the genus Cedrus. However, most landscape trees called cedar are members of the cypress family (Cupressaceae) and are more prone to bronzing. These “false cedars” include the widely planted Arborvitae (Thuja) and various Junipers (Juniperus).
Arborvitae species, such as ‘Emerald Green,’ are the most common trees to shift from deep green to a russet or brownish-purple hue in cold weather. Certain Juniper species also exhibit this winter change, often taking on a plum or bronze coloration. In contrast, true Cedars, like the Cedar of Lebanon (Cedrus libani), typically maintain their green or glaucous (blue-green) foliage throughout the winter with less dramatic bronzing. The distinct winter appearance of these different genera reflects their individual strategies for surviving the cold season.
The Biological Mechanism of Winter Bronzing
The shift to a bronze or purplish color is a natural, protective process in certain conifers, triggered by freezing temperatures and bright sunlight. This physiological adaptation shields the tree’s photosynthetic apparatus from damage. When temperatures drop below freezing, the enzymes necessary for photosynthesis slow down dramatically, but the foliage continues to absorb light energy.
If this excess light energy cannot be processed efficiently, it can lead to the formation of reactive oxygen species, damaging the chlorophyll molecules. To prevent this photodamage, the tree breaks down some of its green chlorophyll pigment, allowing other, less-visible pigments to become dominant. These pigments are primarily carotenoids, such as rhodoxanthin, which safely dissipate the excess light energy as heat.
The appearance of these orange, yellow, or reddish-brown pigments acts as a form of internal sunscreen for the foliage. This protective process reduces photosynthetic efficiency during the winter, effectively putting the tree into dormancy. The bronzing is temporary, and the tree will resume normal chlorophyll production and return to its green color once warmer temperatures arrive in the spring.
Knowing When Browning is a Problem
Distinguishing between natural winter bronzing and browning caused by injury or disease requires careful visual inspection. Normal winter bronzing is typically uniform across the entire canopy, or at least across parts exposed to direct sunlight. The color change is a subtle shift to a consistent bronze, purple, or russet color, and the affected foliage remains pliable and attached to the branch.
Browning that indicates a problem is often patchy, localized, and appears as a dry, tan, or straw-brown color, signaling tissue death. A common injury is winter desiccation, or “winter burn,” which occurs when foliage loses water through transpiration faster than frozen roots can absorb replacement moisture. This damage is often most severe on the south or west sides of the tree, where sunlight and wind exposure are highest, leading to distinctly one-sided browning.
Other signs of unhealthy browning include discoloration of only the innermost needles, known as seasonal needle drop. While this is a normal process in late fall, it can look alarming. If browning appears randomly throughout the year, is limited to a single branch, or is confined to the tips of the foliage, it may indicate a fungal blight, insect infestation, or root issues. Browning that appears in late spring or summer, long after the cold season, is almost always a sign of underlying stress, such as drought or poor soil drainage.