Arborvitae (Thuja) are popular evergreen conifers chosen for their dense, year-round foliage. When winter arrives, many owners notice a distinct shift in color, often leading to concern about the plant’s health. Arborvitae frequently change color, but the cause determines if this is a natural, temporary adaptation or a sign of permanent tissue damage. Understanding this difference is important for proper winter plant care. This article explains the two distinct types of winter color change and provides specific steps for prevention.
Is the Browning Normal or Damage?
The change in foliage color during winter can be categorized into two separate phenomena. The first, known as seasonal bronzing, is a completely normal, protective reaction that affects certain cultivars. Varieties like ‘Green Giant’ or ‘Techny’ arborvitae naturally shift their color from deep green to a reddish-brown or bronze hue as temperatures drop. This color change is triggered by the breakdown of green chlorophyll pigments, revealing underlying reddish pigments that help protect the foliage from intense winter light and cold. This bronzed appearance is not damage; the foliage remains pliable and will revert to its vibrant green color once warmer weather returns in the spring.
The second type of browning indicates genuine injury, often referred to as winter desiccation or winter burn. Unlike seasonal bronzing, true damage presents as dead, brittle, and dry foliage that will not recover. This irreversible browning frequently appears in distinct patches, often concentrated on the side of the plant most exposed to the sun and wind. The affected area feels entirely desiccated to the touch, and the dead foliage will remain brown even after the arrival of the growing season.
The Mechanism of Winter Desiccation
True winter browning is a direct consequence of water loss that the plant cannot replenish, a process known as desiccation. As evergreens, arborvitae foliage continues to transpire, meaning they lose moisture through their scaled leaves into the dry winter air. This moisture loss occurs whenever the air temperature is above freezing, even on clear, sunny winter days. Normally, this lost water would be replaced by the roots drawing moisture from the soil.
However, when the ground freezes solid, the water in the soil turns to ice and becomes unavailable for root uptake. The plant is then in a state of physiological drought, where its roots are surrounded by water it cannot access. This imbalance, where water loss through transpiration exceeds water absorption by the frozen roots, causes the foliage cells to dry out and die.
The resulting damage is most pronounced on the south and west-facing sides of the plant where solar radiation is strongest, increasing transpiration rates. Drying winter winds further accelerate this moisture loss by constantly stripping the thin layer of humid air surrounding the foliage, intensifying the desiccation effect. A lack of insulating snow cover can also allow the soil to freeze deeper and faster, prolonging the period during which the roots are unable to function. When the plant tissue dries out completely, the resulting brown color is permanent.
Preventing Winter Browning
Proactive steps focus on mitigating the environmental factors that contribute to desiccation.
Hydration
The most important preventative measure is ensuring the plant is fully hydrated before the ground freezes for the season. Provide deep, thorough watering late into the fall, continuing until the soil is no longer workable. This saturates the root zone, allowing the plant to store as much moisture as possible before its water supply is cut off.
Mulching
Applying a thick layer of organic mulch, such as wood chips, around the base of the arborvitae helps insulate the soil. This mulch layer moderates soil temperature fluctuations, slowing the depth and rate of freezing, which can extend the period of water availability for the roots. It is important to keep the mulch a few inches away from the trunk itself to prevent moisture buildup and pest issues.
Physical Protection
For young or highly exposed specimens, physical protection is highly effective against wind and sun exposure. Temporary burlap screens or wraps can be installed on the windward side to reduce the drying effects of strong air currents and shade the foliage from intense winter sunlight. Furthermore, a late-fall application of an anti-transpirant spray can coat the foliage, forming a thin barrier that physically reduces the rate of water vapor escape.