Leyland Cypress trees are a popular choice for homeowners seeking a fast-growing, dense evergreen screen for privacy and windbreaks. These hybrid conifers, known as x Cupressocyparis leylandii, are valued for their vigorous growth rate, often exceeding three feet per year. Their foliage should maintain a rich, blue-green color throughout all seasons, including winter. Widespread browning during colder months indicates the tree is under significant environmental or biological stress, not experiencing a natural seasonal change.
The Natural State of Leyland Cypress During Colder Months
A healthy Leyland Cypress retains its feathery, scale-like foliage and deep green coloration throughout the winter, characteristic of evergreen species. These trees do not naturally shed all their needles or turn entirely brown in response to cold temperatures. They are considered winter-hardy in USDA zones 6 through 10, typically withstanding temperatures down to negative 8 degrees Fahrenheit without major damage.
The only normal browning is seasonal needle drop, or interior needle shed, which typically happens in autumn or early winter. This process involves the oldest needles, located deep within the canopy near the trunk, turning yellow or brown and eventually dropping. This localized shedding is a resource-management mechanism and confirms the tree is efficiently cycling its older growth, distinguishing it from browning caused by disease.
Environmental Stressors That Cause Winter Browning
The most frequent cause of abnormal winter browning is winter desiccation or windburn. This occurs because the tree loses moisture through its needles on windy or sunny winter days, but the frozen ground prevents the shallow root system from taking up replacement water. The resulting water deficit causes the foliage, often starting at the tips or on the windward side, to turn brown and dry out.
Sudden, extreme drops in temperature can also lead to cold damage, sometimes appearing as browning in the spring when new growth begins. These rapid temperature shifts can kill the cambium layer or water-conducting tissue, resulting in sporadic branch dieback. Trees planted too close to roads or walkways can also suffer from salt damage, where exposure to de-icing salts leads to foliage burn and subsequent browning.
Non-Environmental Causes of Browning and Dieback
Beyond weather-related stress, biological and soil-related issues can cause Leyland Cypress browning, often exacerbated during winter dormancy. The most serious pathological problem is Seiridium canker, a fungal disease that enters the tree through wounds or stress points. This disease causes distinct, localized branch dieback, turning the affected area reddish-brown from the tip inward. It is identifiable by small, sunken cankers on the branches.
Another frequent cause is root rot, related to poor soil conditions, specifically insufficient drainage or overwatering. Saturated soil deprives the roots of oxygen, preventing the uptake of water and nutrients. The resulting moisture stress causes the entire canopy to turn brown, often mimicking severe drought damage. Pests such as bagworms also contribute to browning by feeding on the needles, causing defoliation that becomes more apparent when the tree is already stressed.
Strategies for Maintaining Winter Health
Proactive measures taken before and during the cold season can significantly reduce the likelihood of winter browning and dieback. Deep, thorough watering late into the fall, before the ground freezes solid, is a crucial anti-desiccation measure. This saturates the soil and hydrates the tree before water uptake is restricted. During dry winter weather, providing supplemental water when the air temperature is above freezing and the ground is thawed can also prevent desiccation.
Applying a two-to-four-inch layer of organic mulch around the base helps maintain consistent soil moisture and regulate soil temperature. This protects the shallow root system from extreme fluctuations. The mulch should be kept a few inches away from the trunk to prevent moisture buildup that could encourage disease. Avoiding late-season pruning is also beneficial because it stimulates tender new growth highly vulnerable to early frost and cold damage.