Some of us were not ready for the warm weather this past couple of weeks and some of the plants were not ready either. We have noticed some of the local Leyland cypress trees are turning brown – usually not the entire tree, just a few branches or maybe the top portion of the tree. If you look closely, the browning starts at the tips and moves inward. This is caused by damage from this past winter. Winter damage is showing up now because of heat stress. Just like us, plants need more water when it is hot.
This is called needle or leaf desiccation. During the winter, evergreen plants continue to lose water by transpiration. Water loss is greatest during periods of strong winds and on bright, sunny days. Even though the air temperature is cold the temperature inside the leaf can be warm. The brighter the sun, the higher the rate of transpiration. Desiccation occurs when water is leaving the plant foliage faster than the roots can replenish lost moisture. Root absorption is reduced or prevented when the soil is cold or frozen. The foliage of plants, such as camellia and boxwood, may turn yellow or orange due to mild desiccation or excessive sunlight. More severe injury is commonly seen as discolored, burned evergreen needles or leaves. In several cases the branches can also become dehydrated and die. Damage is normally worst on the side of the plant facing the wind or sun or near a reflective surface (white house, concrete paving, snow cover). In severe situations, whole plants may die.
Some branches have cracked or split because of recent heavy snow and ice. This has caused damage to the water conducting vessels of the tree. Branches that are cracked and split cannot transport enough water. The tips turn brown because of lack of water
If your trees have winter damage, the best thing to do is prune it out. However, wait before pruning because even dead-looking plants may still be alive. The extent of winter damage can best be determined after new growth starts in the spring. Damaged plants may be delayed in starting spring growth. Broadleaved evergreens showing leaf damage will usually produce new leaves if branches and vegetative leaf buds have not been too severely injured. Damaged leaves may drop naturally or be removed by hand.
Prune all dead twigs or branches back to within 1/4 inch above a live bud or flush with the nearest live branch. Do not remove branches that reveal a green layer underneath when scraped with a knife. They should eventually develop new growth.
Leyland cypress are unusual because they can survive pruning better than most species in the cypress family. On the other hand the Leylands seem to have suffered more winter damage that our local native evergreens. Leaving damaged limbs makes it easy for diseases to enter and possibly kill the tree. If more than one-third of the tree is showing damage, replacing the tree is the best solution.
A special thanks to Craig Adkins, Area Specialized Nursery Crops Agent, for his technical advice on winter damage.