Protecting evergreens from winter injury
Evergreen plants often experience injury in winter as harsh winds pull moisture out of leaves that cannot be replenished.
Evergreens, by their nature, are at the mercy of winter’s environmental stresses. Exposed leaf tissue of spruce, pine, fir, holly and rhododendron are often injured by low temperatures, sudden changes in temperature, drying winds, or low soil moisture. Each spring as temperatures warm, evidence of winter injury comes to light as needles and leaves turn reddish-brown in landscapes and nurseries.
Winter injury to evergreens commonly results from desiccation (loss of water) of foliage. A dry autumn can leave plants with inadequate stored moisture going into the winter. As the ground freezes, roots cannot obtain more moisture from the soil to replenish water lost to normal plant processes and drying winds. Damage appears as browning at the tips and margins of leaves and needles. Moisture loss from plants in winter increases when conditions are sunny, warm and windy. This type of injury is most dramatic on the side of the plant exposed to predominant winter winds. Providing moisture to evergreens when conditions are dry in the fall prior to the ground freezing can reduce damage from winter desiccation.
Sudden changes in temperature can also damage leaves and buds. Warm temperatures in late fall or early spring can leave plants susceptible to injury if temperatures drop quickly. The location of plants can contribute to this damage, especially low lying areas where cold air settles. These areas are called frost pockets and evergreens in these areas are often damaged in spring when temperatures drop quickly. Poorly timed fertilization of plants in late summer into early fall can delay the hardening off period predisposing tender growth to low temperature injury. If you are considering fertilizing woody plants in the fall, wait until after a hard freeze.
Site selection is critical for tender evergreens like rhododendrons and hollies that are very susceptible to damage from winter winds. Select locations where these plants will be protected by structures and other plantings. Protective burlap barriers can also be used to screen plants from winter winds reducing loss of moisture from leaves. Burlap screening is a protective wall reducing the impact from winter winds without wrapping the plant in burlap. For more information on burlap barriers visit the following Extension site: http://www.extension.umn.edu/distribution/horticulture/dg1411.html.
Though you may be eager to remove branches with damaged leaves and needles, it is best not to be too hasty. Damaged leaves and needles do not always mean that buds are injured. Until temperatures begin to warm, you may not know the extent of the damage. Damaged buds, leaves and stem tissue will darken as temperatures begin to climb. Often buds are not damaged and the loss of old foliage will be replaced by new healthy growth as plant growth resumes in the spring. A couple years of good growth can hide past damage to plants.