Winter Injury on Woody Ornamentals
PP056 (2/03R)
By Dr. Sharon M. Douglas
Department of Plant Pathology and Ecology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8601 Fax: (203) 974-8502
Email: Sharon.Douglas@po.state.ct.us
Weather conditions during the past few winters in Connecticut have been
conducive for plant injury. The impact of these conditions is evident in
ornamental plantings in the landscape as well as in woodlands throughout the
state. The factors that cause this type of injury are diverse and problems often
do not appear on woody ornamentals until the spring and summer. In addition, the
extent and severity of the injuries that develop are often more extreme on
plants weakened by drought stress.
Winter injury results from many environmental factors that have little in common
other than that they occur during the winter. Examples of these diverse factors
include late spring frosts, cool summers followed by warm autumns and sudden
drops in temperature, dramatic temperature fluctuations, freeze-thaw cycles,
lack of snow cover, unusually warm midwinter temperatures, extended periods of
extreme or abnormally cold temperatures, and drying winds. For example, when
freezing conditions follow a gradual drop in temperature, they are seldom
harmful to most woody ornamentals provided the tissues are mature and have
started to go dormant. However, if plant tissues are not mature or have not
entered dormancy, they are unable to withstand freezing conditions and injury
results. The injury is often confined to the immature, succulent shoots and the
damage appears as a dieback of the terminals once growth begins in the spring.
Winter injury is important in and of itself, but it also predisposes and weakens
plants and subsequently makes them more vulnerable to secondary or opportunistic
pests. Among these secondary problems are unusually high incidences of branch
and twig diebacks such as those caused by Botryosphaeria spp. and Phomopsis
spp., fungi that are normally not considered to be aggressive pathogens. Another
important characteristic of winter injury is that, quite often, the symptoms are
not evident until some time after the injury has occurred. This
can make accurate diagnosis very difficult. For example, winter damage to
sapwood of lilacs may not be evident until spring or early summer when branches
suddenly collapse and die.
Winter injury can occur on a wide range of plants. However, it is often
particularly problematic on evergreens in the landscape. This includes
broadleaved evergreens such as rhododendron, mountain laurel, and holly and
needled evergreens such as hemlock, arborvitae, chamaecyparis, pine, and
juniper. Deciduous trees and shrubs such as flowering cherry and almond, maple,
and dogwood are also damaged by winter injury, as are groundcovers such as ivy
and pachysandra.
SYMPTOMS:
Symptoms of winter injury are quite variable and depend on the type
of plant. Woody plants that are damaged by winter injury will often show tip and
branch dieback, foliar browning, sunscalding, and bark splitting. One type of
winter injury is excessive drying. This is quite common on evergreens and
results from factors that create a water deficit in a plant. This type of injury
occurs when water evaporates from leaves or needles on windy or warm, sunny days
during the winter or early spring. Drying occurs because this water is not
replaced since the roots cannot take up enough water from cold or frozen soil.
On broadleaved evergreens such as rhododendron and mountain laurel, the most
familiar leaf symptom is characterized by a marginal browning and longitudinal
rolling along the midvein. In some cases, entire branches or shrubs can be
affected. Needled evergreens exhibit a slightly different symptom with browning
of the tips or center portions of the needles, chlorotic flecking, needle drop,
and tip and twig dieback. In extreme cases, an entire shrub or tree may turn
brown or appear off-colored.
On deciduous trees and shrubs, bark may be injured or split by cold weather.
Cracks and dead areas appear in the bark and the bark begins to peel away from
the trunk as the tree grows in spring and summer. This type of damage is common
on many of the thin-barked species such as crabapples, cherries, and maples.
Frost cracks during dormancy result from the expansion and shrinkage of bark and
wood that causes internal mechanical stress and cracking and splitting of wood
and slipping of bark at the cambium layer. Extremely cold winter temperatures
also result in damage to flower and leaf buds. Buds can also be damaged by
periods of unusually warm winter temperatures that trigger them to begin to
break dormancy. When the normal temperatures return, these tender buds are
injured. As a consequence of both types of injury, deciduous trees and shrubs
may not flower or may fail to leaf-out properly in the spring. Cold temperatures
occasionally cause sub-lethal or lethal damage to cambial tissues. This type of
injury usually does not appear early in the season, but causes new branches to
suddenly wilt and begin to die back by early to midsummer. Injured tissues
apparently cannot keep up with the water demands of actively growing plants and
the plants collapse.
STRATEGIES FOR CONTROL:
Although the weather cannot be controlled, and there are no
"cures" once the damage is done, there are steps that can minimize the
effects of winter injury. These include:
1) select the appropriate site for planting and use sound cultural practices to
maintain vigor;
2) select native plants or match plants to the site; for example, avoid planting
broadleaved evergreens in open, windy locations where they will be subjected to
drying winter winds;
3) have sufficient moisture in the root zone before the soil freezes—this can
be accomplished by giving the shrubs (especially evergreens) a deep watering
before the ground freezes in the fall; mulching also helps to increase moisture
retention in the winter;
4) avoid late summer and early fall fertilization—this stimulates and
encourages growth late in the season that may not harden-off properly for the
winter;
5) prune and remove any dead twigs or branches that can serve as sites for
secondary invaders or opportunistic pests;
6) provide physical protection from water loss and drying winds—this is
especially important for new transplants or plants in exposed locations; burlap
wraps and sprays of anti-transpirants can be used.
Summary
Winter injury can occur on a broad range of evergreen and deciduous plants. Damage results from many environmental factors including temperature extremes, freeze-thaw cycles, drying winds, lack of snow cover, and warm autumn temperatures that promote late-season growth. Symptoms are quite variable and depend on the type of plant and the causal factor. This fact sheet discusses the symptoms, causal factors, and methods to minimize these injuries on woody ornamentals.