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There are several ways in which the winter environment can adversely
affect trees and shrubs. Included are: direct low temperature injury and
frost injury; desiccation injury; winter sunscald; frost cracks; frost
heaving; and snow and ice breakage.
Figure 1: Snow weight lifted
root ball out of the ground. |
Figure 2: Cold temperature
damage to new growth of
spruce. |
Plants frequently injured by low winter temperatures are
those which are planted in areas north of their appropriate hardiness
zone. Such species cannot harden off at an appropriate rate or to an
extent sufficient to withstand prevailing winter temperatures. However,
ever hardy plants can be injured during unusually cold periods or when
temperatures drop rapidly or oscillate frequently. If hardy plants are not
managed properly (not properly fertilized, pruned, watered, etc.) they may
also suffer. Flower buds, vegetative buds, branches, stem, crowns, bark,
roots, or even whole plants may be injured. Containerized plantings are
particularly vulnerable to low winter temperatures since their roots are
not protected by being below ground.
Late spring and early autumn
frosts can injure metabolically active tissues that are insufficiently
"hardened" to withstand the cold temperatures (Figure
2). This type of injury may occur on native or exotic
plants although the latter are usually more valuable. A result of late
spring frosts can be the death of dormant but , particularly, expanding
flower buds on species such as magnolia or lilac, or the death of young,
succulent, actively growing shoots. Cold temperature injury that occurs
during winter may not be evident until injured tissues fail to grow the
following spring.
Management:
Avoid planting exotic
species north of their plant hardiness zones unless unique microclimates
in the landscape are such so as to guarantee winter survival.
Containerized plants should be placed in protected areas, sunk into the
ground, grouped together, or heavily mulched to avoid low temperature
injury to roots. To allow proper hardening of plant tissues avoid heavy
applications of nitrogenous fertilizer in late summer to plants in-ground.
Mulch around the bases of root-tender plants to help protect their crowns
and roots from freezing temperatures. Even with good management, injury to
young growth or insufficiently hardened tissues may still occur as a
result of unusual weather patterns. Little can be done to prevent injury
in these instances.
Injured and dead tissues should be pruned and
discarded or destroyed to discourage invasion of the plants by disease
organisms. Replace plants which are completely killed with species adapted
for the appropriate plant hardiness zone.
This type of injury, called "winter drying" or "winter
burn", is usually observed in late winter or early spring on evergreen
plants. Broadleaved evergreens, such as rhododendron, exhibit browning or
even total necrosis of their leaf margins (leaf scorch) depending on the
extent of injury. Narrowleaved evergreens, such as white pine, exhibit
slight browning of needle tips when injury is slight. Extensive injury may
result in browning and premature abscission of entire needles. The injury
occurs during sunny and/or windy winter weather when plants lose water
from their leaves through transpiration faster than it can be replaced by
roots which are in frozen soil.
Management:
Plants
which are properly watered during dry periods in late autumn are better
equipped to withstand this type of injury. Thoroughly watering the soil
around plants once every two weeks (once per week for new transplants)
during extended dry periods throughout the growing season will also prove
helpful. Placing a protective barrier of burlap over or around plants to
protect them from winter winds and sun will help to reduce the incidence
of this injury. Antidessicant sprays applied once in late autumn and again
in mid-winter may also prove helpful.
This type of injury occurs when the sun warms tree bark
during the day and then the bark rapidly cools after sunset. These abrupt
fluctuations are most common on south or southwest sides of trunks and
branches, and they may kill the inner bark in those areas. Young and/or
thin-barked trees are most susceptible to winter sunscald.
Management:
Wrapping trunks of susceptible trees with
protective "tree wrap" is the most effective way to minimize this type of
winter injury.
Frost cracks are splits in bark and wood of a tree that
result from rapid drops in temperature. They may be associated with
internal defects resulting from previous injury to the trunk years prior
to splitting. Defective wood does not contract as readily as the outer
layers of healthy wood do when winter temperatures plunge rapidly. The
strain between the outer, contracting layers of wood and the inner defect
causes the outer layers of wood to crack. The initial crack is often
accompanied by a loud snap. In winter, the crack may become wider or
narrower during colder or warmer periods. Such frost cracks often close
and callus over during the summer only to open again in subsequent
winters. This callusing and recracking may lead to the formation of large
"frost ribs" on the sides of affected trees.
Management:
Avoid wounding trees when they are young. Be particularly careful not
to bump trees when mowing near them. Mulch around young trees to eliminate
the need for close mowing and to help prevent lawnmower injury. Large
frost ribs can be braced to prevent reopening during the winter, thus
enhancing callusing and healing. frost cracks in trees are ideal sites for
the entrance of wood decay organisms. Affected trees should be checked
regularly to insure they are free from serious decay and, therefore, not a
hazard to surrounding buildings and living things.
Frost heaving of new transplants and small shrubs during the
winter will expose plant roots to sever above-ground winter conditions
which include cold temperatures and drying wind and sun. Freezing and
drying injury to roots, if extensive enough, can result in the death of
plants which are heaved.
Management:
Proper mulching
around the base and entirely over the root zone of plants will help
prevent the soil from frequent freezing and thawing conditions which are
most responsible for heaving. Replant heaved plants quickly if possible
and mulch around them. Wait until spring to determine the extent of injury
and need for replacement.
Heavy snow or ice on weak limbs with foliage (as in the case
of evergreens) can result in breakage. Even strong healthy limbs of
deciduous trees and shrubs can be broken if the weight of ice or snow is
extremely heavy. If enough weight is placed on the upper portion of a
tree, it can lift the root system right out of the ground. (Figure
1)
Management:
Prune trees and shrubs to
reduce the amount of snow and ice they will collect and/or to eliminate
those branches which will be inherently weak. Branches with a wide angle
to the main stem are generally stronger and can support more snow and ice
than those with a narrow or acute angle. cabling and bracing of weak limbs
on specimen trees by commercial arborists may be helpful. However, removal
of such limbs may be the only truly safe measure in many instances. Plant
trees and shrubs away from places where snowmelt from roofs will drip on
them. Otherwise, the dripping water may freeze on the plants and
accumulate sufficiently to break branches. Wooden barriers may be built
over small shrubs to allow snow and ice to slide off rather than
accumulate.
Last updated, KLS, 11/99
______________________________________________________________________________
The Plant Disease Diagnostic Clinic at
Cornell University is located at 334 Plant Science Building,
Ithaca, NY, 14853. Phone: 607-255-7850, Fax: 607-255-4471, Email:
kls13@cornell.edu |
