Winter Injury to Trees and Shrubs
William
M. Fountain, PhD
Extension
Professor of Arboriculture and
Landscape Management
The welcomed warming temperatures of spring and early
summer are a relief from the cold winter temperatures of 2013-14. The USDA Plant Hardiness Zone Map (PHZM) for
Kentucky (http://planthardiness.ars.usda.gov/phzmweb/Images/72dpi/KY.jpg)
places most of the state in zone 6 (–10° to 0°F). The far western counties are in Zone 7a (0°
to 5°F). By this data, Kentucky was on
average no colder than we have historically experienced. The visible indication of dead plants and
utility bills indicated that something was different.
Temperatures associated with the PHZM are based on the 30
year average of the lowest winter temperature experienced in a
region. This is not the lowest
temperatures experienced over the last 30 years, just the average. The map does not indicate the duration of the
cold, soil moisture, humidity, solar radiation, topography or wind. If you look at the map for the entire country
(http://planthardiness.ars.usda.gov/PHZMWeb/Maps.aspx)
you will note that Kentucky shares Zone 6 with places that have dramatically
different environments (e.g. Texas panhandle, coastal New England and the
Alaskan panhandle). While there are
shortcomings to this mapping tool, it is still a valuable aid in making the
decision of what to plant and where it should be located.
The winter of 2013-14 did not reach the historical lows
of approximately -20°F experienced in parts of Zone 6 Kentucky in recent
decades. The extensive damage now being
observed in landscape plantings are the result of factors not recorded in the
PHZM. Our recent winter was different in
many aspects.
Duration of Cold
Winter temperatures fell as would be expected in a normal
winter. In a continental climate like
Kentucky’s you can expect wide swings in temperature throughout the winter
months. A few days after a cold front
pushes through either a warm front from the south replaces it or clear skies
following the cold front allow the sun to warm the air and ground. What was different this year is that the
temperatures stayed consistently low for extended periods. This allowed the soil to freeze deeper and
stems to remain frozen longer than in previous winters.
Wind, Humidity and Sun
The majority of the water lost by plants is from their
leaves. Deciduous plants drop foliage in
the fall to reduce their need for water.
Water loss continues through winter but at significantly lower rates. The small amounts of water lost from dormant
stems must be replaced to prevent damage.
Sometimes this is not possible if the soil or stems are frozen.
The winter of 2013-14 had numerous days when the wind,
coupled with low humidity (often below 20 percent) resulted in more water loss
from foliage and twigs than the plant could absorb and transport through frozen
stems. When this occurred in the
presence of bright winter sun the rate of transpiration (water loss from the
plant) increased even higher. With
water in the soil and stems frozen, the pull of transpiration resulted in embolisms
(air pockets) developing in xylem cells (conducting tubes that move water from
the soil to the top of the plant). Like
the air pocket in a syphon, plants could not move water through these damaged
xylem tubes. This was compounded when
frozen stems were physically shaken to remove ice and snow or bent by ice, snow
and wind. Bending frozen xylem cells can
fracture the cell walls reducing the plants ability to conduct water and
mineral elements.
What Has Happened?
Many evergreen plants turned brown in late winter and
early spring. Many of these broadleaf
evergreens are marginally hardy in our climate.
They are from milder climates where they retain evergreen foliage all
year long. Some of these species
survived previous winters because of the milder than normal temperatures
experienced. Examples include:
Monkey grass (Mondo japonica)
Southern magnolia (Magnolia grandifolia)
The southern magnolia is a good example of a species with
a wide provenance. Though they may look
identical, southern magnolias originally from the upper parts of the south are
more winter hardy than those that were originally from the deep south. Some native species can be found from the
Gulf Coast north into Canada. Becasue
they are the same species does not mean that all individuals in this group have
the same genetic level of winter hardiness.
One severely damaged plant I encountered was an eastern red cedar (Juniperus
virginiana), a common Kentucky native.
Only after asking the right questions did it come out that this
individual had been transplanted from a native stand in south Georgia.
The Japanese maple (Acer palmatum) is another
species that frequently suffers the ill effects of our continental
climate. As the common name indicates
this plant is native to Japan, a series of islands bathed in the warming
currents flowing north from the equatorial Pacific. The moderate climate of east Asian islands do
not experience the wide swings in temperature common to Kentucky. This is especially true for the late spring
frosts and freezes. After a brief warm
spell our Japanese maples leaf out prematurely only to be damaged by a late
spring frost.
Other broadleaf evergreens may look green and healthy as
the coldest of winter temperatures transition into the warmth of spring
followed by the first hint of summer heat waves. Unseen is significant damage to the xylem
cells (long tubes that conduct water upward).
Up to this point the individual plant has been able to supply its
foliage with sufficient water. But, the
limited amount of healthy conductive tissue has been working at maximum
efficiency to supply the plant’s water needs.
With a few days of 80°F in early spring the ability of the plant to
absorb and translocate water as rapidly as it is lost becomes a water deficit. The result is leaf and stem death as if it
were much hotter and dryer. Examples of
species where this has been common include:
Boxwood (Buxus spp.)
Japanese aucuba (Aucuba japonica)
Laurel cherry (Prunus laurocerasus)
Snow and ice are a common form of winter
precipitation. Many gardeners are afraid
that the slight bending will result in stem breakage. If the xylem in a branch
freezes and then bends downward as a
result of the ice or snow load, the ice crystals can result in the rupturing of
the xylem cells. This type of damage is
made worse by strong wind or when the owner of the plant shakes the snow or ice
off of the plant thinking that they are helping the plant.
Sometimes provenance (where it originated) is the major
issue for winter injury. Other times it
is the location, specifically failure to match the plant to its site. In winter the sun is very low on the horizon. Locating broadleaf evergreens on the north
side of a structure will help to protect them from the warming rays of the
sun. These same plants also need to be
protected from the drying effect of the wind.
Though we may have fond attachment to them, marginally hardy southern
plants do not belong in most Kentucky landscapes.
Sunscald
 |
| Sunscald on Red Maple |
In late May or June, well after the cold winter months
have passed from our minds we begin to see the development of another type of
winter injury. This problem is called
sunscald. It is not only disfiguring but
often leads to a long, slow mortality spiral.
It usually appears only on the south or southwest side of trunks and
only on recently planted trees. The
first indication is a small vertical crack in the bark. These cracks often run from close to the soil
line up to the lower branches. As the
crack opens the bark begins to peel back exposing the wood. The damage that caused this injury occurred
during winter when the cambium died. As
the bark peels back exposing the wood fungi attack the xylem and insects are
attracted to the open wound. It is
common for 40% of the trunk’s circumference to be damaged by sunscald. This is more than a disfiguring problem. It results in the loss of conductive tissues
essential for growth and development.
Trees can no longer move water and mineral elements from the roots to
the foliage nor supply the roots with sugars and other organic chemicals
necessary for growth. At best, trees
stressed by sunscald will reestablish more slowly and are more susceptible to
diseases and insects. While these trees
may ultimately survive, replacement trees outgrow severely damaged trees.
Sunscald is almost always limited to young, recently
installed landscape trees. It is not seen
on mature trees or those in forests. It
is most common on species with thin bark than trees with thick or exfoliating
bark. Problematic species include:
Maple (Acer spp.)
Linden (Tilia spp.)
Pear (Pyrus calleryana)
Crabapple (Malus spp.)
Cherry, plum (Prunus spp.)
Willow (Salix spp.)
The most common species associated with this problem is
red maple (Acer rubrum). This is
in part because it is the most widely planted (over-used) species and because
it is a floodplane species. While this
problem can develop on any tree, it is more problematic on species with high
water demands. Understanding the
multiple causes of this problem is the solution to preventing it. Installing trees with larger soil balls
(containing more roots) and watering during the winter months helps prevent
water deficiencies leading to sunscald.
Though not seen until later in the growing season, this
damage occurred during the colder parts of winter. Though the air is well below freezing, the
intense winter sun warms the thin bark and the cambium below it. This is most likely to occur in late
afternoon when the low angle of the sun results in sunlight hitting the trunk
directly. The intense sunlight causes
the cambium cells to begin dividing. As
a cloud moves across the sun or the sun sets below the horizon, the trunk
quickly returns to sub-freezing temperatures and the cambium freezes and dies.
Sunscald can be prevented by shading the trunks of young,
newly planted, thin-barked trees. A
double layer of plastic or fiberglass windowscreen is an easy and economical
way to accomplish this. Wrap the double
layer of screen around the trunk. Hold
the two ends of screen and staple them together (not to the tree). Leaving excess screen will prevent girdling
damage to the tree. Screen is better
than plastic pipe or paper wraps sometimes sold for this purpose. Windowscreen allows moisture to evaporate
from the trunk. This protective covering
should be removed after the tree begins to become established, usually one or two
years. White latex paint diluted 50:50
with water is also effective but is messy, unsightly and remains for many years
after it is needed.
Cold Damage to Roots
The least hardy part of any plant is its root
system. Roots grow in the ground where the
insulating effect of soil buffers the roots against extremes of heat and
cold. Plants growing in above ground
containers and plants being transplanted (balled and burlapped, bare root or
container grown plants) lack the temperature moderating protection of
surrounding soil. When roots are
subjected to low temperatures they can be killed even though the above ground
portions of the plant are hardy and remain alive. As a rule of thumb, roots are two USDA Plant
Hardiness Zones less hardy than the rating assigned to the above ground
portions. As spring growth begins the
buds begin to pop open but fail to put out new foliage. The green stems quickly turn brown, and die. This occurs because the roots were killed
killed by cold and were unable to absorb water essential for growth.
Winter injury to landscape plants appears with multiple
visual symptoms. While these injuries
are associated with low temperatures, injury is usually the result of a
combination of different environmental and cultural conditions (low
temperatures, duration of cold, lack of soil moisture, low humidity, wind and
sun). Healthy landscapes are not an
accident. It is important to always
match the plant to the site conditions.
This helps ensure that your investment will have every opportunity to
thrive and return aesthetic dividends for years.
Managing Winter Injury on
Landscape Plants
Rule number one in diagnosing winter injury and making
recommendations is don’t be impatient.
If the foliage or the tips have been damaged but the stems and buds are
still green, give the plant the opportunity to put out new growth. Sheering dead foliage will immediately
improve the appearance of the plant.
Pruning should not be done until after the chance of the last frost has
passed.
Spring fertilization is not recommended, especially for
plants suffering winter injury. The
addition of nitrogen can encourage more growth than the damaged stems can
supply with water during the hot, dry summer months. The addition of water during dry periods is
more beneficial than the addition of fertilizer. When necessary, fertilization of woody
landscape plants should occur in late fall after leafdrop (e.g. Thanksgiving to
Christmas).
Broadleaf evergreens that are established and exposed to
winter sun can be protected from the intensity of winter sun and wind. Cover these plants with light-colored cloth
or burlap prior to the onset of winter.
Spray moisture on the cloth prior to the onset of extremely windy
sub-freezing temperatures. Water frozen
on the cloth will further reduce the effect of the wind. The best long-term approach is to match the
plant to the site. This can involve
using hardy needled evergreens where evergreens are desired and deciduous
species that originated in our climatic zone.
Damage to plants will vary widely depending on exposure
and location in the state. The following
tables will offer suggestions on degree of damage and cultural advice. The degree of damage varies widely across the
state and even locally depending on exposure, vigor of the plant and genetic
adaptions.
Table 1.
Broadleaf
evergreens
|
Comments:
|
Abelia (Abelia x
grandifolia)
|
Foliage burn and stem
dieback, can be cut back almost to the ground (3 inch stubs).
|
Japanese aucuba (Aucuba
japonica)
|
Severe foliage dieback
and stem dieback. This marginally
hardy evergreen should only be grown in full winter shade.
|
Barberry (Berberis
juliana)
|
Moderate foliage and
twig death
|
Boxwood (Buxus spp.)
|
Foliage damage, can be
sheered back
|
Camellia (Camellia
japonica)
|
Foliage and twig death.
Even the cold hardy cultivars are not reliably hardy.
|
Threadcypress,
Chamaecyparis (Chamaecyparis spp.)
|
Moderate foliage damage
variable by species and cultivar. These plants do not generally tolerate
heavy sheering.
|
Bigleaf wintercreeper (Euonymus
fortunei)
|
Foliage damage and twig
dieback. This plant will rebound (unfortunately).
|
Japanese euonymus (Euonymus
japonicus)
|
not reliably hardy, cut
back to the ground
|
Spreading euonymus (Euonymus
kiautschovicus)
|
not reliably hardy, cut
back to the ground
|
Foster holly (Ilex x
attenuata)
|
moderate foliage
damaage with some twig death
|
Japanese holly (Ilex
crenata)
|
variable by cultivar
but moderate foliage damage
|
American holly (Ilex
opaca)
|
slight to moderate
foliage damage in sunny locations.
Hard sheering often does not regrow.
|
Blue holly (Ilex
x meserveae)
|
numerous cultivars with
slight to moderate damage. Do not
prune unless severely damaged
|
Privet, ligustrum (Ligustrum
spp.)
|
Damage to foliage and
stems. Plants resprout from the base.
(Note: this species is considered invasive and not recommended.)
|
Liriope, bigblue
lilyturf (Liriope muscari)
|
Trim back dead foliage
being careful not to damage the crowns.
|
Southern magnolia (Magnolia
grandiflora)
|
Severe foliage to twig
death. Most cultivars are of southern
provenance. Damaged plants will often produce sprouts on the trunk.
|
Oregon grapeholly,
Mahonia holly (Mahonia bealei, M. aquifolium)
|
severe damage to death
of the entire plant. These species are
marginally hardy and should only be grown in areas protected from wind and
winter sun.
|
Nandina (Nandina
domestica)
|
Severe damage with many
plants killed to the ground or killed completely. Cut dead canes back to 3-inch stubs.
|
Laurel cherry (Prunus
laurocerasus)
|
Severe damage to
foliage and stems. Plants that have
not been killed will sprout back from the base. Plants should be protected
from wind and sun during severe winters.
|
Azalea, rhododendrons (Rhododendron
spp.)
|
Damage is variable by
species and location. Damage is from
slight foliar damage to complete death of the plant. Protect evergreen forms from winter wind
and sun.
|
Yew (Taxus x
spp.)
|
Damage is variable
depending on location and species. Sheering will remove dead foliage though
most Taxus do not tolerate hard pruning.
|
Willowwood viburnum (Viburnum
x rhytidophylloides ‘Willowwood’)
|
Foliage and twig death.
Plants generally come back from the base.
|
Leatherleaf viburnum (Viburnum
rhytidophyllum)
|
slightly less winter
hardy than Willowwood
|
Table 2.
Marginally hardy
deciduous species
|
Comments:
|
Mimosa (Albizia
julibrissin)
|
Killed to the
ground. Trees will sprout back as
shrubby or multi-trunk forms. (Note: this species is considered invasive)
|
Orange-eye butterfly
bush (Buddleia davidii)
|
Severe dieback to the
ground. (Note: this species is often listed as invasive.)
|
European hornbeam (Carpinus
betulus)
|
Dieback of stems;
expect flatheaded borers
|
Border forsythia (Forsythia
x intermedia)
|
Flower buds of less
hardy cultivars are often killed by sub-zero temperatures
|
Crape myrtle (Lagerstroemia
indica)
|
Many cultivars have
been killed to the ground. Cutting
dead plants to the ground will generally result in regrowth from the
base. These plants will be shrubs or
multi-trunk trees unless trained back to a single leader.
|
Knockout rose (Rosa
x spp.)
|
Stem damage or killed
to the ground.
|
