Plant Pathology
Winter Stresses on Trees and Shrubs
Winter 2000/2001 has been the coldest and harshest winter in many years. Several conifer
samples showing symptoms of winter browning have come into the Iowa State University Plant
Disease Clinic. Winter conditions like this year's can cause damage in several ways. Some
symptoms you might see are needle browning (sometimes yellowing), cracking or death of the
bark and animal feeding.
Winter browning is often referred to as desiccation injury. On sunny, windy days during the
winter when temperatures are above freezing, needles lose (transpire) water. If the soil is frozen,
the transpired water cannot be replaced and the tree suffers desiccation stress (needle browning
and tissue death). However, water loss alone in midwinter is not the only cause of needle
browning. A combination of environmental stresses can cause needle browning on conifers.
Winter browning may result from the interaction of extreme low temperatures and frequent
fluctuations between freezing and thawing. These conditions are usually found on the south or
southwest side of conifers, since those areas generally receive most of the sunlight during winter
months. Damage may also appear on the windward side, which may be to the north. Dry
conditions during the summer months of 1999 and 2000 may also be a contributing factor to
damage we are seeing this season. Trees went into winter already stressed.
Avoid pruning browned, burned areas from evergreen trees and shrubs in the early spring since
these branches may still have viable buds that will produce new foliage when growth resumes. If
the buds did not survive, prune dead branches back to living tissue. Some plants may appear to be
fine for a while, and then begin to decline when conditions become stressful in the summer. This
may be evidence that there was some root damage.
Proper placement of winter injury sensitive plants, such as yew or arborvitae, can reduce the
chance for winter damage. Avoid planting them on the south or southwest side of buildings or in
exposed areas. Constructing a physical barrier can also protect the plant from the elements.
Sunscald injury may occur on the bark of trees. It is characterized by an elongated sunken, dried,
or cracked area of dead bark, usually on the southwest side of a tree. On cold days the sun can
heat the bark to the point where that area breaks dormancy. When the sun sets or is shaded by
clouds, the active tissue freezes and dies. Young trees, newly planted trees, and trees with thin
bark are most susceptible to sunscald. Trunks can be wrapped in the fall with tree wrap or plastic
tree guards to prevent sunscald. Remove the protective covering in the spring.
Besides environmental stresses, mice, rabbits and deer can cause severe damage to plants in the
winter. Animals can girdle trees and shrubs and eat shrubs down to the ground when food
supplies are low during a harsh winter. Placing a _ "mesh cylinder around the trunk can protect
trees. The cylinder should extend 2 - 3 inches below the soil line for mice and 18 - 24 inches
above the anticipated snow line for rabbit protection. If the mesh hardware cloth is to be left on
all year long, it must be large enough to allow for tree growth. Chicken wire around shrubs will
protect them from rabbit damage. Deer can be excluded by fencing, but it must be well
constructed and high. Repellants may also be used to give woody material an undesirable taste or
smell. Animals may not be deterred by repellents if feeding pressure is high. Supplying animals
with supplemental food may be effective in preventing damage to plants. However, be aware that
this method may attract more animals to the area.
When spring finally arrives and plants begin to resume growth, provide adequate moisture and
balanced fertilization. You may need a soil test to determine if there is a nutrient deficiency. Most
important of all, enjoy the season of new life!
This article originally appeared in the March 9, 2001 issue, pp. 22-23.
Prepared by Barbara Ambruzs, extension program specialist, Department of
Plant Pathology
Iowa State University, Ames, Iowa.
This information subject to a usage policy.
[Search]
[Horticulture and Home Pest
News]
[Integrated Pest Management]
Last updated 3/8/2001 by John VanDyk
http://www.ipm.iastate.edu/ipm/hortnews/2001/3-9-2001/winterstress.html