Winter Protection for Landscape Plants.pdf

OKLAHOMA COOPERATIVE EXTENSION SERVICE

F-6404

Winter Protection

for Landscape Plants

David Hillock

Extension Horticulture Specialist

Oklahoma Cooperative Extension Fact Sheets

are also available on our website at:

http://www.osuextra.com

Mike Schnelle

Extension Horticulture Specialist

The irst step in avoiding winter damage is to select plants

that are winter-hardy to the area. Plant mail-order catalogues

and plant material literature will usually state the cold hardi-

ness of each plant listed. Cold hardiness is normally based

on the USDA cold hardiness recommendations. The state of

Oklahoma is divided into zones 6 to zone 7 (Figure 1).

If you choose to use a plant that is marginally hardy,

choosing the correct site may determine the survivability of

the plant. Buildings may or may not offer winter protection

for plants. West walls relect heat, which can cause plants to

be damaged by daily freezing and thawing (Figure 2). While

the same plants growing in the shadow of north walls may

be damaged less because they thaw more slowly.

Protecting plants from dry winter winds is also important

for some plant species. Placing the plant on the downwind

side of a wall or windbreak of other trees and shrubs will help

reduce the incidence of desiccation or drying out.

The second step is to keep plants healthy during the

growing season. Plants in poor health or poorly adapted

Figure 2. Relected heat from a building may cause

thawing and freezing on a winter day which may result

in plant damage.

Highway 81

Highway 48

Panhandle

West

Central

East

°F Z ones

North

-5 to -10

6 a

0 to -5

6 b

I-40

5 to 0

7 a

South

10 to 5

7 b

Figure 1. USDA Plant Hardiness Zone Map of Oklahoma.

Division of Agricultural Sciences and Natural Resources • Oklahoma State University

species are the irst to suffer during any weather stress. Few

landscape plants recommended for Oklahoma die directly

from cold weather during an average winter. Generally, many

factors contribute to what is commonly called winterkill. To

keep plants healthy avoid late-summer fertilization and pruning,

supply plants with adequate moisture, and mulch to keep

moisture and temperature levels even.

Plants should be watered thoroughly in the fall to prepare

them for the winter months. If the soil is heavy clay, water

should be applied very slowly. To speed water absorption

or to get water into the soil on a sloping bank, holes may

be punched or drilled a few inches deep. These holes may

be left open or back-illed with peat or mulch to aid water

absorption.

During dry winters, broadleaf evergreens such as hollies

should be watered about once each month. Do not forget

those growing in above ground planters protected from rain.

They need watering even in a wet season. Also, remember

to water plants that are located under the eaves of a building

or home since they often receive little natural precipitation.

Most container-grown plants today are produced in a

soil-less mix. These need water more often, even after plant-

ing, than ield-grown plants. This is because the soil-less

growing mix holds very little water. Until the roots grow into

the surrounding soil the plant can dehydrate even though the

bed soil is damp. This is especially true of plants like hollies

and other broadleaf evergreens planted in fall and winter.

When watering new container-grown plants, slowly apply

the water directly at the center of the plant.

Fertilizing

Fertilizing is best done between early spring to mid-

August while the plants are actively growing. It is during this

time that plants can best utilize the nutrients available in fertil-

izers. Even though plant roots continue to grow during the

winter months when soil temperatures are favorable (above

40 o F), much of the elemental nitrogen can be lost due to

leaching or vaporization. However, if plants seem to be weak

or if nutrients are deicient as determined by a soil test, then

a fall application of low nitrogen fertilizer can be beneicial.

Nutrient-starved plants should be fertilized to correct deicien-

cies after frost, but before freezing weather if possible. Lack

of proper nutrition makes all plants more subject to winter

damage. Proper decisions about fertilizer requirements and

rates needed can be made by having the soil tested by the

Oklahoma Cooperative Extension Service.

In case of deiciency, use a fertilizer according to label

directions. If trees or shrubs are growing in turf, wait until

ten days after frost. Before fertilizing plants in beds, cultivate

shallowly around plants except such shallow rooted plants

as azaleas. Water after fertilizing if suficient rain is delayed

for more than one or two days after application.

To correct a micronutrient deiciency like iron, apply a

liquid or chelated product according to label directions and

water it in.

Protecting Young Trees

Trunks of some newly planted trees, especially those

with green trunks or thin-bark, require protection from direct

sunlight during all seasons. They are especially susceptible

to sunscald (blistering and cracking of the bark) during winter

months when leaves are absent. Protect the trunk with a com-

mercial tree wrap such as a polyurethane spiral wrap (Figure

3) or paper (kraft) wrap. The wrap should be applied in the fall,

but should be removed prior to trunk expansion each spring.

The most commonly reported damage from trunk protective

wraps is trunk girdling or constriction because the wrap was

too tight or left on too long. Generally, a tree will only need

to be wrapped the irst season or two after planting.

Tie the wrap irmly, but not tightly. Polyurethane wraps

expand without binding the trunk. Start at the ground

and wrap up to

the irst branch

slightly overlap-

ping as you go

(Figure 4). Do not

attach wraps with

wire, nylon rope,

plastic ties, or

electrical tape.

Pruning and Fall Cleanup

Pruning from mid-August to killing frost is discouraged.

Some species such as crape myrtle might be stimulated to

grow and low temperatures could damage the tender stems.

Also, many lower buds of spring-lowering shrubs and trees

could be destroyed.

However, dead, diseased, or insect-infested plant parts

should be removed anytime they are noticed.

Refrain from removing lower limbs of young, newly

planted trees the irst year or too many of the lower limbs of

thin barked trees at one time. Removing too many too quickly

could result in winter damage from the wind and sun. For

proper timing and pruning techniques see fact sheet F-6409,

“Pruning Ornamental Trees, Shrubs, and Vines.”

Haul all infested debris away. Many insects and diseases

over-winter in dead plant parts. However, most leaves and

clippings can be composted or shredded and used as mulch.

Do not allow fallen leaves to remain piled on lawn grasses.

Figure 3. Plas-

tic tree guards

(polyurethane

spiral wraps)

can be reused

for many years,

allow good air

m o v e m e n t

between bark

and guard, and

will not girdle

the trunk.

Water and Soil Moisture

Lack of adequate soil moisture is often a major cause

of winter damage. All plants, but especially narrowleaf and

broadleaf evergreens, use water during winter. Moisture must

be available below the frost line or frozen soil.

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Figure 4. Apply paper wraps starting at the base of the

tree, overlapping as you go, up to the irst branch.

Plants prone to winter desiccation, such as broadleaf

evergreens, when planted in open windy areas may require

additional protection. Temporary protective barriers such as

sheets of burlap, lathe fencing, bales of hay etc. can be con-

structed to provide protection from the drying winds (Figure

5).

Protect young trees and shrubs from animal damage.

Polyurethane wrap, wire mesh collars (Figure 6) or rodent

repellent paint can be used. Holly, honeylocust, elm, and

fruit trees are particularly susceptible. Remember snow will

change the height of the bite.

Figure 5. Temporary barriers may help protect evergreen

plants from dry, cold winter winds.

Mulching

Wait until after killing frost to apply winter mulch. In

addition to insulating plant stems and roots from freezing,

mulch also prevents luctuation in soil temperatures. Mild,

sunny weather warms the soil. Some unmulched shrubs

such as roses and many perennial lowers then begin to

grow only to be damaged when freezing temperatures return.

Newly planted plants are often lifted out of the soil due to the

constant freezing and thawing motion of the soil that causes

upheaval and damage to new roots. Keep mulches a minimum

of six inches from the plant stems or trunks. Mice or vermin

often overwinter in mulch and may nibble the bark off. Deep

mulches or mounds should be completely removed by March

1 in most of the state.

Many materials can be used for mulching. These include,

but are not limited to, bark, old hay, straw, sawdust, cotton

burrs, and grass clippings. If hay or straw is used presprout

weed seeds by soaking with water for two or three weeks

before use. This should be done while the weather is still mild.

If sawdust is used, as much as one cup of urea per bushel

may be needed to prevent nitrogen deiciencies.

The iner textured the mulch, the thinner the layer required.

For example, one or two inches of sawdust will insulate as

much as four or ive inches of hay or straw. Leaves should

not be used unless they have been shredded or composted.

Avoid using diseased tree leaves, etc. for mulching. Use

leaves or grass clippings with caution as they can pack and

prevent air and water from entering the soil.

Figure 6. Chicken wire

mesh can be used to

protect trunks from

animal damage.

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Using polyethylene or plastic under mulches is generally

undesirable since it also reduces air and water penetration.

Fabric landscape barriers are beneicial under mulches.

Broadleaf Evergreens - Even though the top may be partially

or completely dead, the plant may grow from the base.

Allow enough time for new growth to appear before

deciding to remove or prune. If top damage is severe

on such plants as abelia, euonymus, nandina, and jas-

mine, cut the top back to within six to 10 inches of the

ground. Such plants as cherry laurel and holly should

be pruned back to side branches having live wood. In

the spring, apply fertilizer at label rates to promote new

growth. Cultivate carefully to avoid damaging ine roots

near soil surface. Water thoroughly after fertilizing.

Flowering Shrubs - Remove all dead wood and part of the

oldest and weak spindly canes or stems of damaged

lowering shrubs. Tree-type shrubs such as althea and

crape myrtle should have only the weak and dead wood

removed. Old and weak plants should be completely

removed. Multiple-stemmed shrubs such as spirea and

forsythia can be cut back to within six to 10 inches of

the ground if severely damaged. Fertilize as noted for

narrowleaf evergreens.

Damaged Plants

During severe winters and when late spring freezes occur,

even well adapted plants may be damaged. Winter damaged

shrubs should have begun growth by June 1 at the latest.

However, freeze damaged shrubs often begin growth early

only to collapse and die with the irst hot days of summer.

Treat freeze-damaged plants as follows:

Narrowleaf Evergreens (juniper, pine, etc.) - Species differ

in their ability to recover from winter damage. Juniper

(cedar), pine, and Arizona Cypress should be removed

if 30 percent or more of the top is dead. Otherwise, cut

out dead branches and prune to reshape. Arborvitae and

yew are the only species of this group with dormant buds.

Thus, they can be cut back severely and recover or be

maintained as a hedge. In the spring, apply fertilizer at

label rates to promote new growth. Do not place fertilizer

against the stems or trunks of plants. Scatter fertilizers

from slightly beyond the branch tips toward the stem.

Oklahoma State University, in compliance with Title VI and VII of the Civil Rights Act of 1964, Executive Order 11246 as amended, Title IX of the Education Amendments of 1972, Americans

with Disabilities Act of 1990, and other federal laws and regulations, does not discriminate on the basis of race, color, national origin, gender, age, religion, disability, or status as a veteran in

any of its policies, practices, or procedures. This includes but is not limited to admissions, employment, inancial aid, and educational services.

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Robert E. Whitson, Director of Cooperative

Extension Service, Oklahoma State University, Stillwater, Oklahoma. This publication is printed and issued by Oklahoma State University as authorized by the Vice President, Dean, and Direc-

tor of the Division of Agricultural Sciences and Natural Resources and has been prepared and distributed at a cost of 64 cents per copy. 1004

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