Winterizing Your Yard in the Pacific Northwest

by Rod Smith, Oregon Certified Nursery Professional

© 1998-2023 Rodney A. Smith
All rights reserved.

Winter weather can cause much damage to a landscape. Winter in the Pacific Northwest can bring excessive rainfall, cold temperatures, snow, ice and drying winds. Normal winter weather stresses plants, and severe weather can kill or disfigure them. However, there are several steps that can be taken to prepare your yard to minimize winter damage.

Pruning. Strong winter winds can break limbs and branches. While fall is not the best time for pruning because it might lead to freeze damage, removing long thin branches that are likely to break is better than having them break off. Thinning out a dense tree in a windy spot might prevent it from being blown over. If nothing else, dead and diseased wood is easier to recognize and remove while living branches still have leaves. Sterilize the pruning tool after each cut with Lysol or Clorox wipes to prevent spreading diseases to the next cut.

Roses should be pruned back to waist high in the fall. Strong winter winds can rock rose bushes and damage the roots. Tree roses should also be cut back about half way to the center of the ball. Then wait until the end of February after the coldest weather is over before giving roses their final pruning. The upper branches help protect the lower branches from freeze damage.

Hydrangeas and other summer blooming deciduous shrubs can also be cut back in the fall or left until late winter. Mophead and lacecap hydrangeas bloom on old wood, so cut them back to about half of their height. Look for pairs of fat buds on the stems and leave at least one pair on each stem. These are the buds that will grow into branches that produce flowers. PG hydrangeas, with the cone shaped flowers, bloom on new wood. You can cut them back to a few inches tall and they will still bloom

Do not prune spring blooming shrubs such as forsythia and lilacs or else many flowers will be cut off. The time to prune spring flowering plants is in the spring after they bloom.

Leaves need to be removed from lawns before grass is smothered, but leaves can be a valuable asset. Healthy leaves can be left on lawns to recycle the nutrients if the leaves are mowed with a mulching mower to chop up the leaves and work them down into the grass. If there are still leaves sitting on top of the grass, mow again and bag the excess leaves. The chopped up leaves can also be spread in tree, shrub and flower beds as a mulch, which is what happens in nature. I always mow up leaves rather than raking them up. I set the mower height one notch lower each time I mow while leaves are dropping. This adds more grass clippings to the leaves so they make better compost. One part grass clippings to three parts leaves is ideal. However, black walnut leaves and Tree of Heaven (Ailanthus) leaves should not be added to compost because chemicals in the leaves can hurt other plants. Oak leaves have a lot on tannic acid which is not the best for compost, but they are very good for using as mulch to prevent weeds. However, if leaves are diseased, they might re-infect the new leaves next year. Send diseased leaves to recycling, or at least move them to the far side of the yard away from susceptible plants.

Rainfall is normally beneficial, but too much at a time can cause trouble. When the soil becomes soggy and soft, it does not provide a solid anchor for plant roots. Large trees might be blown over in a strong wind. Also, roots are not able to gather water and nutrients when soil is saturated with water. There are cases of plants wilting while surrounded by standing water. There was plenty of water, but the roots could not absorb it because they were starved for oxygen. Plants will stay healthier through the winter if the soil has good aeration and drainage.

Lawns may need to be aerated every year to every four years, depending on the soil and amount of foot traffic. The best kind of aerator cuts a plug of soil every few inches and drops it on the ground. Running a rotary mower over the lawn will shatter the plugs and return them to the soil. Then a thin layer of sand or Profile Soil Conditioner can be applied and raked into the lawn to fill the holes.

A Ross Root Feeder can be used to aerate around existing trees and shrubs, but the best time to improve aeration and drainage for trees, shrubs, perennials and bulbs is at the time of planting. The planting hole should be three to five times the width of the root ball and about the same depth. Organic matter, such as compost, and concrete sand or Soil Conditioner should be mixed into the soil. Plants that need extra drainage, such as Japanese maples, daphne, heather and tulips, should be planted on a slope or a mound.

For really wet areas, it may be necessary to bury drain pipe about a foot deep to drain away excess water. The tricky part is finding a place to drain the water. Many local building codes prevent connecting drain pipes to downspout drains. Dumping the water on the neighbor's yard is also a bad idea. Taking the drain to the curb is usually best. Make sure that there is an even slope to the trench so mud doesn't clog up the low spots in the pipe. Another option is to turn wet areas into a bog with plants that like wet soil such as canna, ferns, Japanese iris, sedge and red twig dogwood.

Freezing is the most common cause of winter damage. The damage is worse when plants thaw out rapidly. The best cold protection will raise the minimum temperatures and reduce rapid changes in temperature. A long cold spell will do more damage than one cold night. Cold weather that comes early or late in the winter will do more damage than cold weather in the middle of winter when plants are fully dormant.

Leaves and twigs are the first to be damaged by freezing. Many tropical plants cannot stand any frost at all. The plants that are native to the Portland area are hardy down to at least 5 degrees. For other plants, look them up in the Calendar of Color section and check the USDA Hardiness Zone number. Winter temperatures in Portland dropped below 5 degrees in three of the winters during the 1980's but have stayed above 13 degrees since then. It gets even colder at higher elevations. However, on clear nights with little wind, cold air drains downhill and collects in frost pockets where cold air gets trapped by fences and hedges. On these cold, still nights, the bottom of a hill might be ten degrees colder than the top of the hill.

Leaves and twigs can be protected by covering them with something. A cardboard box, garbage bag or a bucket works very well on small plants. Larger plants can be wrapped in a bed sheet or in plastic. Clear plastic needs to be removed when the sun comes out so the plants don't overheat. Black or milky plastic is better than clear plastic, although they still need to be removed as soon as freezing weather is over. Even better products are Garden Blanket or Remay, which breathe and can be left on as long as necessary. If it is windy, cover plants all the way to the ground.

Tree trunks sometimes develop sunscald, which is dead areas and cracked bark on the southwest side of the tree trunk. In freezing but sunny weather, the sun will warm up the trunk enough that the cambium layer will begin growth. When the sun stops shining, the new cells quickly freeze and die. Young trees, with few branches and thin bark are the most likely to get sunscald. Maples, cherries, plums and fruit trees are especially susceptible. The best protection is to wrap the trunk with paper or spun fiber tree wrap.

The roots of a plant are not as hardy as the top. Roots will often be damaged if the soil temperature reaches 20 to 25 degrees. Fortunately, moist soil is a fairly good insulator against cold. Water to keep the soil moist during freezing weather.

The roots of a plant in a pot or planter will be exposed to the cold much more than plants in the ground. Roots are mostly against the inside of the pot and plastic or clay pots offer no protection, although wood gives some protection. Pots and planters should be moved to a protected place, such as an unheated garage or shed, or next to the house under the eaves. If a planter is too heavy to move, it can be wrapped with bubble wrap or a sheet of packing foam.

Roots of plants in the ground may be protected with a layer of mulch such as straw, compost or bark dust. Mulch protects the roots from summer heat as well as winter cold. A layer of mulch one to three inches deep should be enough over the root zone, but the mulch should not be more than an inch deep around the trunk since it may encourage crown rot. The branches of low plants, such as azaleas, should be lifted out of the mulch. Mulch can be mounded over roses during short periods of freezing weather, then leveled again so the rose canes do not stay wet.

The best mulch for strawberries is Garden Blanket. It protects from the cold as well as straw and it can be left on in the spring to encourage an earlier harvest of berries. It can also be used on tomatoes and other vegetables to protect them from early frosts.

Winds also cause damage during in the winter, especially the strong, cold, dry winds that blow from the east. Shallow rooted trees are toppled much easier when the soil is soggy. Long branches are broken, especially if they are weighed down with snow and ice. Regular pruning to remove weak crotches and to keep trees compact will prevent breakage. Deep watering once a week in the summer encourages deeper root systems.

Wind chill increases the effects of the cold on evergreens. Wind can also dry out leaves quicker than plants can absorb moisture. This is called freeze burn and can happen to lawns as well as trees and shrubs. The best protection is to spray the leaves with Wilt Pruf. This product puts a natural coating on the leaves that seals in the moisture to prevent freeze burn. Broadleaf evergreen plants such as daphne, rhododendrons and camellias in windy locations should be sprayed in late fall.

Be prepared. Start early to protect your plants from winter weather. Supply good aeration and drainage. Wrap young trees with tree wrap. Apply a good layer of mulch. Spray tender leaves with Wilt Pruf. When freezing weather is predicted, move planters to a protected area, cover tender plants, and make sure the soil is well watered.

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Seasonal Pruning Guide
Trees and Shrubs in the Landscape
Water Wise Gardening

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