By Sarah Browning, Extension Educator in Lancaster County
Many homes have patios and decks complete with large pots containing shrubs or trees. These containers provide a feeling of permanence and beauty to the area. We see pictures in garden magazines — from more temperate parts of the county — of beautiful winter containers with boxwood, yew, arborvitae or holly. But unfortunately, containerized plants usually die during Nebraska winters if not provided with good winter protection.
WHAT CAUSES WINTER DAMAGE?
In containers, the roots of plants are exposed to below-freezing temperatures on all sides. Containers lack the temperature buffering and insulating effect soil provides to plant roots in ground plantings.
Winter soil temperature inside an above-ground container can be very close to air temperature, meaning the soil can freeze solid and your plant roots along with it. This will kill your plants.
Finally, sudden temperature changes can also damage the container itself, causing it to crack, especially clay and ceramic containers.
PROVIDING WINTER PROTECTION
Small plants can easily be moved into a cool garage or basement. Temperatures should be in the upper 30’s or lower 40’s, but not below freezing. Woody evergreen plants do need sunlight, and all plants require periodic watering. Check the soil as you would in summer and water when it gets dry.
Protecting large plants is a bigger challenge, but it can be done. Covering the plant and the container thoroughly can help protect the plant. However, if the plant is too tender for our climate or if the winter is unusually harsh, even these measures may not be adequate.
Two methods to provide winter protection for containers, includes wrapping and “planting” them.
WRAPPING THE CONTAINER
After the first hard frost, water the plant thoroughly and mulch the top of the soil with several inches of straw or leaves.
Next, make a cylinder around the outside of the container with chicken wire. The cage should be tall enough to enclose the entire plant, down to the base of the container — remember, you need to protect the roots, too. Fill the cage with straw or leaves, working carefully so no branches are broken in the process.
Finally, wrap the outside of the cage with burlap or shade cloth and secure it with twine. Leave yourself a way to check soil moisture throughout winter. You’ll need to water whenever the soil is dry to prevent plant desiccation.
Once the danger of late spring freeze is past, remove the burlap or shade cloth wrap and the cage. Prune broken or damaged branches, and remove any other unnecessary growth. Select a cloudy day to remove coverings so the tree can acclimate gradually.
New products in the garden market make winter protection a little easier. Gardeners Supply, http://gardeners.com, now offers a Pop-up Plant Protector or Shrub Jacket. Both can be used for multiple years and stuffed with leaves or straw to provide the extra winter protection container plants need to survive. Similar products are available from other companies also. Just make sure the outer covering material is thick enough to offer a good level of temperature protection.
“PLANTING” AN OUTDOOR CONTAINER
Another good method of winter protection is to bury the decorative container in the ground. This method takes advantage of the soil’s temperature buffering abilities to protect your plant.
In fall, before the soil freezes, dig a hole large enough to accommodate the entire container all the way to the upper lip. When daytime temperatures drop into the 30s, place the container in the soil and backfill around the edges with more soil. Apply a thick 6–12 inch layer of wood chips, leaves or straw over the soil surface of the container and the surrounding area. If winter conditions are dry, periodically check soil moisture in the container and water as needed. In spring, the container can be lifted from the soil, washed and returned to its location for another summer.
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Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by Nebraska Extension is implied. Use of commercial and trade names does not imply approval or constitute endorsement by Nebraska Extension. Nor does it imply discrimination against other similar products.