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. Unfortunately for those of us in Iowa, containerized plants often experience severe winter injury and often death if unprotected. In containers, the roots of the plants are exposed to below-freezing temperatures on all sides. As temperatures fluctuate, the soil thaws and refreezes causing the plant to heave out of the soil. This tears the roots and can expose the roots to drying winds. Branches can be broken directly by strong winds or by the container tipping over. Sudden temperature changes can also damage the container itself causing it to crack.
Small plants can easily be moved into a cool garage or basement. Temperatures should be in the upper 30's or lower 40's. 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, these measures may not be adequate.
To aid in the success of the plant, select plants hardy for our area and make every effort to be sure the plant is going into the winter in a healthy state. Continue watering the plant through the fall. Do not fertilize after mid-summer. Woody plants should be encouraged to gradually cease growth and harden off in preparation for winter. After the first hard frost and the plant has lost most of its leaves, begin the process of winter protection. Gently tie together the branches so they won't be damaged when you pack insulating material around them. Water the tree thoroughly and mulch the top of the soil with several inches of straw or leaves. Make a cylinder around the outside of the tub with chicken wire or other type of garden fencing. Make the cage tall enough to enclose the entire plant. Fasten the wire fencing to a stake with wire or staples to add support. Fill the cage with straw or leaves working carefully so no branches are broken in the process. Wrap the outside of the cage with burlap or shade cloth and secure it with twine. This prevents the stuffing material from blowing away. As a last step, cover the cage with plastic or roofing paper and tuck in the edges. Tie over the top to prevent it from blowing off.
When spring arrives, unwrap the tree gradually. Remove the plastic or roofing paper cover first. Gently pull out the leaves or straw from around the branches and untie them. The stuffing can be used as a garden mulch around perennial flowers or in the vegetable garden. Leave the fencing and the outside wrap in place. Water if the soil is dry. Once spring has truly arrived, 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. Store the fencing and burlap away for next year.
Containerized trees and shrubs add a great deal to our landscapes. With proper winter protection, the same plant can provide beauty for many years.
This article originally appeared in the September 15, 1995 issue, p. 136.