Growing plants in containers is very popular. Almost anything can be grown in containers, including trees and shrubs. Rhonda Ferree, Horticulture Educator with University of Illinois Extension, says that containers provide a feeling of permanence and beauty to an area.
"Unfortunately for those of us in central Illinois, 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.
Ferree says that very large containers should weather fine without any extra protection. Smaller containers, on the other hand, need extra protection if overwintered outdoors. "Plant breeders are producing smaller and smaller plants to grow in containers, and some of these need extra winter care." "These include bonsai trees, dwarf fruit trees, miniature berry shrubs, and more."
Small plants can easily be moved into a protected location such as 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. Cover the plant and the container thoroughly to 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.
Ferree offers these tips 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.
Since the real danger to these plants is from the frozen rootball, work to protect the root system. Wrap the container with burlap or straw to protect it. Some people will bury the entire container into the ground so that the ground can insulate it. Others will entirely encase the upper and lower plant in a "cage" of chicken wire stuffed with leaves or straw. Nurserymen often "mulch-in" container grown plants in the winter by laying them against each other and packing the container area with mulch.
"Containerized trees and shrubs add a great deal to our landscapes." "With proper winter protection, the same plant can provide beauty for many years."