The Cold, Hard Facts
Winter container gardening is tough–you have to protect plants from wind, harsh sunlight and drying out. The biggest challenge, though, is guarding against root damage caused by rapidly fluctuating temperatures.
Even plants that are hardy to your zone can be hit hard when planted in a container in the winter. Although the top part of a plant has the ability to go dormant, the roots don’t.
“Essentially any type of container exposes the roots to ambient temperatures,” says Dr. Hannah Mathers, assistant professor in nursery and landscape extension at Ohio State University in Columbus. Mature roots can gradually get used to the cold, but young, immature roots can’t. In containers, young roots grow on the outer part of the rootball. When exposed to the cold, young roots are unable to acclimate and die back. And, young or old, the roots are usually not has hardy as the plant’s top.
American holly (Ilex opaca) is hardy to USDA Zone 5. The top part (stems and foliage) of the plant will survive to a temperature of about -20 degrees F, but immature roots die at 23 degrees above zero, and mature roots at nine degrees. In the ground and insulated by the earth, that’s usually no problem for the roots of hollies in Zone 5 where the average minimum temperature is -10 to -20 degrees F. But in a container, root damage in American holly would begin to occur at 23 degrees if left unprotected–a drastic difference from -20 degrees.
To protect delicate root systems, consider these tips from the experts:
Avoid exposing plants to the freeze/thaw cycle. Rapidly fluctuating temperatures (from cold to hot and back) can cause significant injury to roots and, as a result, may even heave the plant out of the pot. To avoid this, place pots on soil instead of pavement. For plants in the ground, the main source of heat for roots is the heat of the earth. For containers on pavement, the heat of the sun can warm the pavement considerably, elevating the temperature of the rootball. This is followed by a drop in temperatures at night. The fluctuation exposes containers on pavement to freezing and thawing.