I don’t grow my trees in pots exposed to coldest temps because the nurseries around here always keep their’s protected by piling up mulch for protection- so I can’t speak from experience because I imitated them after reading that the relative vulnerability of roots to cold is unpredictable. I don’t know if the issue has ever been studied of trees not used as ornamental plants in large pots on a regular basis. The issue varies from species to species.
“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 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 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.
