Many kinds of trees and shrubs, normally, hardy and capable of surviving severe winter wheather, usually need protection if growing in containers. It is because they are more susceptible to freezing injury than those in the ground.
Since containers are above-ground, the temperature of the soil in the container will be identical to the airir temperature. Moreover, the roots of the plant are significantly less hardy than the aerial portions.
For example, Laland Firethorn (Pyracantha coccinea 'Lalandii') stems and leaves can survive -15 degrees F. when fully acclimated, but mature (woody) roots are killed at 2 degrees F. and young roots cannot survive 22 degrees.
This demonstrates that there can be a hardiness difference of almost 40 degrees between the aerial plant parts and the young roots. When field-grown, the low hardiness of the root system isn't a problem because the soil temperatures do not get low enough to cause injury.
In addition to freezing injury, low container temperatures can cause desiccation injury, according to recent research by Cornell University, Department of Floriculture and Ornamental Horticulture.
Once the water in the container soil is frozen, conditions are the same as having a dry soil. Obviously, ice cannot be taken up by the roots. Watering the containers beforehand does not help this situation much because the extra water will freeze also. The aerial portions of plants (especially evergreens, but also, to a lesser extent, decidous species) are continuously transpiring (giving off moisture) and this can lead to serious desiccation injury.
Pyracantha is not unique with respect to having sensitive young roots. Actually, it is typical of many plants. In several instances, with plants screened last year at Cornell, the difference in the killing points of young roots and mature roots varied as much as 20 degrees.
What it all means, according to Cornell researchers, is that the plants have to be given extra protection during the winter if they are to survive. They will have to be kept where it will not be too cold for young root survival and not too warm for them to remain dormant.
"At first thought, one would expect that freezing injury in plants can be likened to the freezing of water in a glass container," says the Cornell report.
"Since water expands when it is frozen with sufficient force to break a glass container, and since plant cells contain a very high amount of water, it is logical to assume that freezing injury is due to the rupturing of plant cells by expanding ice cyrstals.
"However, there is much evidence to indicate that this is not the case and, in fact, individual plant cells actually contract rather than expand when frozen.
"Upon exposure to low temperatures, the water between the cells will freeze first and ice crystals will form in the spaces between the cells.
"Following ice crystal formation, liquid water will be drawn out of the cells to the growing ice crystals causing the cells to shrink. Thus, the water inside the cell actually remains unfrozen.
"The important factor is that the decrease in temperature is slow enough to accommodate equilibration of the plant tissue water by freezing outside the cell. If the drop is too rapid then the water cannot leave the cell fast enough and will freeze inside the cell, however, temperature drops of this magnitude are not normally encountered under natural conditions."