The last two winters have shown conclusively that trees and shrubs growing outdoors in tubs, barrels and other containers may be damaged or killed unless given some protection.
The coming winter may be a severe one also, according to some of the forecasts.
The major difference between growing a plant in a container and in the ground, as far as overwintering is concerned, is the soil temperature. Field or garden soils retain heat stored during the summer at lower levels. If the air temperature goes down to 10 or 15 degrees the soil temperature in the container in likely to do the same.
If the plant were in the ground, the soil temperature at a 6-inch depth would be about 30 degrees when the air temperature is 10 to 15.
It is known that roots cannot survive low temperatures that the top of the plant can. Mock-orange tops (Philadelphus virinalis), for example, can survive temperatures down to 44 below zero, whereas the roots may be killed by temperatures of 24 degrees above.
Not only are the roots less hardy than the stems, but there is also a differential hardiness in the root system itself. The young, active roots never develop hardiness: the older roots do. For example, the young roots of the yew (Taxus) are killed at 25 degrees while the older ones are not damaged until the temperature reaches about 10 degrees.
If only the young roots are killed, it may not cause death of the plant. However, it may delay growth in the spring until active roots have been regenerated. This may explain why plants in containers are often slower to start growth in the spring than those in the ground.
Another possible consequence of lower soil temperatures in containers is lack of available moisture due to soil freezing. Although field soil moisture also freezes, it is not frozen as completely nor very deep nor for long periods of time. Thus, the possibility of winter desiccation of plants in containers is much greater.
The significance of the hardiness of the young roots in containers without protection is that they are almost certain to be damaged if the winter is severe.
If you have a lot of plants growing in containers, your best bet is to lay the plants on their sides and cover them with Microfoam. In Maryland, the critical period is about the 1st of December. Some of the larger garden centers have this material.
If you have only one or two plants, they can be stored in a garage or basement where temperatures range from 25 to 45 degrees and where they will get light, which they need for food production. The temperature must stay below 45 to satisfy their chill requirement.
Another way to protect container plants is to pile straw or hay around the sides of the container to help insulate the roots against extreme cold. Containers in which the roots are root-bound particularly should get such protection.
Continue watering plants in containers outdoors until the soil begins to freeze. After that, do not water because alternate freezing and thawing may break the container.