Why don’t some cold hardy plants make it in pots over winter?
I was really excited when we got this question, since most people don’t think to ask, but just assume that their container plant died from the top freezing back. When plants are in containers, the roots are actually more exposed to temperature and other climatic extremes.
Soil is actually quite insulating, so being planted in the ground provides a measure of protection from the elements, at least, for the roots. If a plant is listed as hardy into your actual USDA zone, that would be appropriate for being planted in the landscape, not in a container. The top may freeze back, but if the plant is able to return from the roots, one of the descriptive characteristics of plants known as horticultural perennials, the winter is easily survived.
In a container, heat is rapidly radiated away. So if you have winter hardy plants in pots that die-back to the ground in winter, consider moving them into your garage for the winter season, or at least overnight on the coldest nights.
And this concept, of soil having insulating properties, should also be remembered when protecting tender landscape plants from extra cold nights. When you cover your plants, be sure to enclose the base of the soil, which will radiate heat into the air around the plant, thus protecting it, and weigh down the sides so that no cold air can get inside your protected “bubble.”