Some growers like to control their source of plants and produce perennials from propagation to finishing. This requires skill in growing as well as overwintering. Because plants are purchased when small, costs are controlled to greater extent by the finished plant producer. If the method you choose involves overwintering container perennials, then the following information may be helpful to you. Cold Hardiness Unlike field grown plants, container-grown plants cannot take advantage of the temperature buffering effect of the soil.
Roots are much less cold-hardy than the shoots.
Since most roots of plants growing in containers tend to spiral around the inside wall of the container, the only insulation these roots have is the thin wall of plastic. Research in Oklahoma has shown that the temperature of the root ball in an unprotected container can approach that of the ambient air. These colder root zone temperatures increase the probability of cold injury to roots of plants otherwise considered hardy when grown in the field. Therefore, winter protection of container plants is essential to their survival. The key to overwintering is keeping the plants cold and alive but not actively growing.