Photo credit: Lois Miklas
Annuals grown in containers are discarded at the end of the season, but many potted perennials, shrubs and small trees can be maintained over the winter if steps are taken to protect the plants and their containers.
The more cold-hardy the plants, the more likely you are to succeed in keeping them alive. The USDA zone hardiness of a plant is based on the plant in the ground. Roots of above-ground container plants can be the same temperature as the winter air. To be safe, go two zones colder with containerized plants, use larger pots (more insulation) and provide some protection. Water the plant well before the soil freezes. Evergreen plants will benefit from an anti-desiccant spray or a screen to block the wind.
Very large wooden or concrete planters can usually be left in place. It’s best not to leave clay, ceramic or glazed pottery pots exposed to the elements since they may crack. If you must leave them out, wrap the pots in bubble wrap or some insulating material covered with plastic to help protect them.
There are several methods you can use to protect smaller containers. First, you can bury the entire pot in the ground and cover it with soil or mulch. The surrounding soil acts as insulation.
A second method is to move the pots to an unheated garage, shed, cold frame, or basement. Check the soil moisture periodically, but don’t overwater or you may get root rot.
A third method is to group the pots close together in a protected location, water them and cover them with straw, compost, shredded leaves, bark mulch, evergreen boughs, or a thermal blanket. Similarly, one or a few pots can be placed inside a box, larger pot, or wire enclosure around which you have added some sort of insulating material such as bark mulch, straw, packing peanuts, or even bubble wrap. Again, check the soil moisture since dry soil freezes faster than moist soil.
In the spring, remove the protective covers gradually to give the plants time to re-acclimate.