The 4 most common reasons why gardeners lose outdoor container plants during the winters are:
1. Sub-freezing temps
2. Soil freezing for long periods
3. Waterlogged roots
4. Dessication from dry winter winds
Roots are not as cold hardy as above-ground shoots, trunks, branches, etc. Evergreen plants become more challenged when the soil media is frozen. Their leaves demand more water and nutrients that the roots are not able to supply from the frozen ground.
Root injury for most woody tree species begins at 22°F. This is soil that has little water in it. Keep in mind that water freezes at 32°F. So, one tip is to keep container plants watered, but not waterlogged. Most plants are injured when soil temps hit 16°F over a 24-hour period.
Pots must provide adequate drainage. Planting in a soil-less media containing high percentages of an organic component such as peat moss and/or compost, plus a gritty substance as coarse sand, tiny pea gravel or perlite for aeration and drainage. The pot should include bottom holes to weep out excess moisture.
Evergreens naturally shed most rain and snow like an umbrella and supplemental watering is critical for these plant compared to deciduous plants. Irrigate containers at least twice monthly, assuming that natural rain and snowfall will supply the rest.