Overwintering your potted perennials

Even if a perennial plant is hardy to your zone, extra steps need to be taken for winter survival if that plant is growing in a pot or other container. The two primary enemies to successful overwintering are, 1) Roots getting too cold, 2) Potted soil media drying out. There are secondary issues to watch out for that will also be discussed.

Providing Cold Protection

First and foremost, you should consider additional cold protection. The more cold hardy the plant, the more likely you are to keep them alive. The USDA Plant Zone Hardiness is based on a plant in the ground. Roots in a potted plant are more exposed to the cold than those in the ground. Depending on your perennial, it may be fine wintering in a pot. For example, much of our area is defined as Zone 5. If your plant is hardy to Zone 3, it is more likely to survive in a pot in Zone 5 than a plant is listed as a 4 or 5. Keep this in mind as you plan how to overwinter your potted perennials.

You have some choices for cold protection. First (and probably the most effective), you can simply dig a hole and bury the pot(s). Alternatively, you can cover the entire pot(s) in straw, leaves, bark mulch or another similar material. It can help to first place your pot in a box, larger pot or wire enclosure to keep the insulating material contained around the pot. Unfortunately, this insulation method can attract rodents that may damage your plants. Even piling snow up around a pot will offer a measure of protection.

A third option is to place the pot in an unheated garage, shed, or basement. The air temperature should stay below 40 degrees Fahrenheit. Avoid swings in the potted soil temperature. This means staying away from the south and southwest sides of structures where reflective heat on a sunny winter day may warm the soil in potted plants. More sensitive plants may need additional insulation to the above ground portion of the plant.

Maintaining Moist Soil

The second biggest threat to overwintering potted perennials is the potting media becoming dry. If you place your pots in an area protected from winter precipitation, you must periodically check to ensure moist soil is maintained. Don’t overwater, however, or you may induce root rot. Plants that are outdoors typically maintain potted soil moisture unless there are persistent dry conditions.

Watering outdoor pots should continue as long as the air temperature is above 40 degrees Fahrenheit. Dry soil will freeze more quickly and get colder than moist soil. Also keep in mind that potted evergreens will require more vigilance in keeping the soil moist as they continue to transpire throughout the winter. Once the soil freezes, don’t water. Ponded water in the pot will freeze and the ice expansion can damage the trunk of trees/shrubs.