If left outdoors over the winter, your container plants and pots alike can suffer from the effects of freezing weather. Porous containers made of clay or concrete may chip, crack, or break into pieces after repeated freezes. Even plastic or metal containers may be cracked or broken apart. For controlling winter damage to plants, the temperature of the potting mix is more important than air temperature.
These measures will give your container gardens their best chance at surviving winter:
- Empty small containers and store them out of the weather.
- Move tender plants indoors for the winter, but be sure they are free of insects. If you find insects on them, spray them with landscape insect killer before you take them inside. Some tender plants tolerate the relatively low light levels, dry air, and warmth of indoor rooms quite as well as houseplants. Others are best on a sunny enclosed porch that remains cool without freezing.
- Take this opportunity to re-pot plants that have outgrown their containers over the summer. Fill the new containers with enriched potting mix, which will also provide the plants with nutrients from plant food that is incorporated in the mix. For best results keep feeding regularly with a liquid plant food.
- Use your garage, enclosed porch, potting shed, or basement to store emptied containers as well as those with permanent, dormant plants. (An attached garage is often an ideal overwintering environment, dry and cool with temperatures that rarely go below freezing.) If you’re potting bulbs this fall, store those here, too.
- Save valuable permanent plants that are difficult to overwinter in containers (such as roses in zones 4 and 5) by transplanting them into the ground this fall. Buy new ones for containers next spring.
- Make sure outdoor containers go into winter’s freezes with moist potting mix, which is far more insulating than dry mix.
- If you have outdoor containers that are just too large to move but need to be protected, you can cover the sides and top with several layers of bubble wrap, followed by a weatherproof tarp and a final layer of less obtrusive burlap.
Winter-proof containers (such as those made with newer man-made “foam” materials) offer a good place to over-winter evergreen shrubs and trees such as boxwood or dwarf Alberta spruce. Especially when several are grouped together on a patio or deck, they will provide a welcome touch of greenery against the snow or drab winter landscape!