Many people garden exclusively in pots and some keep hardy trees, shrubs and perennials in containers year-round as a way to provide winter interest during Chicago's cold, and often gray, winter season. Container gardening in winter depends on a lot of controllable factors, but realistic expectations are important and success begins with smart plant and container choices.
Check the labels on your plants to know which will have the best chances for survival in the container you select. The USDA has a hardiness zone map that labels Chicago as "Zone 5." Roots are naturally less hardy than stems, so subtract one zone to choose a Zone 4 plant for your container. Zones 2 and 3 would be even better.
As for the container, bigger is better with pots because a large soil mass means more insulation and protection for the root system. I generally recommend 24- by 24- by 24-inch containers, with 18- by 18- by 18-inches as an absolute minimum to have any chance of plant survival over winter.
Think about the life of the pot, too. Even materials like concrete ( now referred to as "cast stone" ) have a shortened life expectancy when they're full of freezing and thawing soil during winter. Ceramic pots outside all winter, full of wet soil, will not last long. Instead, investigate wonderful new plastics that truly resemble terra cotta or cast stone, yet offer long life-expectancy and the added bonus of being lightweight and easy to move.
If your pot is too heavy to move, and you don't care about a not-so-pretty view all winter, you might consider buying roll insulation and wrapping the pot ( not the plant ) for the winter. Anything you do to reduce cold air movement around the pot will increase the plant's survival chances. Even something as simple as moving the container with one side against a building will decrease the chances of soil freezing solid. Our most damaging winter winds are generally from the northwest, so pots along the east side of a building will be more apt to survive damage-free than those along a north or west side.
Consider moving plants that are marginally hardy ( Japanese maples or tree roses, for example ), or specimens that don't have a lot of winter interest, to an unheated garage or storage shed. If you can keep them at around 35 degrees, so the root ball doesn't freeze solid, you may be able to save them.
Dehydration is as damaging as cold temperatures, especially for evergreens in pots. After all, even dormant plants need water to survive. Check every few weeks over the winter ( whether the potted plant is outdoors or in storage ) and give them a drink if the soil is dry. Water will eventually soak in, especially if you water plants on an "above freezing" day.
Like all things in life, have realistic expectations. No matter what lengths you go to, your plants may or may not be alive come spring. So, think of them as annuals and if they survive to grace your landscape another year, then that's a wonderful bonus.
Tony Fulmer is with Chalet, a landscape/nursery/garden center in Wilmette. See www.chaletnursery.com .
