Container Gardening in Cold Climates: Temperature Control
This week’s topic is one that will allow you to cheat your growing zone and be able to grow much warmer crops in your area: temperature. This topic is critical for the success of growing cold climate fruits successfully in containers. Using containers can allow you to manipulate the plants’ growing environment into thinking your growing season is longer both in the spring and fall. But before we get into specifically how/when to protect your plants from the cold, we must get into a little bit of biology to talk about why we need to in the first place.
Plants have developed a wide variety of ways to withstand cold climates, but most of them revolve around the fact that the ground is significantly warmer than above-ground. The above-ground tissue may be exposed to beyond freezing temperatures, but between dormancy and the warmth of the ground, the plants are either able to support the above ground tissue, or die back to the roots which will stay warm in the ground to resprout in the spring. While plants may get this benefit from very large containers, the soil in smaller containers is far more likely to still freeze through, resulting in death of the roots and killing the plant. With plants suited to your growing zone, you may be able to keep them outside in large containers. However, those same plants will likely struggle if given a small container because the soil is able to freeze. This last winter, I had accidentally left some strawberries outside in 6” pots which had survived temperatures down to 0 degrees, but were not able to survive a blast of -20 degrees, but plants in larger containers had survived. With plants that are much hardier than your area, you may be able to get away with smaller containers.
With all that being said, by using your plants’ dormancy to your advantage, you can grow plants that are supposed to be grown in warmer climates. I live in zone 5b which can be a problem when some fruits that I really like to grow would prefer zones 6 or 7. However, if the plant goes dormant and you have access to a shed or a garage, you have the benefit of moving the plants outside during the random warm periods in the late winter while also protecting it from the coldest parts of winter. However, once you start the dance of moving your plants in the late winter you will need to keep an eye on the weather and move the plants accordingly. In zone 5b, March will still have a few deep freezes but will mostly be done with sub 20 degree temperatures. At this time, I will typically leave my cold-loving plants like blueberries, brambleberries, honeyberries, and currants outside even in the cold swings, but I may throw bug netting over them on cold nights for a little protection. For example, this next week we are supposed to get hit with a 10 degree night temp, which will cause me to huddle the container plants together next to the house and cover it with insect netting. I will also be moving in my plants in smaller containers and plants with early bud break that would be able to survive this easily in-ground, but I have concerns about the buds and roots dying so they will get moved into the garage. One of the most labor intensive parts of my gardening year is the late winter/early spring when I am juggling my containers in and out of the garage. I have been doing it for years and have been able to successfully grow many different berries even when only renting.
As I had previously mentioned, there can be a few different ways that you can protect your plants during the cold spells after moving them outside as well as prolonging the growing season in the fall! Keeping the containers close to your home or any heated building will allow for them to stay several degrees colder than the night temps. Additionally, using a south-facing wall will get extra heat from the sun during the day and again provide a few degrees of warmth. Huddling the plants together is another way you can get the benefits of having a large garden bed or raised bed by conserving the heat from the soil. Another option that I have already mentioned in this article is covering the plants with insect netting to provide slight protection from the wind and allow the plants to have a warmer microclimate. I have used various combinations of these techniques to be able to do things like keep a lemon tree outside until December and move it outside as early as March.
While this time of year can certainly be more labor intensive with the need to move my container plants several times, but it really is the best cheat code for northern gardeners.
