Best plants for growing in containers on balconies facing west with a lot of sun which will survive Toronto winters, zone 5/6. Balcony is on the 4th floor, one with roof , the other without.
What type of planters are best to use for plants to survive the winter weather?
Thank-you for contacting Toronto Master Gardeners with your question. As you anticipate, there are some definite challenges with overwintering plants in your balcony containers.
Extreme cold can be a major problem with containers, but even more lethal are the freeze/thaw cycles. A warm break can melt some of the ice in the pot and then the water freezes solid when the temperature plummets. Few plants can withstand these fluctuations. Even very hardy plant roots struggle in a solid block of ice! Small volumes of soil are more susceptible to freezing than larger volumes (which is why plants generally overwinter more successfully in the ground). The larger the container, the better the chance of survival. Of course, weight is also an issue on a balcony so some winter resilient materials may not be suitable. Clay, ceramic and cheap plastic pots can crack when frozen. Sturdy plastic or fiberglass/composite pots, and even wooden half-barrels are better. Metal can be used but it is a poor insulator and may be too heavy for the balcony.
A previous TMG response on a similar question suggests the following:
We can give it extra help by providing a more hospitable micro-climate, by minimizing sudden fluctuations in soil temperature, soil moisture, air temperature, and air movement. Since your planters are not insulated, would you be able to provide the plants a more sheltered environment during the colder months? Perhaps you can cluster them in a more sheltered part of your terrace, or erect temporary barriers to block the north-westerly prevailing winds. Just as you want to avoid sudden cooling, you want to avoid sudden warming; site the planters away from vents and any spot subjected to reflected heat (e.g. immediately next to a light-coloured wall facing south or south west). You want the temperature in the soil and around the plant to remain as constant as possible, with only gradual or mild fluctuations.
Both perennials and shrubs can work in containers, but the more extreme cold and wind conditions in pots and on a balcony will limit the suitable choices. The general rule of thumb for container planting is to look for plants that will grow in climates 2 zones colder than garden plants. In Toronto (zone 6b), this would be zone 4 at a minimum. You are fortunate that you have plenty of sun. The choices for a shady situation are more limited. All types of plants will need to be well watered going into winter but conifers, in particular, require watering right up to the first deep freeze. It is also necessary to ensure that the plants do not remain overly wet. If you can put them on “feet” of some kind, that will help to ensure that good drainage is maintained. You may want to put your “container cluster” under the roofed section of your balcony to minimize the wet.
Some container plant recommendations you might wish to consider are listed in the following Fine Gardening article. Pay attention to the hardiness zones cited (a couple of the plants will not work in our climate, but most will).
Conifers can be more cold hardy and also provide winter interest. The false cypresses, Chamecyparis obtusa ‘Nana Gracilis’ (Hanoki cypress ‘Nana Gracilis’) and Chamaecyparis pisifera ‘Golden Mop’ (Sawera cypress ‘Golden Mop’) are attractive options. They stay fairly small relative to many of the conifers. Some other useful suggestions on conifers can be found in the following piece by Art Drysdale:
The following links are to general articles on overwintering container plants:
Best of luck with your container plantings!