We have several perennials in large pots on an exposed terrace.
Cedars, Pine, Hydrangeas and Dog Wood.
We made a mistake and didn’t create insulated containers.
We need to find a box/product or some way to insulate the pots to prevent any freeze/thaw damage over the winter.
We are also thinking about using burlap to wrap the cedars – is that a good idea? Do you recommend anything else.
I really appreciate that you guys exist! And look forward to your response.
Thank you for writing to the Toronto Master Gardeners with your questions; we are glad to be of help.
In the abstract, a plant can survive in a container if it is rated at least 2 USDA hardiness zones lower than the zone it is in. 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.
While deciduous woody plants go completely dormant during winter, conifers do not; they slow down, but they still “breathe”. The needles still need moisture from the roots, especially if they are being dried out from winter winds and/or afternoon sun. Yet once the ground freezes plant roots are not able to take in moisture from the soil. Therefore, you want to make sure that the roots of your cedars and pine have as much reserve moisture as possible going into winter; do not stop watering them until freeze-up. Then, place a thick layer of mulch over the soil to keep it all nicely insulated. Do the same for the deciduous woody plants; although they don’t have leaves to worry about, they still appreciate the extra moisture to help keep the rest of the plant alive over winter.
Wrapping the cedars in burlap is a great idea–just make sure that the burlap is not touching the needles (otherwise they will suffer more frost damage than unprotected). One way to do this is to put stakes 8-12 inches around the plant and wrap the burlap around this scaffold. The burlap shelter will protect the needles from being desiccated by the freezing winds or scorched by the aggressive south-west sun in late winter/early spring.
Here are links to some past answers that are also relevant to your question:
In addition, there is a Gardening Guide: Container Gardening: A Toronto Master Gardener’s Guide: https://www.torontomastergardeners.ca/gardeningguides/container-gardening-a-toronto-master-gardeners-guide/
We wish you the best of luck in overwintering your containerized woody plants.