I was told that to overwinter perennials in pots, I should insulate the inside of my pots with 1″ styrofoam. However, how could I do this with a round pot, since those insulation sheets aren’t flexible? Do you recommend a different insulating material that might work? Some of my pots are taller and rounder, and some are shorter, shallower bowls. I especially don’t want to take up too much soil space in the smaller containers with super thick insulation (and yes, I have heard that the bigger the pot, the better the chances of overwintering, but I am willing to risk it in my smaller containers and see what happens).
Thank you for contacting Toronto Master Gardeners regarding insulating and overwintering garden pots, a subject pertinent for many gardeners in our region. Insulating containers can benefit summer planting too, especially if the planters themselves have a heat-absorbing dark surface such as black cast metal urns.
For the contents of the container, it is recommended to select plants hardy to two zones colder than your hardiness zone, and to have them healthy and well watered as they transition into winter. Continue to water containers through the fall into freeze up, and water again through April.
An easy and good form of insulation is to put your pot into a larger size one that will leave sufficient room to put in the bottom and stuff around the sides of your container an appropriate insulating material, such as straw, mulch, or shredded leaves. Alternatively you may sink the pot to just below the rim in the garden. It is preferable to set your insulated container on soil rather than cement or other hard surface which may heat up during the sunshine and cool dramatically overnight. If it is sitting on a hard surface, such as a deck, raise your pot on feet to provide good drainage and to protect the deck’s surface.
Assemble your containers together in an area sheltered from the winter winds which are not only cold but desiccating too.
An informative website is this: https://www.hgtv.com/outdoors/gardens/garden-styles-and-types/the-cold-hard-facts-on-protecting-potted-plants
You are sure to have success in overwintering your planted containers because you are carefully and thoughtfully planning ahead. We wish you great success with your container garden.