I have a 7th floor balcony which is exposed to SW and NW winds as it is on the corner. I have heard that you can line a container with styrofoam to help plants survive the winter. Do you just line the sides or do you line the bottom of the container as well and drill holes in the styrofoam?
Thank you for contacting Toronto Master Gardeners. A comprehensive response to a similar question on our website can be found here and it should provide you with all the details you require. The bottom line is that styrofoam *may* help containerized plants survive the winter. However, careful selection of both plant (hardiness zone) and container (material) is essential, even more so given that you are located on the 7th floor, which means that plants/containers will be exposed to more cold and wind during the winter than if you were at ground level.
If you choose to line the bottom of your container with styrofoam, use the drainage hole of the container as a marker for where to drill or cut out the drainage hole in the styrofoam.
A few additional sites that may be useful are listed below:
Overwintering Potted Plants (Brooklyn Botanic Garden)
Winterizing Container Grown Trees and Shrubs (University of Illinois Extension)