Hi again, thank you for your answer to my question on insulating round planters. However, some of your suggestions can’t apply as I did not provide enough detail about my situation in my question. I am in an apartment, so do not have a garden bed in which to sink my pots over the winter. They will be sitting on a concrete balcony all the time, unfortunately. Also, some of them are so heavy (concrete), and wide shallow urns, so I think would be very difficult, especially once filled with soil, to lift them into a larger pot to add insulation around the outside of the pot. Maybe there is something I could wrap around the outside of the pot over the winter instead? I have also read that you can line the inside of the pot with bubble wrap (and holes poked through the bottom for drainage) before filling with soil. Would that be as effective as the hard sheet foam that most people suggest? Thank you!
The key to overwintering perennials successfully is you need to keep the plants dormant and provide a winter environment that’s within their hardiness zone. That is why when initially choosing your perennials it is advisable to choose ones that are hardy to two zones colder.
Raising your containers onto feet off the concrete which may heat up during the sunshine and cool dramatically overnight is preferable. Garden centers now carry movable flowerpot base holders in various sizes, making it easy to group the containers together for the winter. Consider asking the help of your neighbours or superintendent.
As for utilizing bubble wrap for insulation there are two different schools of thought. One website Gardening with Bubble Wrap suggests building a wire cage around the container then filling it with bubble wrap. However, another website Garden Fundamentals states the following “I’ve seen people suggest that wrapping plants in bubble wrap will keep them warm on a balcony – that’s not true. The only way such wrapping works is if you extend the bubble wrap right to the ground so that you trap the warmth coming from the soil. On a balcony, there is no warmth to trap.
It is true that bubble wrap acts like a small greenhouse and traps the warmth of the sun during the day, but that probably does more harm than good. It does not take much sunshine to overheat the plant and if that happens they start to come out of dormancy. A bit of cold will then kill them. Bubble wrap should be covered with a light proof material.”
If you do decide to try utilizing bubble wrap, please write us back and let us know how your perennials fared over the winter.