I had a question awhile back about wintering over native plants (asters and coneflowers) which didn’t get planted. The person calling was wondering if they could simply store them in their unheated garage until spring and then transplant them.
I’m no expert, so when it comes to answering this question, I can only draw upon past experience. Here’s how I answered that question.
I suggested they leave the plants outside in whatever containers they were in (plastic pots, plastic grocery bags, etc) and cover them with leaves.
The same thing has happened to me in the past. Because of the summer heat and lack of rain, I have occasionally held off planting a partial tray of plants from the plant sale or plants dug from a rescue. Then mid-September arrives too soon and it’s too late to get them into the ground to establish a good root hold before winter sets in. So, in the fall I cover them with a huge pile of leaves and store them outside, in a spot where they can be sheltered from the high noon heat.
I discovered this method works because, sometimes in the past, I have had no choice but to over-winter late season rescued plants in their grocery bag greenhouses. Out of necessity, I have simply covered them with fall’s discarded leaves, and they’ve come back to life in the spring without fail. This gave me a chance to transplant them into much softer, more adaptable ground in the spring, and eliminated the need to worry about frost heave from having planted them the previous fall.
Storing them in an unheated garage might also work, but they would need to be watered now and then. Most native plants in the Midwest need a winter rest, but they also need some moisture to get them through to spring wake-up. Putting them outside means Mother Nature will water these plants and give them a sufficiently cool rest.