Not too long ago, a reader emailed me and asked,
I bought some hostas and dwarf bleeding hearts to plant. Shortly thereafter I hurt my knee and I can’t go out there and plant them. They are all planted in one gallon plastic pots. How can I safely winter them? If I put them in my garage they will still freeze.
It is the roots you are worried about freezing. The rule of thumb is that roots in a pot will effectively be in a situation two zones colder than plants in the ground. So if the plant tag says zone 5 and you are in zone 7 they should be fine.
But I’m guessing you live in a colder place. Definitely keep them outside until the tops die down. You want them to go dormant. After that, you want to put them in a situation where they will stay dormant but not get colder than two zones warmer than the hardiness zone on the tag. That will be different depending on what is available at your place. The garage might be okay, if it gets cold, but not as cold as outside. You could put them up against the house, and surround them with bags of leaves for insulation.
The other thing to consider is the greater the volume of soil in the container, the more insulation the roots will have. A one gallon container is not that big, and won’t have much insulating soil mix for the roots, so you might err on the side of caution and make that three zones warmer than the tag.
I have a drafty, dirt floor cellar that barely stays above freezing. I have put dormant plants in the coldest corner of the cellar and pulled them through the winter. They did start growing sooner than they should have, and made some pale spindly growth, but I very carefully hardened them off and planted them after all danger of frost. It was a pain in the neck but better than losing them.