Starting a Garden: Microclimates
When contemplating what zone Chicago is--one thing you may want to take into consideration is the effects of microclimates & the urban heat island. A microclimate can allow you to push the boundaries of what plants you can plant in a garden in Chicago and reasonably except to survive the winter.
This photo is of a plant called "Elephant Ears" that generally isn't left in the ground in our area over the winter. You'll see these plants in planters, pots and gardens throughout Chicago in the spring and summer. In the fall they are lifted and stored indoors either in pots or with bare roots. A few years ago I was helping a neighbor plant a garden and in the fall when I was lifting out the tender and tropical plants I left some dahlias, gladiolas and the Elephant Ears in the ground.
When these tender bulbs and corms returned in the spring and summer I wasn't surprised because I had a hunch they would because of the microclimate of the garden. The bed received full exposure (at least 6 hours of direct sunlight) throughout the day and was against the foundation of a brick building next door. I had also observed a neighbor, one house down, who planted cannas (sometimes called Canna Lily, although not a lily) in a raised bed made of bricks and who never lifted and stored them in the winter.
I'm not sharing this to advocate you leaving your tropical and subtropical plants in the ground and in the hopes they'll survive a winter in Chicago. I'm just posting this in the hopes that you'll become more aware of the climate in your garden and to illustrate how garden conditions in Chicago can vary from one house to the next. With the exceptions of some gladiolas the same garden experiment in my garden around from these two gardens was a failure.
What have you planted in your garden that surprised you when it returned?