A plant's hardiness refers to plant's ability to tolerate air temperature. Every plant has a floor of what temperatures they can tolerate. It is important to know your zone before you pick new trees to plant. Most of Illinois in in Zone 5. This zone has a minimum winter temperature of -10 degrees to -20 degrees. Much of Cook County was reclassified in 2012 for Zone 6a. This zone can has a minimum winter temperature of -5 degrees to -10 degrees. These differences can be important when selecting new plantings. All trees are classified in these zones, so it should be easy to check if it works for you.