If you want to be a successful gardener, there’s one thing you absolutely must know: your hardiness zone. Having a successful garden in Chicago means knowing when the first and last frost is likely to take place and how cold it gets in the winter.
The modern hardiness zone, sometimes called a growing zone, was developed by the USDA. They assign each hardiness zone by observing 30 years of annual minimum temperature data at each location in order to determine the likely lowest temperature each winter. Knowing how cold it gets and when the last and first frost dates are for the year can mean the difference between a successful garden and a lot of wasted time and money.
Chicago’s USDA hardiness zone
Chicago’s USDA hardiness zone is 5b and 6a. That means winter temperatures can drop as low as -15 degrees Fahrenheit. Much of Chicago is in zone 6a, meaning temperatures drop to only -10 degrees Fahrenheit, but outlying areas and the suburbs fall into zone 5b. Freezing temperatures tend to begin in October and extend all the way until April.
Illinois’ USDA hardiness zone map
Illinois’ USDA hardiness zones range from 5a, where temperatures can drop to as low as -20 degrees Fahrenheit, to 7a, where they only hit about 0 degrees Fahrenheit in the winter. Above is a map for the entire state to help you determine your USDA hardiness zone.
First and last frost date in Chicago
According to your hardiness zone, 6a, the final frost date in spring is likely to be around April 25th. Keep in mind that this is an estimation based on the observation of 30 years of data. It is not a guarantee. It is possible for frost to occur after this date so be prepared to cover your plants in the off chance you get an unexpected frost after this date.
Your first frost in the fall is likely going to take place around October 13th. Just as with your final spring frost, your first fall frost can come a little bit earlier or a little bit later. This date is not an exact guarantee.
When to plant in Chicago
Below is a helpful table for determining when to start different types of vegetables indoors and when exactly to sow them outdoors.
| Vegetable | Spring Indoors Start | Spring Crop Transplant Date | Fall Indoors Start | Fall Crop Transplant Date |
| Artichokes | Feb 28 | Apr 25 | Spring only | Spring only |
| Beans | Direct Sow Only | Apr 25 | Direct Sow | Aug 4 |
| Beets | Feb 28 | Apr 11 | Direct Sow | Aug 4 |
| Broccoli | Feb 28 | Apr 11 | Jun 25 | Jul 25 |
| Brussels Sprouts | Feb 28 | Apr 11 | May 16 | Jun 15 |
| Cabbage | Feb 14 | Mar 28 | Jun 17 | Jul 15 |
| Carrots | Direct Sow Only | Apr 4 | Direct Sow | Jul 19 |
| Cauliflower | Feb 28 | Apr 11 | Jun 25 | Jul 25 |
| Celery | Feb 7 | May 2 | May 26 | Jun 23 |
| Chard | Feb 28 | Apr 11 | Direct Sow | Aug 9 |
| Collards | Feb 21 | Mar 28 | Jun 18 | Jul 18 |
| Corn | Direct Sow Only | May 2 | Spring only | Spring only |
| Cucumbers | Direct Sow Only | Apr 25 | Spring only | Spring only |
| Eggplant | Mar 14 | May 9 | Spring only | Spring only |
| Gourds | Direct Sow Only | May 9 | Spring only | Spring only |
| Kale | Feb 14 | Mar 28 | Direct Sow | Jul 20 |
| Kohlrabi | Feb 14 | Mar 28 | Direct Sow | Aug 14 |
| Leek | Jan 31 | Apr 11 | May 21 | Jun 25 |
| Lettuce | Feb 2 | Mar 28 | Jul 5 | Aug 4 |
| Melons | Direct Sow Only | May 9 | Spring only | Spring only |
| Okra | Mar 14 | May 9 | Spring only | Spring only |
| Onions | Jan 17 | Mar 28 | Jun 25 | Aug 4 |
| Peas | Direct Sow Only | Feb 28 | Direct Sow | Jul 5 |
| Peppers | Feb 28 | May 2 | Spring only | Spring only |
| Pumpkins | Direct Sow Only | May 9 | Spring only | Spring only |
| Radishes | Direct Sow Only | Mar 21 | Direct Sow | Sep 3 |
| Spinach | Feb 7 | Mar 14 | Jul 25 | Aug 17 |
| Squash | Apr 11 | May 9 | Spring only | Spring only |
| Tomatoes | Mar 6 | May 2 | Spring only | Spring only |
| Turnips | Direct Sow Only | Mar 21 | Direct Sow | Aug 14 |
| Watermelon | Apr 11 | May 9 | Spring only | Spring only |
