Plant tomatoes in Chicago after May 15, when the last frost date has passed

Chicago's last spring frost typically falls around May 15, which is the date you need to know before putting tomato seedlings in the ground. Tomatoes cannot survive a frost—even a light one will kill the plant or stunt it permanently. If you plant before May 15, a late frost will destroy your work. If you wait until after that date, you give your tomatoes a full growing season to mature and produce fruit before the first fall frost, which usually arrives around October 10.

The reason the date matters so much is that tomatoes are warm-season plants. They need soil temperature of at least 60°F to grow, and they thrive when it stays above 70°F. Planting too early means the seedling sits in cold soil and either rots or grows so slowly that you lose weeks of the growing season. Waiting the extra week or two after May 15 is not wasted time—it is the difference between a plant that produces fruit and one that does not.

Key Takeaways

  • Chicago's last frost date is around May 15, and tomatoes planted before this date risk being killed by a late freeze.
  • Soil temperature should reach at least 60°F before planting, and ideally 70°F or warmer for best growth.
  • Starting seeds indoors 6 to 8 weeks before May 15 gives you transplant-ready seedlings without exposing them to frost risk.
  • Fall frost arrives around October 10 in Chicago, giving you roughly five months for tomatoes to grow and produce fruit.
  • Hardening off seedlings for 7 to 10 days before planting helps them adjust to outdoor conditions and reduces transplant shock.

Starting seeds indoors in late February or early March

If you want to grow tomatoes from seed rather than buying seedlings, start them indoors 6 to 8 weeks before May 15. That means sowing seeds in late February or early March. Seeds germinate best in warm soil—around 70°F to 80°F—so use a heat mat under your seed trays or place them on top of the refrigerator where it is warm. Most tomato seeds sprout within 7 to 10 days under those conditions.

Once seedlings emerge, move them to a bright location under grow lights or in a south-facing window. They need 14 to 16 hours of light per day to grow stocky and strong. Without enough light, seedlings become leggy—tall and thin with weak stems—and they will not transplant well. Water when the soil surface feels dry, but do not let the trays sit in standing water, which causes damping off, a fungal disease that kills seedlings at the soil line.

By mid-May, your seedlings should be 6 to 8 inches tall with their first true leaves (the second set of leaves that look like actual tomato leaves, not the initial rounded seed leaves). At this size, they are ready to harden off and then plant outside.

Hardening off seedlings before transplanting

Hardening off means gradually exposing indoor seedlings to outdoor conditions so they do not go into shock when planted. Start this process 7 to 10 days before your planting date. On the first day, place seedlings in a sheltered, shaded spot outdoors for 2 to 3 hours. Each day, increase the time outside and gradually move them into more sunlight. By day 7 or 8, they should be outside all day and all night if temperatures stay above 50°F.

This slow transition toughens the plant's leaves and stems so they can handle wind, direct sun, and temperature swings without wilting or dying back. Seedlings that go straight from an indoor shelf to full outdoor sun often turn purple or brown and stop growing for weeks. A hardened-off seedling bounces back quickly and starts producing roots and new growth within days of planting.

Soil temperature and ground preparation

Before planting, check your soil temperature with a soil thermometer. Push it 4 inches into the ground in the spot where you plan to plant. If it reads below 60°F, wait a few more days. Soil warms up gradually in spring, and planting into cold soil is one of the most common reasons tomatoes fail to thrive in Chicago.

While you wait for the soil to warm, prepare the planting area. Tomatoes need full sun—at least 6 to 8 hours per day, ideally 8 or more. They also need well-draining soil rich in organic matter. Mix compost or aged manure into the top 8 to 10 inches of soil. Tomatoes are heavy feeders and will exhaust poor soil quickly. A soil test from the University of Illinois Extension can tell you whether your soil needs lime or other amendments, but adding compost is almost always helpful.

Planting depth and spacing

When you plant, bury the seedling deeper than it was growing in its pot. Tomatoes can develop roots all along the buried stem, which creates a stronger root system. Bury the seedling so that only the top 2 to 3 sets of leaves are above ground. Remove the lower leaves before planting so they do not sit in soil and rot.

Space plants 24 to 36 inches apart, depending on the variety. Determinate varieties (bush types that grow to a fixed size) can be closer—24 inches. Indeterminate varieties (vining types that grow all season) need more room—36 inches. Good spacing allows air to circulate around the plant, which reduces fungal diseases like early blight and septoria leaf spot that are common in Chicago's humid summers.

Protecting against late frosts

Even though May 15 is the average last frost date, frost can still occur after that date in Chicago. If the forecast calls for frost or temperatures below 40°F within a week of planting, cover your plants with row covers, blankets, or even upturned buckets on the coldest nights. Remove the covers during the day so the plants get sunlight and air circulation.

Frost protection is temporary—you only need it for a night or two if a cold snap arrives. Once nighttime temperatures consistently stay above 50°F, frost risk is essentially over and you can leave covers off permanently. By late May, frost is rare enough that most gardeners stop worrying about it.

Choosing varieties for Chicago's growing season

Chicago's growing season is about 150 to 160 days from the last spring frost to the first fall frost. That is enough time for most tomato varieties, but early and mid-season varieties are safer than very late varieties. Early varieties like 'Early Girl', 'Stupice', and 'Sub-Arctic Plenty' produce fruit in 50 to 65 days. Mid-season varieties like 'Brandywine' and 'Cherokee Purple' take 70 to 85 days. Very late varieties like 'Mortgage Lifter' can take 85 to 100 days and may not finish ripening before the first frost.

If you want to grow a late variety, start seeds earlier (in January or early February) so seedlings are larger when planted. You can also use black plastic mulch or row covers in early fall to extend the growing season and help late fruit ripen before frost arrives.

Frequently Asked Questions

Can I plant tomatoes before May 15 if I use row covers?

Row covers help, but they are not foolproof. A hard frost can still kill plants even under covers. It is safer to wait until after May 15 and use covers only if an unexpected frost threatens after planting. Starting seeds indoors and hardening them off gives you a head start without the frost risk.

What if I buy tomato seedlings from a garden center in April?

Keep them indoors or in a sheltered location until after May 15. Garden centers often sell seedlings before it is safe to plant them outside. Hardening them off for 7 to 10 days before planting will help them adjust to outdoor conditions.

Do I need to use fertilizer when planting?

If you mixed compost into the soil before planting, you do not need fertilizer at planting time. Tomatoes will use the nutrients in the compost for the first few weeks. Once they start flowering, side-dress with compost or use a balanced fertilizer every 2 to 3 weeks to keep them producing.

What happens if I plant too early and frost kills my seedlings?

You can replant, but you will lose 2 to 3 weeks of growing time. The new seedlings will mature later in the season, and you may not harvest as much fruit before the first fall frost. Waiting until after May 15 avoids this setback entirely.

Can I plant tomatoes in containers instead of the ground?

Yes. Containers warm up faster than ground soil in spring, so you can sometimes plant a week or two earlier. Use containers at least 5 gallons for determinate varieties and 10 gallons for indeterminate varieties. Fill them with potting mix, not garden soil, which compacts in containers and drains poorly.