Plant tomatoes in Indiana after the last spring frost, which falls between May 10 and May 20 depending on your county

Indiana's last frost date is not the same everywhere. The southern counties around Evansville see frost risk drop by mid-May, while northern counties near Michigan City stay at risk until late May. Tomatoes planted before the soil warms will sit in cold ground and rot or fail to grow. Waiting until after your local frost date — and until soil temperature reaches 60°F, ideally 65°F or higher — gives seedlings the conditions they need to thrive.

You can start seeds indoors 6 to 8 weeks before your expected transplant date. That means starting seeds in late March or early April if you plan to move plants outside in mid-May. Seedlings started too early will become leggy and stressed while waiting for warm soil outdoors.

Key Takeaways

  • Indiana's last frost date ranges from May 10 in the south to May 20 in the north, and tomatoes should not go in the ground until after that date.
  • Soil temperature matters more than air temperature — wait until soil reaches at least 60°F, preferably 65°F or warmer.
  • Start seeds indoors 6 to 8 weeks before your transplant date if you want a head start, but do not move seedlings outside too early.
  • A fall crop is possible in southern Indiana if you transplant seedlings by late June, giving plants time to mature before the first fall frost in October.

Find your exact frost date by Indiana county

The Purdue University Cooperative Extension publishes frost dates for each Indiana county. Your county's last spring frost date is the average date when the last freeze occurs — but frost can happen a few days after that date in some years. Using the average date as your target gives you a reasonable safety margin without waiting unnecessarily long.

Northern Indiana counties (Lake, Porter, LaGrange, Steuben) typically see their last frost between May 15 and May 20. Central counties (Marion, Hamilton, Boone) fall between May 10 and May 15. Southern counties (Warrick, Spencer, Perry) often clear frost by May 5 to May 10. Check your specific county rather than relying on a regional estimate — the difference between May 10 and May 20 can mean the difference between a thriving plant and one that never recovers from cold shock.

Check soil temperature before planting outdoors

A soil thermometer costs $8 to $15 and removes the guesswork. Push it 4 inches into the ground in the morning and read it after 5 minutes. Tomatoes germinate slowly in soil below 60°F and may rot before sprouting. At 65°F or warmer, they grow steadily. Most Indiana gardeners see soil reach 65°F by late May, sometimes earlier in southern counties or in raised beds that warm faster than in-ground soil.

If your frost date has passed but soil is still cold, wait another week or two rather than planting into cold ground. A tomato planted in 55°F soil will sit dormant for weeks while one planted in 65°F soil will grow when ready. The delay from waiting for warmth is shorter than the delay from cold damage.

Starting seeds indoors gives you larger plants by transplant time

Tomato seeds take 6 to 8 days to sprout under grow lights or on a sunny windowsill. Seedlings then need 6 to 8 weeks of growth before they are large enough to transplant outdoors — roughly 6 to 8 inches tall with a few true leaves. That timeline means starting seeds in late March or early April for a mid-May transplant.

Seedlings started too early (February or early March) will become tall, thin, and weak while waiting for warm soil. They may also outgrow their containers and become root-bound. Start seeds close enough to your transplant date that seedlings are vigorous and ready to move, not exhausted from waiting indoors.

Harden off seedlings before moving them outside

Seedlings grown indoors are tender and will wilt or die if moved directly into full sun and wind. Hardening off — gradually exposing plants to outdoor conditions over 7 to 10 days — toughens the stems and leaves. Start by placing seedlings in a shaded, sheltered spot for 2 to 3 hours, then increase sun exposure and time outdoors each day until they can handle a full day in direct sun.

This step is not optional. A seedling that looks healthy indoors can collapse within hours if transplanted directly into a hot, windy garden. Budget the hardening-off period into your timeline so seedlings are ready to plant, not recovering from shock.

A fall crop is possible in southern and central Indiana

If you want a second harvest, transplant seedlings outdoors by late June in southern Indiana or early July in central Indiana. This gives plants 12 to 14 weeks to grow and fruit before the first fall frost, which typically arrives in early to mid-October. Determinate varieties (bush types that ripen most fruit at once) work better for fall crops than indeterminate types (vining plants that produce all season).

Fall crops often produce less total fruit than spring crops because days shorten and temperatures cool as plants mature. However, fall tomatoes often taste sweeter because cool nights concentrate sugars. Start fall seedlings indoors in late April or early May so they are ready to transplant by late June.

Frequently Asked Questions

Can I plant tomatoes before the last frost date if I use row covers or cloches?

Row covers and cloches trap heat and can protect plants from light frost, but they do not solve the core problem: soil that is too cold. Tomatoes planted in 55°F soil will not grow even if air temperature is protected. Wait for soil to warm, then use covers if frost threatens after planting.

What if frost happens after I plant my tomatoes?

A light frost (32°F to 35°F) may kill leaves but not kill the plant if it is established. A hard freeze (below 28°F) will kill tomato plants outright. If frost is forecast after planting, cover plants with blankets, cardboard, or row covers in the evening and remove them the next morning. This works best for plants already in the ground with some root development.

Is it better to buy transplants from a garden center or start seeds indoors?

Both work. Transplants from a garden center are ready to plant when ready and save you 6 to 8 weeks of indoor growing. Starting seeds indoors costs less per plant and gives you more variety choices. Either way, wait until after your frost date and until soil is warm before planting outdoors.

How deep should I plant tomato seedlings?

Bury tomato seedlings deeper than they grew in their containers — up to the first true leaves. Tomatoes will root along the buried stem, creating a stronger root system. This is one of the few cases where planting deeper than the original depth is beneficial.