Plant tomatoes in Kansas after the last spring frost, which falls between mid-April and early May depending on your location

Kansas has two distinct growing windows for tomatoes. The main planting window opens after your area's last frost date passes — typically mid-April in southern Kansas and early May in the north. Soil temperature matters as much as air temperature: tomato seeds and transplants need soil at least 60°F to germinate and grow, though they thrive when soil reaches 70°F or warmer. Planting too early into cold soil causes seeds to rot and transplants to sit dormant and stressed.

You can also plant a second crop in mid-summer (late June through July) for a fall harvest before the first frost arrives in October. This second planting extends your season and often produces fruit with fewer pest and disease problems because plants mature as temperatures cool.

Key Takeaways

  • Spring planting in Kansas begins after the last frost date, which ranges from mid-April in the south to early May in the north — check your specific county's date rather than relying on the state average.
  • Soil temperature must reach at least 60°F for seeds and transplants to grow; waiting until soil hits 70°F reduces transplant shock and speeds establishment.
  • A second planting in late June or early July produces a fall crop that matures before the first frost in October and often has fewer disease problems.
  • Hardening off transplants by exposing them to outdoor conditions for 7 to 10 days before planting prevents wilting and sunscald when they go into the ground.

Finding your county's last frost date

Kansas State University's Research and Extension office publishes frost date maps by county. Your last spring frost date is the average date when the final freeze occurs in your area — frost can still happen after this date, but it is less common. Southern Kansas counties like Cowley and Comanche have last frost dates around April 15, while northern counties like Cheyenne and Sherman fall closer to May 10.

You can also contact your local county extension office directly, or search online for "[your county name] Kansas last frost date." Having the exact date for your county matters because planting a week too early into a late frost can kill transplants or rot seeds.

Soil temperature and when to actually plant

Air temperature and soil temperature are not the same. A warm day in April does not mean the soil is warm enough. Tomato seeds need soil at least 60°F to germinate; transplants tolerate 60°F but grow slowly and may yellow. Waiting until soil reaches 65 to 70°F produces faster growth and stronger plants.

Check soil temperature 4 inches deep with a soil thermometer (available at garden centers for $10 to $20) in the morning before the sun warms the surface. If you do not have a thermometer, wait until nighttime temperatures stay above 50°F for at least a week — that usually means soil has warmed enough. In central Kansas, this typically happens 1 to 2 weeks after the last frost date.

Starting seeds indoors versus buying transplants

Starting seeds indoors 6 to 8 weeks before your last frost date gives you more variety and lower cost, but requires grow lights and careful watering. Sow seeds in seed-starting mix (not garden soil) in late February or early March, keep them at 70 to 75°F, and provide 14 to 16 hours of light daily. Transplants are ready to move outdoors when they have their first true leaves (the second set of leaves that appear) and soil temperature is warm enough.

Buying transplants from a garden center or nursery is faster and simpler if you only need a few plants. Choose stocky, dark-green plants with no flowers yet — plants already flowering have used energy that should go into root development. Harden off transplants by setting them outside in a sheltered spot for 1 to 2 hours on day one, then gradually increasing time and sun exposure over 7 to 10 days before planting in the garden.

Spacing and planting depth for transplants

Space transplants 24 to 36 inches apart in rows 3 to 4 feet apart, depending on the variety. Determinate (bush) types like 'Roma' and 'San Marzano' need less space; indeterminate (vining) types like 'Brandywine' and 'Sungold' need more room for air circulation. Bury the transplant deeper than it was growing in the pot — tomatoes develop roots all along the buried stem, creating a stronger root system. You can bury the stem up to the lowest true leaves.

Water transplants well before planting and water the soil after planting. Do not fertilize at planting time; wait 3 to 4 weeks, then feed with a balanced fertilizer or one slightly higher in phosphorus (the middle number) to encourage flowering rather than excessive leaf growth.

Summer planting for a fall crop

Plant a second crop in late June through mid-July to harvest before the first fall frost. Kansas's first frost typically arrives in early to mid-October, giving a summer-planted tomato about 12 to 14 weeks to mature. Choose faster-maturing varieties that reach full size in 60 to 75 days — 'Early Girl', 'Stupice', and 'Glacier' are reliable choices for fall crops.

Summer planting faces heat stress and lower germination rates if soil temperature exceeds 85°F. Start seeds indoors in May and transplant in late June when soil begins to cool slightly, or buy transplants and plant them when ready. Water more frequently during the hottest weeks of July and August, and provide afternoon shade cloth if temperatures stay above 95°F for extended periods.

Common mistakes and how to avoid them

Planting too early is the most common error. Cold soil causes seeds to rot and transplants to sit stunted for weeks, wasting time and resources. Wait for soil temperature, not just air temperature. Overcrowding plants reduces air circulation, trapping moisture and encouraging fungal diseases like early blight and septoria leaf spot — space plants according to variety size and prune lower leaves once plants are established.

Overwatering, especially early in the season, causes root rot and weak root systems. Water deeply but less frequently — once or twice weekly depending on rainfall and heat — rather than light daily watering. Mulch around plants with 2 to 3 inches of straw or wood chips to retain moisture and keep soil temperature stable, but keep mulch 3 to 4 inches away from the stem to prevent rot.

Frequently Asked Questions

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

Row covers protect transplants from frost but do not warm the soil. If soil temperature is still below 60°F, transplants will not grow even if protected from air frost. Wait for soil to warm, then use row covers only if an unexpected late frost is forecast after planting.

What if my tomato transplants already have flowers when I buy them?

Pinch off the flowers before planting. Flowering transplants have diverted energy from root development, and removing flowers redirects that energy underground. The plant will flower again once roots are established, and you will harvest more fruit overall.

Is it too late to plant tomatoes in August in Kansas?

Yes, for most of Kansas. Plants need 60 to 85 days to mature depending on variety, and the first frost arrives in early to mid-October. An August planting would not have time to produce ripe fruit. Stick to late June or early July for a fall crop.

Do I need to harden off seeds started indoors?

No, only transplants need hardening off. Seeds germinate indoors and stay there until they have true leaves and soil is warm. Hardening off applies to transplants that were grown entirely indoors under lights and need gradual exposure to outdoor sun and wind.

What temperature kills tomato plants?

Frost (32°F and below) kills tomato plants. A single frost night can destroy an entire crop. Plant after your last frost date and monitor the forecast in fall — if frost is predicted, cover plants with blankets or burlap in the evening and remove them the next morning once temperatures rise above freezing.