November 3, 2026

When to start planting tomato seeds for your yard's last frost

Start tomato seeds indoors on the date you get by counting the weeks on your seed packet back from your own garden's last frost. The published last frost for your area is a regional average. A garden that runs colder than the forecast on spring nights keeps freezing after that date passes, so its true last frost lands later. Our published last frost is April 28. Siltwell learned from our weather station that our nights run 7°F below the forecast. It applies that offset and shifts our last frost to May 20.

The steps below set a sow date tied to your site, then get the seed into the mix and out to the bed.

1. Find your published last frost date

Start with the average last frost date for your area from your extension service or a frost date lookup. Treat it as a starting point. It comes from weather records at sites that may sit higher, warmer, or windier than your yard. We explain why it often runs early in your last frost date is probably wrong.

2. Shift it by your site's offset

On clear, still nights the ground radiates its heat to the sky. Because it weighs more than warm air, the chilled air slides downhill and collects in dips. A garden in one of those spots runs colder than the forecast, and its frost dates move with it.

Siltwell moves your first and last frost dates by your microclimate offset, by at most 30 days. On a free account you enter the offset yourself under Settings, Garden, Edit garden, Microclimate offset, using a negative number for a colder site. With a connected station on Pro, Siltwell learns the offset from the station's outdoor air temperature after about 14 days of overlapping station and forecast readings.

If you have no station, the microclimate checker compares nearby home weather stations with the forecast model over recent nights and returns a range for your area. That range describes your neighborhood. Your own bed can sit colder or warmer than it.

3. Count back the weeks on the packet

Your seed packet prints how many weeks before the last frost to sow. Count that number back from your shifted date. Suppose your packet says seven weeks. Counted from our published April 28, that lands on March 10. Counted from our shifted May 20, it lands on April 1.

Those extra weeks indoors are where seedlings go wrong. They outgrow their cells, their roots circle, and their stems stretch while they wait for safe nights.

4. Fill the cells with seed-starting mix

Use a fine, sterile seed-starting mix and wet it before it goes into the cells. Dry peat sheds water at first, so premoistening keeps the seed out of a dry pocket. Garden soil packs down in a small cell and can carry the fungi behind damping off. Our guide to seed-starting soil mix covers what goes into a good one.

5. Sow shallow and keep the mix warm

Sow each cell with more than one seed and cover them with a thin layer of moist mix. A seed buried deep spends its stored energy pushing up through the mix before its leaves can make more. Warmth speeds germination, so a heat mat under the tray shortens the wait. Cover the tray to hold moisture, and take the cover off when the first green loops break the surface.

Once the seedlings are up, keep the strongest one in each cell. Snip the others at the soil line with scissors. Pulling them tears the roots of the one you keep.

6. Give the seedlings strong light

Move the tray under bright light the day the sprouts appear. A seedling in weak light stretches its stem toward the source and grows thin. A grow light hung close above the leaves gives steadier light than an early spring window. Water from below when the surface of the mix dries, which keeps the base of each stem dry.

7. Harden off and plant out by your shifted date

Leaves grown indoors have never met wind or full sun, and they scorch when they do. Over several days, set the plants outside for longer stretches before they stay out overnight. Plant out once your shifted last frost has passed and the nights stay mild.

We transplanted our tomato seedlings into a raised bed on May 18, two days before our shifted date. That bed gave us 1.7 lb across 30 fruits between August 17 and September 7. Our plant-out guide covers the bed side of that move.

What can go wrong

• Leggy seedlings. A sow date counted from too early a frost date, or too little light, stretches the stems until they flop. Bury a leggy stem deeper at transplant, since tomato stems grow roots where they touch soil.
• Damping off. Seedlings pinch and collapse at the soil line. The fungus behind it thrives in cold, wet mix with still air, so water from below and let the surface dry between waterings.
• Chilling after planting out. Tomatoes stall and their leaves discolor on cold nights well above freezing. We cover the mechanism in cold damage above freezing.
• A late frost. One night at or below freezing can kill an unprotected tomato. Our guide on protecting tomato plants from frost covers what to put over them.

A shifted frost date tells you where your site's last frost tends to fall across many springs. It cannot tell you how this particular spring will end. Our recorded last frost in 2026 came on April 30, nearly three weeks before our planning date.

Before you sow, set your offset and write the count-back date on the seed packet. After you plant out, keep frost alerts on. Frost alerts run for every garden, free and Pro, and they use the same offset that moved your frost date.