Grow tomatoes from seed: warm germination, sturdy transplants, heavy harvests

Tomatoes reward precision: warm germination, bright light, gentle handling, and a calm transition outdoors. Do those four and you’ll get thick stems, early flowers, and baskets of fruit instead of sad, leggy plants.

Background & common issues

• Identity. Garden tomato is Solanum lycopersicum, a warm-season crop documented by USDA PLANTS USDA PLANTS.
• Germination & heat. Seeds germinate fastest near ~70–85°F; at cooler soil temps they may take much longer UW–Madison, University of Minnesota Extension.
• Hardening off. Gradually acclimate seedlings outdoors for ~7–14 days to reduce shock University of Maryland Extension, Iowa State Extension.

“Sow tomato seeds indoors 5 to 6 weeks before the intended outdoor planting date.” — Iowa State University Extension Yard and Garden – ISU Extension

Useful stat: Under optimal warmth, tomato seeds typically sprout in about 6–12 days UW–Madison, while cooler media can stretch that to ~40+ days University of Minnesota Extension.

Key terms

• Soilless mix: sterile seed-starting medium that drains well and reduces damping-off.
• Potting up: moving seedlings to larger cells or pots once they have true leaves.
• Hardening off: gradual exposure to sun, wind, and outdoor temps so transplants toughen before planting out.

Framework: start, raise, harden, transplant

1) Start seeds warm and clean

• Containers: use clean trays or cells with drainage; label varieties.
• Medium: fill with moistened soilless mix; sow ~⅛ inch deep, cover lightly, and mist to settle UW–Madison.
• Temperature: keep media near the low-to-mid 70s°F for quickest sprout; a heat mat helps University of Minnesota Extension.
• Light: after emergence, give strong light 14–16 hours daily; keep lamps just above the canopy to prevent legginess Penn State Extension.

2) Pot up and grow sturdy seedlings

• Transplant into individual cells or small pots when first true leaves appear; handle by leaves, not stems UMN Extension.
• Keep evenly moist, not soggy; provide a light, balanced feed once seedlings are established.
• Run a fan gently across seedlings to strengthen stems.

3) Harden off methodically

• About a week or two before planting out, place seedlings outdoors in bright shade for a few hours, bring in at night, and gradually increase sun and time each day University of Maryland Extension, Colorado State University Extension.
• Avoid days below ~45°F or very windy conditions during hardening University of Maryland Extension.

4) Transplant deep into warm soil

• Site: full sun, fertile, well-drained soil; add compost for tilth.
• Depth: remove lower leaves and bury stems deeper to encourage adventitious roots; tomatoes are an exception that like deep planting Rutgers NJAES.
• Spacing: many gardens plant roughly ~3 ft between plants for indeterminates; adjust by variety and support NC State Extension.
• Support: install stakes, cages, or trellis at planting; prune and tie as you grow Cornell CALS.

Care: water, feed, prune, prevent

• Water: keep moisture even, especially at flowering and fruit set; water at the base to keep foliage dry.
• Feeding: use a balanced program; avoid excessive nitrogen that drives leaves over fruit.
• Pruning indeterminates: remove small “suckers” when ~2–4 inches long; tie stems with figure-8 loops to avoid girdling Cornell CALS.
• Disease hygiene: mulch soil, space for airflow, and avoid overhead irrigation to reduce blight risk NC State Extension.

Tips & common mistakes

• Cold media. Sowing into cool mix slows or stalls germination; warm it up for reliable sprout University of Minnesota Extension.
• Leggy seedlings. Caused by weak light and excess heat; lower the lights and reduce temperature after emergence Penn State Extension.
• Skipping hardening off. Sudden full sun and wind cause leaf scorch and stall growth University of Maryland Extension.

FAQ

When should I start tomato seeds?

Count back ~5–6 weeks from your safe planting date for your area, then sow indoors. ISU gives that same timing guidance Iowa State Extension.

How warm should the seed-starting mix be?

Target roughly ~70–85°F for fastest germination. At around ~50°F, sprouting can take more than a month University of Minnesota Extension, UW–Madison.

Should I plant transplants deeper than they grew in pots?

Yes, tomatoes are an exception: bury some stem to spur extra rooting and stability Rutgers NJAES.

Stake, cage, or sprawl?

Stake or cage for cleaner fruit, better airflow, and easier harvest. Prune indeterminates and tie regularly for tidy growth Cornell CALS.

Sources

• Homegrown Tomatoes – University of Wisconsin–Madison (wisc.edu)
• Starting Seeds Indoors – University of Minnesota Extension (extension.umn.edu)
• Successful Tomatoes From Seed – Penn State Extension (psu.edu)
• Hardening Vegetable Seedlings – University of Maryland Extension (umd.edu)
• When should I sow tomato seeds indoors? – Iowa State University Extension (iastate.edu)
• FS787: Starting Vegetable Seeds Indoors – Rutgers NJAES (rutgers.edu)
• Growing Tomatoes for the Home Garden – NC State Extension (ncsu.edu)
• Tomato Growing Guide – Cornell CALS (cornell.edu)
• Solanum lycopersicum – USDA PLANTS Database (usda.gov)