Summary
Starting tomatoes from seed is far cheaper than buying transplants (seeds cost $3–$5 for 30 plants vs $5–$8 per plant at a nursery) and gives you access to 300+ heirloom varieties. This 6-week plan covers: timing, soil mix, sowing depth, bottom heat, lighting, first transplant, hardening off, and garden planting.
Produce healthy, stocky tomato transplants ready to go into the garden after all frost risk has passed — with multiple varieties and zero damping-off.
Step-by-step Guide
Timing — count back from your last frost date
Start seeds 6–8 weeks before your average last frost date. Find your date at Dave's Garden Frost Dates tool or Old Farmer's Almanac. Starting too early makes leggy, pot-bound plants. Starting too late wastes the advantage of seed starting.
Gather your supplies
72-cell or 128-cell seed trays with dome lids, seed-starting mix (NOT potting soil — too dense), heating mat for germination, grow light or south-facing windowsill (grow light is far superior), small plastic labels and a marker.
Fill cells and sow seeds
Fill cells with moistened seed-starting mix (damp, not waterlogged). Sow 2 seeds per cell at 1/4" depth. Cover lightly. Mist with a spray bottle. Put the dome lid on to retain humidity.
Germinate with bottom heat
Set the tray on a seedling heat mat. Tomatoes germinate fastest at 70–80°F soil temp. With bottom heat, you'll see sprouts in 5–7 days. Without it, germination takes 10–14 days and is less consistent. Keep the dome on until the first sprout appears.
Move under lights immediately after sprouting
As soon as the first seedlings emerge, remove the dome and position grow lights 2–4" above the tops. Lights on 14–16 hours per day (use a timer). Without adequate light, seedlings stretch and become "leggy" — tall, weak stems that will struggle in the garden.
Water correctly
Bottom-water when the surface feels dry: place the tray in a shallow pan of water for 20–30 minutes, then drain. Top-watering splashes wet leaves and promotes damping-off (a fungal disease that kills seedlings at the soil line). Keep the medium moist but never waterlogged.
Thin to one plant per cell at true-leaf stage
The first leaves are cotyledons (seed leaves). The second set of leaves are the first "true leaves" — they look like tomato leaves. At this stage, snip the weaker seedling in each cell at the soil line. Don't wait.
Pot up to 4" pots at 3–4 weeks
When seedlings have 2–3 sets of true leaves, pot up into 4" containers with regular potting mix + fertilizer. Bury the stem up to the lowest leaves — tomatoes root all along the buried stem. This is why tomatoes can go from leggy to stocky with one good pot-up.
Harden off for 1 week before planting out
One week before transplanting, start putting plants outside in a sheltered spot for 1 hour/day, increasing by 1 hour each day. This acclimates them to outdoor wind, UV, and temperature swings. Skipping hardening off causes transplant shock and stalled growth.
Tools & Materials
Safety & Legal Warnings
Troubleshooting
Seedlings are tall and floppy (leggy)
Light was too far away or hours too short. Lower the light to 2" above canopy. You can pot up and bury the leggy stem to compensate.
Seedlings dying at the soil line (damping-off)
Fungal. Remove affected seedlings immediately. Improve air circulation (small fan), reduce watering frequency, water from the bottom only.
No germination after 2 weeks
Check soil temperature — may be too cold. Verify seeds aren't too old (most seeds are viable 3–5 years). Try a fresh seed batch or check germination rate by placing 5 seeds on a damp paper towel.
What the Video Didn't Cover
Related Resources
- Old Farmer's Almanac frost date tool (almanac.com)
- Epic Tomatoes by Craig LeHoullier — definitive heirloom variety guide
- r/vegetablegardening — seed starting megathread (pinned each January)
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