When to sow tomatoes indoors: exact timing and cultivars

TL;DR

Sow tomatoes indoors mid-February through mid-March (ideally February 15 to March 15). This gives 6-8 weeks growth before outdoor planting (mid-May in temperate zones). Light and heat are essential. Determinate (bush) and indeterminate (vining) tomatoes sow on the same date. NEVER before January (seedlings stretch). NEVER after late March (too late for good harvest).

Why sow indoors instead of directly outdoors?

Tomatoes are heat-loving. They grow outdoors only after all frost danger passes (mid-May in temperate zones). This is too late for direct sowing: you would not harvest until October. By sowing indoors (February/March), you have strong seedlings ready to grow vigorously in May and flower. This gives July/August harvest instead of October.

Also: direct outdoor sowing fails badly. Seed rots in cold, wet soil. Small seedlings have no chance against weeds and insects. Indoor-grown seedlings are far stronger when they go outside.

For home gardeners, indoor sowing is therefore essential.

Month-by-month sowing schedule

January (NEVER): Too early. Seedlings become huge and stretched (leggy) before outdoor planting season. Useless.

February weeks 1-2 (February 1-10): Still early, but possible. For very early cultivars (Stupice, Sub-Arctic Plenty) or if you want many plants.

February weeks 3-4 (February 11-28): IDEAL. This is the standard sowing date for most gardeners. 6-8 weeks growth until May planting.

March weeks 1-2 (March 1-14): Perfect, still ideal. Slightly later than February but completely safe.

March weeks 3-4 (March 15-31): Cautious. Still okay, but you risk seedlings too small for May. Only if you have good light supplementation.

April (NEVER): Too late. For May planting you need seedlings ready now. April sowing gives insufficient growth.

Light: critical factor

Indoor sowing without good light = stretched, weak seedlings. This cannot be tolerated. You must therefore:

Grow light. An LED grow light (15W+) at 5-10 cm above seedlings. 14-16 hours per day.

Without light: sunny windowsill. South-facing essential. North or west-facing window does not work. Even south window may be insufficient in February (short days).

Heat mat. Seedlings grow best at 20-24 degrees Celsius. A heat mat under propagation trays helps enormously. Without warmth, nothing grows.

Cultivar selection

Determinate (bush/patio cultivars)

Moneymaker, Ailsa Craig, Tiny Tim (small, early): Sow early March. Good for pots/balcony.

Roma, San Marzano (sauce tomatoes): Sow February. Need more plants for harvest.

Indeterminate (vining/indeterminate growth)

Grosse du Coeur, Brandywine, Black Krim: Sow February. Grow all season. Huge harvest.

Moneymaker, Tomate de France, Rutgers: Sow February. Mid-season, reliable harvest.

Cherry tomato (Gardeners Delight, Sweetie): Sow February or early March. Bear all summer.

How to sow tomatoes indoors step-by-step

Preparation

1. Choose seed. Order seed in January (many varieties sell out by late January). Choose from reliable suppliers.

2. Seed starting mix. Sterile seed starting mix (not compost, not garden soil). Seed mix is fine-textured and disease-free. Alternatives: perlite + peat moss, or coco coir.

3. Containers. Small seed pots (6-10 cm) or propagation trays with drainage holes. Drainage is essential.

4. Heat. Heat mat (electric) under trays. This is nearly essential. Tomato seeds germinate only at 20C+.

5. Light. LED grow light over seedlings. 14-16 hours daily. This prevents stretching.

Sowing process

Step 1: Fill containers with seed mix. Press lightly, dampen. Should be moist, not soggy.

Step 2: Plant seed. Place each seed at roughly 1 cm depth. Do not plant deeper: germination poor deep. Cover very lightly (0.5 cm seed mix on top).

Step 3: Set on heat mat. Place trays on heat mat. Without warmth (minimum 20C) nothing happens. With warmth: germination in 5-10 days.

Step 4: Cover with plastic. Cover trays with plastic film or propagation dome. Holds moisture. Germination percentage rises dramatically.

Step 5: Daily check. Every day look. Is mix dried out? Very gently mist-spray (spray bottle, not watering can). Too wet = fungus = dead seedlings.

Step 6: First seedlings. As seedlings break soil (usually 5-10 days), remove plastic. Turn on grow light (14h daily, 5-10 cm above). Lower temperature to 18-20C (no more heat needed).

Step 7: First true leaves. When seedlings get their first true leaves (not the first two, those are seed leaves; next two are true), growth accelerates. Now more nutrition needed.

Step 8: Thin out. If two seedlings occupy one spot, remove the weaker. Or very gently separate roots. Must happen before roots tangle.

Step 9: Feeding. Seedlings consume seed mix nutrients. After 2-3 weeks you must feed. Very gently: half-strength tomato fertilizer, once weekly.

Step 10: Pot up. When seedlings are 4-5 cm tall and have two true leaf pairs, pot them up (into larger 9-10 cm pots). Gives more root growth.

Aftercare until outdoor planting

Water: Water carefully. Top moist, bottom well-draining (holes!). Do not pour from above (fungus). Use watering can with spout applied below.

Feeding: Very gently. Half-strength tomato fertilizer, once weekly after 3 weeks.

Light: 14-16 hours daily. Grow light stays on.

Hardening off: 2 weeks BEFORE planting outdoors, start "hardening off" seedlings. This means: gradually lower temperature, more air, time outside in sun (start 1 hour, work up to 6-8 hours).

Step-by-step

Step 1: Buy seed and supplies

January/February: order seed, seed mix, containers, heat mat.

Step 2: Prepare setup

Have heat mat ready, grow light rigged above, trays prepared.

Step 3: Sow

Mid-February through mid-March: sow seed in seed mix, cover lightly.

Step 4: Wait for germination

Set on heat mat (20-24C). Cover with plastic. Wait 5-10 days for seedlings.

Step 5: Turn on light

Seedlings above soil? Remove plastic, turn on grow light (14h daily, 5-10 cm above).

Step 6: Water and feed

Keep moisture right. After 3 weeks: half-strength feed once weekly.

Step 7: Pot up

After 4-5 cm tall and two true leaf pairs: pot up into 9-10 cm pots.

Step 8: Hardening off

2 weeks before outdoors: start hardening. Each day a bit colder and lighter.

Step 9: Plant outdoors

Mid-May (after last frost): seedlings outside in garden or containers.

Frequently asked questions

Can I sow seeds directly outdoors?

No, this does not work in temperate zones. Tomatoes want warmth. Outdoors is too cold until May, and seed washes away. Indoors is necessary.

What if I have no grow light?

Very difficult. Seedlings stretch without light. A sunny south-facing windowsill may suffice in March, but February is too dark. If you have no light, sow later (March).

My seedlings are tall and thin. What now?

This is called "stretching" from insufficient light. They can still be saved: place grow light close above. By outdoor planting time they strengthen somewhat. Or next year start earlier (more growth weeks).

Can I use regular potting soil instead of seed mix?

No. Potting soil is too coarse and can carry fungal disease. Sterile seed mix is essential.

How many seedlings should I sow?

Rule: 1.5-2 seedlings per desired final plant (for losses). If you want 10 final plants: sow 15-20 seeds.

My seedlings have brown spots. What is this?

Probably damping off (fungus). Happens if too wet. Prevention: remove plastic, improve air, water carefully, not from above.

Can I sow seedlings late (May-June)?

Yes, in theory. But they will have much smaller flowering potential. Prevention: sow on time (February/March).

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