Complete Guide to Starting Tomatoes from Seed Indoors

Why Start Tomatoes Indoors?

Starting tomatoes from seed gives you access to countless varieties and a head start on the growing season. Follow these nine steps for strong, healthy transplants.

Step‑by‑Step Process

1. Get Fresh Seeds

Buy from reputable suppliers. Seeds less than four years old germinate best, though older seeds stored cool and dry may still work.

2. Use a Quality Seed‑Starting Mix

Avoid garden soil—it compacts and harbors pathogens. Choose a commercial mix of peat moss, vermiculite, and perlite (e.g., Jiffy Mix, Pro‑Mix). Moisten thoroughly until it feels like a wrung‑out sponge.

3. Select Containers with Drainage

Any container with drainage holes works. Sterilize reused pots with a 10% bleach solution. Cell flats, plastic cups, or yogurt containers are all suitable.

4. Time Your Sowing

Tomato transplants need 6–8 weeks before the last frost. Determine your average last frost date, then count back. Planting outdoors 1–2 weeks after that date is ideal.

5. Sow Seeds Shallowly

Plant seeds ⅛ inch (3 mm) deep in damp mix. Firm gently for good contact. Cover with plastic to retain moisture, but allow some air circulation.

6. Provide Warmth for Germination

Keep at 70–80°F (21–27°C). Seeds sprout in 5–10 days. Light isn't needed yet, but move seedlings to bright light immediately after emergence to prevent leggy growth.

7. Give Strong Light

Seedlings need 16–18 hours of intense light daily. Fluorescent shop lights kept inches above the plants work well. A heated greenhouse or cold frame is even better.

8. Grow On and Transplant Up

Maintain 65°F (18°C) with good air circulation. Water thoroughly, then let the mix nearly dry. When true leaves appear, transplant into larger pots, burying stems deeper to encourage root growth. Fertilize very sparingly with a weak solution.

9. Harden Off Before Planting Out

Gradually expose plants to outdoor conditions over 7–10 days. Start in shade, avoid wind, and bring them in if temperatures drop below 40°F. Remove early blossoms to focus energy on root establishment.

Common Problems and Fixes

• Leggy seedlings: Increase light, reduce heat/fertilizer, repot deeper, use a fan.
• Damping off: Use sterile mix, avoid airtight covers.
• Slow growth: Check temperature and nutrient levels.
• Stuck seed coats: Mist with water or apply a tiny drop of saliva to soften.
• Over‑watering: Water deeply, then wait until nearly dry.
• Over‑fertilizing: One or two dilute applications are plenty.