How to Plant Tomato Seeds: Step-by-Step Guide for Success

Plant tomato seeds 6-8 weeks before your last frost date, 1/4 inch deep in seed starting mix at 70-80°F. Keep soil moist but not soggy, provide 14-16 hours of light daily, and transplant seedlings when they have 2-3 true leaves. Proper spacing (24-36 inches apart) and hardening off before outdoor planting significantly increase success rates.

Starting tomatoes from seeds gives you access to hundreds of varieties you won't find at nurseries and saves money compared to buying established plants. Whether you're growing beefsteak tomatoes for sandwiches, cherry tomatoes for salads, or heirloom varieties for their unique flavors, proper seed planting technique sets the foundation for a productive harvest. This guide walks through each step with science-backed methods that maximize germination rates and produce vigorous seedlings ready for your garden.

Essential Supplies for Planting Tomato Seeds

Before you begin, gather these basic supplies. You don't need expensive equipment to start seeds successfully, but certain items make a significant difference in germination rates and seedling health.

Item	Why It Matters	Cost-Effective Alternative
Seed starting mix	Provides proper drainage and aeration without pathogens found in garden soil	Mix equal parts perlite, vermiculite, and peat moss or coconut coir
Containers with drainage	Prevents waterlogging that causes damping-off disease	Yogurt cups or egg cartons (poke drainage holes)
Heat mat	Maintains optimal soil temperature (70-80°F) for faster germination	Top of refrigerator or near heating vent (less consistent)
Grow lights	Prevents leggy seedlings by providing 14-16 hours of consistent light	Southern-facing window (less ideal, may cause stretching)

When to Plant Tomato Seeds: Timing Is Everything

The single most common mistake new gardeners make is planting tomato seeds too early. Tomatoes grow quickly once germinated, and seedlings kept too long indoors become root-bound and stressed before transplanting.

Calculate your planting date by counting backward from your area's last expected frost date. The USDA Plant Hardiness Zone Map helps determine your average last frost date. Most gardeners should plant seeds 6-8 weeks before this date. For example, if your last frost date is May 15, start seeds between March 15 and March 31.

Tomato Seed Germination Timeline

Understanding this progression helps you identify problems early:

• Days 1-3: Seeds absorb water and begin metabolic activity
• Days 3-7: Radicle (first root) emerges, followed by shoot
• Days 7-10: Cotyledons (seed leaves) open and turn green
• Days 14-21: First true leaves develop
• Weeks 4-6: Seedlings ready for hardening off
• Week 8: Ready for garden transplanting

Source: University of California Agriculture and Natural Resources

Step-by-Step Planting Process

1. Prepare Your Containers

Filling containers properly creates the ideal environment for germination. Moisten seed starting mix until it feels like a damp sponge (squeezing should produce no water). Fill containers leaving 1/2 inch space at the top. Gently firm the mix without compacting it.

2. Plant Seeds at Correct Depth

Place 2-3 seeds per container 1/4 inch deep. Tomatoes need light to germinate, so avoid burying them too deeply. Cover lightly with mix and mist with water. Proper depth prevents seeds from either drying out or becoming waterlogged.

3. Maintain Optimal Temperature

Tomato seeds germinate best between 70-80°F (21-27°C). Below 50°F, germination slows dramatically; above 95°F, it stops completely. A heat mat set to 75°F typically produces germination in 5-7 days. Without a heat mat, room temperature (65-70°F) may take 10-14 days.

4. Provide Consistent Moisture

Keep the seed starting mix consistently moist but not soggy. Water from below by placing containers in a tray of water, allowing the mix to absorb moisture upward. This prevents disturbing seeds and reduces damping-off disease. Check twice daily, especially when using heat mats which accelerate drying.

5. Introduce Light Immediately After Germination

Once seedlings emerge, provide 14-16 hours of light daily. Position grow lights 2-3 inches above plants, raising them as seedlings grow. Without adequate light, seedlings become tall and spindly within days. If using a windowsill, rotate containers daily to prevent leaning toward the light source.

Common Planting Mistakes and How to Avoid Them

Even experienced gardeners sometimes make these preventable errors that reduce germination rates and produce weak seedlings:

• Planting too early: Seedlings kept indoors more than 6-8 weeks become root-bound and stressed. Calculate backward from your last frost date.
• Using garden soil: Contains pathogens and compacts in containers. Always use sterile seed starting mix.
• Overwatering: Creates conditions for damping-off disease. Water only when top of mix feels dry.
• Insufficient light: Causes leggy, weak plants. Provide 14-16 hours of direct light daily.
• Skipping hardening off: Transplant shock kills many seedlings. Gradually expose plants to outdoor conditions over 7-10 days.

When NOT to Plant Tomato Seeds

Understanding these limitations prevents wasted effort:

• When soil temperatures are below 50°F: Seeds won't germinate properly and may rot. Use a soil thermometer to verify.
• In areas with less than 6 hours of direct sunlight: Tomatoes require full sun for healthy growth.
• Without proper indoor lighting setup: Windowsills rarely provide enough consistent light for strong seedlings.
• After your last frost date has passed: Direct sowing outdoors works but produces later harvests than starting indoors.

Source: Cornell University Cooperative Extension

Transplanting Seedlings to the Garden

Before moving seedlings outdoors permanently, they need to be hardened off. This 7-10 day process gradually acclimates plants to outdoor conditions:

1. Start with 1-2 hours of morning sun in a sheltered location
2. Gradually increase exposure by 1-2 hours daily
3. Expose plants to gentle wind and cooler nights
4. After 7 days, leave plants outdoors overnight if temperatures stay above 50°F
5. Transplant when seedlings have 2-3 sets of true leaves and outdoor soil has warmed to at least 60°F

When planting in the garden, bury the stem up to the first set of true leaves. Tomatoes develop roots along buried stems, creating stronger plants. Space plants 24-36 inches apart depending on variety (determinate varieties need less space than indeterminate).

Troubleshooting Common Seed Starting Problems

Identify and address these issues quickly to save your seedlings:

• No germination after 14 days: Seeds may be too old (tomato seeds remain viable 4-6 years), planted too deep, or temperatures too low.
• Damping-off disease (seedlings collapse at soil line): Caused by fungal pathogens from overwatering. Improve air circulation and avoid overhead watering.
• Leggy, stretched seedlings: Not enough light. Increase light duration or intensity immediately.
• Yellowing cotyledons: Normal as true leaves develop, but widespread yellowing indicates nutrient deficiency or overwatering.
• Mold on soil surface: Harmless but indicates excess moisture. Reduce watering frequency and improve air circulation.

How deep should I plant tomato seeds?

Plant tomato seeds 1/4 inch deep in seed starting mix. Planting too deep prevents germination, while too shallow may cause seeds to dry out. Cover lightly with mix and mist with water to ensure good seed-to-soil contact.

What's the ideal temperature for tomato seed germination?

Tomato seeds germinate best between 70-80°F (21-27°C). At 75°F, most seeds sprout in 5-7 days. Below 50°F germination slows significantly, and above 95°F it stops completely. A heat mat helps maintain consistent soil temperature.

How long before transplanting should I start tomato seeds indoors?

Start tomato seeds 6-8 weeks before your last expected frost date. Starting too early produces leggy, stressed plants. Calculate backward from your local last frost date, which you can find using the USDA Plant Hardiness Zone Map for your area.

Why are my tomato seedlings tall and spindly?

Leggy seedlings are caused by insufficient light. Tomato seedlings need 14-16 hours of direct light daily. If using natural light, a south-facing window may not provide enough intensity. Position grow lights 2-3 inches above plants and raise them as seedlings grow to maintain proper distance.

Should I soak tomato seeds before planting?

Soaking tomato seeds isn't necessary but can speed germination by 1-2 days. If you choose to soak, use room temperature water for no more than 12 hours. Over-soaking can damage seeds. Most gardeners achieve excellent results by simply planting in moist seed starting mix and maintaining consistent moisture.